VIMOS Ultra-Deep Survey (VUDS): Witnessing the Assembly of a Massive Cluster at z~3.3
B.C. Lemaux, O. Cucciati, L. A. M. Tasca, O. Le Fèvre, G. Zamorani, P. Cassata, B. Garilli, V. Le Brun, D. Maccagni, L. Pentericci, R. Thomas, E. Vanzella, E. Zucca, R. Amorin, S. Bardelli, P. Capak, L. Cassarà, M. Castellano, A. Cimatti, J.G. Cuby, S. de la Torre, A. Durkalec, A. Fontana, M. Giavalisco, A. Grazian, N. P. Hathi, O. Ilbert, C. Moreau, S. Paltani, B. Ribeiro, M. Salvato, D. Schaerer, M. Scodeggio, V. Sommariva, M. Talia, Y. Taniguchi, L. Tresse, D. Vergani, P.W. Wang, S. Charlot, T. Contini, S. Fotopoulou, R.R. Gal, D.D. Kocevski, C. López-Sanjuan, L.M. Lubin, Y. Mellier, T. Sadibekova, N. Scoville
aa r X i v : . [ a s t r o - ph . C O ] D ec Astronomy & Astrophysicsmanuscript no. AA / / (cid:13) ESO 2018August 2, 2018
VIMOS Ultra-Deep Survey (VUDS) ⋆ : Witnessing the assembly of amassive cluster at z ∼ . ⋆⋆ B. C. Lemaux , O. Cucciati , , L. A. M. Tasca , O. Le Fèvre , G. Zamorani , P. Cassata , , B. Garilli , V. Le Brun , D.Maccagni , L. Pentericci , R. Thomas , E. Vanzella , E. Zucca , R. Amorín , S. Bardelli , P. Capak , L. P. Cassarà ,M. Castellano , A. Cimatti , J. G. Cuby , S. de la Torre , A. Durkalec , A. Fontana , M. Giavalisco , A. Grazian ,N. P. Hathi , O. Ilbert , C. Moreau , S. Paltani , B. Ribeiro , M. Salvato , D. Schaerer , , M. Scodeggio , V.Sommariva , , M. Talia , Y. Taniguchi , L. Tresse , D. Vergani , , P. W. Wang , S. Charlot , T. Contini , S.Fotopoulou , R. R. Gal , D. D. Kocevski , C. López-Sanjuan , L. M. Lubin , Y. Mellier , T. Sadibekova , and N.Scoville Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, France e-mail: [email protected] INAF–Osservatorio Astronomico di Bologna, via Ranzani,1, I-40127, Bologna, Italy Instituto de Fisica y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Breta˜na 1111, Playa Ancha, ValparaísoChile INAF–IASF, via Bassini 15, I-20133, Milano, Italy INAF–Osservatorio Astronomico di Roma, via di Frascati 33, I-00040, Monte Porzio Catone, Italy University of Bologna, Department of Physics and Astronomy (DIFA), V.le Berti Pichat, 6 / INAF–IASF Bologna, via Gobetti 101, I–40129, Bologna, Italy Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, 75014 Paris,France Institut de Recherche en Astrophysique et Planétologie - IRAP, CNRS, Université de Toulouse, UPS-OMP, 14, avenue E. Belin,F31400 Toulouse, France Department of Astronomy, University of Geneva ch. d’Écogia 16, CH-1290 Versoix, Switzerland Geneva Observatory, University of Geneva, ch. des Maillettes 51, CH-1290 Versoix, Switzerland Centro de Estudios de Física del Cosmos de Aragón, Teruel, Spain Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125, USA Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, D-85741, Garching bei München, Germany Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan Department of Physics, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA University of Hawai’i, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055, USA Laboratoire AIM, CEA / DSM / Irfu / SAp, CEA-Saclay, F-91191 Gif-sur-Yvette Cedex, FranceReceived March 14th, 2014 / Accepted July 29th, 2014
ABSTRACT
Using new spectroscopic observations obtained as part of the VIMOS Ultra-Deep Survey (VUDS), we performed a systematic searchfor overdense environments in the early universe ( z >
2) and report here on the discovery of Cl J0227-0421, a massive protocluster at z = .
29. This protocluster is characterized by both the large overdensity of spectroscopically confirmed members, δ g al = . ± . ∼ × M ⊙ at z ∼ .
3, which, evolved to z = / VVDS galaxies in lowerdensity field environments at similar redshifts. We find tentative evidence for an excess of redder, brighter, and more massive galaxieswithin the confines of the protocluster relative to the field population, which suggests that we may be observing the beginningof environmentally induced quenching. The properties of these galaxies are investigated, including a discussion of the brightestprotocluster galaxy, which appears to be undergoing vigorous coeval nuclear and starburst activity. The remaining member galaxiesappear to have characteristics that are largely similar to the field population. Though we find weaker evidence of the suppression ofthe median star formation rates among and di ff erences in the stacked spectra of member galaxies with respect to the field, we deferany conclusions about these trends to future work with the ensemble of protostructures that are found in the full VUDS sample. Key words.
Galaxies: evolution - Galaxies: high-redshift - Galaxies: active - Galaxies: clusters - Techniques: spectroscopic - Tech-niques: photometric Article number, page 1 of 23 & Aproofs: manuscript no. AA / /
1. Introduction
Large associations of galaxies provide an excellent laboratoryfor investigating astrophysical phenomena. The most massive ofthese associations, galaxy clusters and superclusters (i.e., clus-ters of clusters), while rare, are useful not only to constrain thedynamics and content of the universe (e.g., Bahcall et al. 2003;Reichardt et al. 2013), but also to study the evolution of galaxies,since the core of galaxy clusters are the regions of the universewhere galaxy maturation occurs most rapidly (e.g., Dressler etal. 1984; Postman et al. 2005). This rapid maturation is a resultof the large number of transformative mechanisms that a clustergalaxy experiences, mechanisms that are less e ff ective or non-existent in regions of typical density in the universe (e.g., Moranet al. 2007). The number of processes a cluster galaxy is sub-ject to is, however, both a virtue and a complication for studyingtheir evolution. While the signs of transformation and evolutionare prevalent among galaxies in clusters that have not alreadydepleted their galaxies of gas, the large number of physical pro-cesses that are e ff ective in overlapping regimes complicates in-terpretation. Furthermore, the e ff ectiveness of such mechanismsappears to have a complex relationship with the halo mass of thehost cluster and the dynamics of the galaxies that comprise it,the density and temperature of the intracluster medium (ICM),local galaxy density, mass of the individual galaxies, and cosmicepoch (e.g., Fujita & Nagashima 1999; Poggianti et al. 2010;Lemaux et al. 2012; Muzzin et al. 2012; Dressler et al. 2013).The lower mass counterparts to galaxy clusters, galaxy groups,also su ff er the same ambiguities.As such, despite nearly a century of study into such associ-ations, the role that environment plays in galaxy evolution andthe dominant process or processes that serve to transform clusteror group galaxies is still unclear. In the local universe, the re-lationship between environment and galaxy evolution has beenrevolutionized over the past decade with the advent of the SloanDigital Sky Survey (SDSS). Observations from this survey havebeen used to great e ff ect to study the properties of both groupsand clusters and their galaxy content (e.g, Gómez et al. 2003;Hansen et al. 2009; von der Linden et al. 2010) and have ledto insight into the nature of environmentally driven evolution inthe local universe. However, these studies alone provide only abaseline for studies of cluster and group galaxies in the higherredshift universe because, in general, the galaxies populatingstructures in the low-redshift universe have come to the end oftheir evolution. Initial investigations of clusters beyond the lo-cal universe found that the fraction of galaxies that displayed asignificant gas content, bluer colors, and late-type (i.e., spiral)morphologies increased rapidly with decreasing cosmic epoch(Butcher & Oemler 1984). Yet, thirty years later, the cause orcauses of such a trend have not been identified definitively. Inintermediate-density environments, such as galaxy groups andpairs or small associations of galaxies, significant progress hasbeen made in the past decade to understand the relative e ff ectof such processes on galaxy evolution due to the emergenceof spectroscopic surveys covering large portions of the sky inthe intermediate-redshift universe ( z ∼
1, e.g., DEEP2, VVDS,zCOSMOS). While such surveys are typically devoid of mas-sive clusters, a testament to their relative scarcity, the large num- ⋆ Based on data obtained with the European Southern ObservatoryVery Large Telescope, Paranal, Chile, under Large Program 185.A-0791. ⋆⋆ Table 2 is only available at the CDS via anonymous ftpto http: // cdsarc.u-strasbg.fr (ftp: // // cdsarc.u-strasbg.fr / viz-bin / qcat?J / A + A / ber of spectroscopic redshifts, wide field coverage, and qualityof both spectroscopic data and associated ancillary data haveled to a variety of insights into the nature of galaxy evolutionin intermediate-density environments (e.g., Cooper at al. 2006,2007, 2008; Cucciati et al. 2006, 2010a, 2010b, 2012, Tasca etal. 2009; Peng et al. 2010; Presotto et al. 2012; George et al.2012; Knobel et al. 2013; Kovaˇc et al. 2014).At similar redshifts, systematic spectroscopic studies of clus-ters and cluster galaxies are somewhat rare. Surveys of clustersextending to several times the virial radius at z ∼ z ∼ ff ect of residing in the harsh clusterenvironment for several Gyr is evident among member galaxies,because the fraction of both red and quiescent galaxies is ob-served to be in excess of that of the field at similar redshifts (e.g.,Patel et al. 2011; Lemaux et al. 2012; van der Burg et al. 2013).Going to higher redshifts, the e ff ect of the environment shouldbe reversed, inducing rather than suppressing star formation asgas-rich galaxies coalesce in the primeval universe. Indeed, ten-tative evidence for the reversal of the correlation between starformation rate (SFR) and galaxy density has already been foundat slightly higher redshifts (Tran et al. 2010; Santos et al. 2014,though see also Santos et al. 2013; Ziparo et al. 2014).Observing the reversal of the SFR − density relation, as wellas contextualizing the massive, red-sequence galaxies (RSGs)observed at z ∼ z > ∼ .
5) clusters (e.g.,Henry et al. 2010; Gobat et al. 2011; Papovich et al. 2010; Stan-ford et al. 2012; Zeimann et al. 2012; Newman et al. 2014) orother overdensities (i.e., protoclusters or protostructures) in theearly universe (e.g., Steidel et al. 2005; Doherty et al. 2010;Toshikawa et al. 2012; Hayashi et al. 2012; Koyama et al. 2013;Hodge et al. 2013). One of the main di ffi culties in such searches,beyond the extreme faintness of the bulk of the member pop-ulations of such structures, is the failure of search techniquesthat are widely used at lower redshifts. Traditional techniques,such as searching for overdensities of RSGs or the presence ofa hot ICM, are predicated on the assumption that a su ffi cientlylong time scale has elapsed over which cluster galaxies can beprocessed. While these techniques can be used to find the mostmassive and oldest structures at any given redshift, such searchesare biased against exactly the types of structures where the rever-sal of the SFR − density relation should be most apparent. Oneway of circumventing this bias is to search for overdensities ofgalaxies lying at the same redshift as estimated by broadbandphotometry (i.e., photometric redshifts), which have now largelysupplanted searches for high-redshift overdensities of red galax-ies. However, the nature of such overdensities cannot be be char-acterized well without dedicated spectroscopic followup.An alternative technique, which is employed especially forsearches of the high-redshift universe, is to perform narrow-band imaging or photometric redshift searches around massiveradio-loud quasars or other types of powerful active galactic nu-clei (e.g., Kurk et al. 2004; Miley et al. 2004; Venemans et al.2004, 2005; Zheng et al. 2006; Overzier et al. 2008; Kuiper etal. 2010, 2011, 2012). Such phenomena are typically associatedwith massive galaxies, which are, in turn, typically associated Article number, page 2 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster with galaxy overdensities. While this technique has been suc-cessful in observing large numbers of structures or protostruc-tures in the high-redshift universe, it is not at all clear whethersuch environments are typical progenitors of lower redshift clus-ters or are exceptional in some way, which limits their useful-ness in contextualizing results at lower redshifts. Additionally,narrow-band and spectroscopic searches of Lyman alpha emit-ter (LAEs) populations in (somewhat) random regions of thesky have revealed protostructures in the very high-redshift uni-verse (e.g., Shimasaku et al. 2003; Ouchi et al. 2005; Lemaux etal. 2009; Toshikawa et al. 2012). However, such surveys coverrather limited portions of the sky and are only e ff ective at ob-serving overdensities of emission line objects, a population that,while being readily observed at high redshift because of the rel-ative ease of obtaining redshifts of emission line objects, is thesubdominant population in the early universe (see, e.g., Shapleyet al. 2003). As such, the structures (or protostructures) foundby such searches are wildly inhomogeneous (see the recent re-view in Chiang et al. 2013). This inhomogeneity, combined witha lack of large, comparable samples of galaxies at more mod-erate (i.e., field) densities at similar redshifts makes interpretingsuch structures di ffi cult.Ideally then, one would require a spectroscopic census ofgalaxy populations residing in both high- and lower-density en-vironments in the high-redshift universe, representative in someway of the overall galaxy population at those epochs. With sucha census it should be possible to make distinctions between evo-lution due to environmental processes and those driving over-all trends observed in galaxy populations as a function of red-shift and to properly connect these galaxy populations to theirlower redshift descendants. The recently undertaken VIMOSUltra-Deep Survey (VUDS; Le Fèvre et al. 2014), an enormous640-hour spectroscopic campaign with the 8.2-m VLT at CerroParanal targeting galaxies over 1 (cid:3) ◦ in three fields at z > ff er the problem of limited dynamic ranges in localdensities. Indeed, despite extensive spectroscopy from varioussurveys in the COSMOS (Scoville et al. 2007), CFHTLS-D1,E-CDF-S (Lehmer et al. 2005) fields, the three fields targetedby VUDS, only a few massive spectroscopically confirmed clus-ters have been found in these fields at z < . ff erences between thesesurveys and VUDS in the way that they relate to a study of thee ff ect of environment on galaxy evolution due to the nature ofgalaxies being probed. LAEs and other star-forming galaxies athigh redshift, both of which are selected in VUDS by virtue ofa photometric redshift selection, are known to be highly clumpypopulations (e.g., Miyazaki et al. 2003; Ouchi et al. 2003, 2004,2005; Lee et al. 2006; Bielby et al. 2011; Jose et al. 2013), mak-ing it possible to observe a wide dynamic range of local densi-ties. In the high-redshift universe, protostructures comprised ofsuch populations are observed (e.g., Steidel et al. 1998; Ouchiet al. 2005; Capak et al. 2011; Tashikawa et al. 2012; Chianget al. 2014) and found in simulations (e.g., Chiang et al. 2013;Zemp 2013; Shattow et al. 2013) to be large in transverse ex- tent. This large extent on the sky allows for sampling a largernumber of members in a single VIMOS pointing than in tradi-tional multi-object spectroscopic surveys of lower-redshift over-dense environments. In addition, as a result of a photometric red-shift selection, galaxies that have more distinguishing features intheir SED, i.e., both a continuum break at ∼ α ,will be more likely to be assigned a accurate photometric red-shift and are thus more likely to be targeted. Such a sample willbe comprised of a mix of quiescent, post-starburst, and starburstpopulations. These populations are instrumental in the investi-gation the e ff ect of environment on galaxy evolution. With thisin mind, we performed a systematic search for overdensities ofgalaxies with secure spectroscopic redshifts in all three VUDSfields. The full results of this search will be published in a fu-ture work. In this paper, we focus on the discovery and study ofthe most significantly detected spectroscopic overdensity in theCFHTLS-D1 field, Cl J0227-0421, a massive forming cluster at z ∼ . Λ CDM cosmology with H = − , Ω Λ = Ω M =
2. Observations
Over the past decade and a half, the 0226-04 field has beenthe subject of exhaustive photometric and spectroscopic cam-paigns. First observed in broadband imaging as one of the fieldsof the VIMOS VLT Deep Survey (Le Fèvre et al. 2004), thisfield was subsequently adopted as the first of the “Deep” fields(i.e., D1) of the Canada-France-Hawai’i Telescope Legacy Sur-vey (CFHTLS) . In this section, we first describe the VIMOSUltra-Deep Survey (VUDS; Le Fèvre et al. 2014) data, whichhave made the discovery of the protostructure reported in thispaper possible. We then briefly review other spectroscopic red-shift surveys of the field, as well as the associated deep imagingdata available in the CFHTLS-D1 field. For a thorough reviewof all data available in the CFHTLS-D1 field prior to VUDS, seeLemaux et al. (2013) and references therein. The primary impetus for the current study comes from the vastspectroscopic data available in the CFHTLS-D1 field, with a par-ticular reliance on recent VIsible MultiObject Spectrograph (VI-MOS; Le Fèvre et al. 2003) spectroscopic observations taken aspart of the VIMOS Ultra-Deep Survey (VUDS; Le Fèvre et al.2014). We therefore begin here by a brief discussion of the spec-troscopic surveys whose data are utilized for this study. http: // / Science / CFHTLS / Article number, page 3 of 23 & Aproofs: manuscript no. AA / / The observations from which a majority of our results are de-rived were taken from VUDS, a massive 640-hour ( ∼
80 night)VIMOS spectroscopic campaign reaching extreme depths ( i ′ < ∼
25) of three well-known and well-studied regions of the sky, ofwhich, the CFHTLS-D1 field is one. The design, goals, and sur-vey strategy of VUDS are described in detail in Le Fèvre et al.(2014) and are thus described here only briefly. The primary goalfor the survey is to measure the spectroscopic redshifts of a largesample of galaxies at redshifts 2 < ∼ z < ∼
6. To this end, unlikeits predecessors that were magnitude limited, the selection ofVUDS spectroscopic targets was performed primarily throughphotometric redshift cuts, occasionally supplemented with a va-riety of magnitude and color − color criteria. These selectionswere used primarily to maximize the number of galaxies withredshifts likely in excess of z > ∼ ff ect, as a largefraction of the galaxies spectroscopically confirmed in VUDShave redshifts z > ∼ all other surveyscombined at redshifts z > ∼
2. The main novelty of the VUDS ob-servations is the depth of the spectroscopy and the large wave-length coverage that is a ff orded by the 50400s integration timeper pointing and per grating with the low-resolution blue and redgratings on VIMOS (R = α emitter (LAE) galaxies, galaxies which dominate other high red-shift spectroscopic samples, but also for redshift determinationfrom Lyman α (hereafter Ly α ) and interstellar medium (ISM) ab-sorption in those galaxies that exhibit no emission line features.Thus, the VUDS data allow for a selection of a spectroscopic volume-limited sample of galaxies at redshifts 2 < ∼ z < ∼
6, a sam-ple that probes as faint as M ∗ + = X2, X3, & X4, where X = , for which the probability of theredshift being correct is in excess of 75%, are considered reli-able (hereafter “secure spectroscopic redshifts”). In total, spectraof 2395 unique objects were obtained on the CFHTLS-D1 fieldas part of VUDS, with 1534 of those resulting in secure spec-troscopic redshifts. This represents only 80% of the final VUDSdata on this field, because one VUDS VIMOS quadrant, cen-tered at [ α J , δ J ] = [02:24:36.1, -04:44:58] has yet to bereduced at the time of publication. For further discussion of thesurvey design, observations, reduction, redshift determination,and the properties of the full VUDS sample see Le Fèvre et al.(2014). X = = = = CFHTLS-D1 Spectroscopic Surveys spec o f S ec u r e R e d s h i f t s VUDS (N=1534, spec =2.50)ORELSE (N=318, spec =0.89)VVDS (N=8748, spec =0.77)
Fig. 1.
Spectroscopic redshift distribution of the 10600 unique objectswith secure spectroscopic redshifts (see text) in the CFHTLS-D1 field.The two lower redshift surveys, VVDS (Le Fèvre et al. 2013) andORELSE (Lubin et al. 2009), are shown as hashed green and orangehistograms, respectively. The higher redshift VUDS (Le Fèvre et al.2014) objects are shown as a black filled histogram. The number of ob-jects with secure spectroscopic redshifts coming from each survey alongwith the median z spec of each sample is shown in the top right corner.For the sake of clarity, the bin size of the histograms for the VUDS andORELSE objects are twice that of the VVDS. Though it is not apparentfrom the diagram, there is a tail of galaxies with z spec > The bulk of the lower redshift ( z < ∼
2) spectroscopy in this fieldwas drawn from observations taken as part of the VVDS “Deep”and “Ultra-Deep” surveys (see Le Fèvre et al. 2005, 2013 fordetails on the survey design and goals) and the Observations ofRedshift Evolution in Large Scale Environments (ORELSE; Lu-bin et al. 2009) survey. The properties of the spectroscopy avail-able in the CFHTLS-D1 field from these surveys is described ex-tensively in Lemaux et al. (2013) and references therein. Thesedata were used primarily in this study to reject lower redshiftinterlopers and to calibrate and extensively test the SED fittingprocess that is described in §2.3. For the VVDS surveys, the cri-terion for a secure spectroscopic redshift was the same as thatof VUDS. For ORELSE, only those objects with quality codes Q = −
1, 3, & 4 , for which the probability that a correct redshiftwas assigned is in excess of 95%, were considered secure. Ac-counting for duplicate observations, a total of 11267, 1120, and Q = − Q = ≥ Q = ≥
450 spectra were taken of unique objects in the CFHTLS-D1field from the VVDS-Deep, VVDS-Ultra-Deep, and ORELSEsurveys, respectively, resulting in 7942, 806, and 318 securespectroscopic redshifts of unique objects from the three surveys.Combining all surveys, we have obtained a secure spectroscopicredshift for a total of 10600 unique objects across the CFHTLS-D1 field spanning from 0 ≤ z spec ≤ .
53. The redshift distribu-tions of those objects with secure spectroscopic redshifts fromthe three surveys are shown in Figure 1.
Of the plethora of optical imaging data available on theCFHTLS-D1 field, the most relevant for this study is the deepfive-band ( u ∗ g ′ r ′ i ′ z ′ ) optical imaging of the entire 1 (cid:3) ◦ fieldobserved with Megacam (Boulade et al. 2003) as part of the“Deep” portion of the CFHTLS survey. Model magnitudes(MAG_AUTO, Kron 1980; Bertin & Arnouts 1996) were takenfrom the penultimate release of the CFHTLS data (T0006, Gora-nova et al. 2009) and corrected for Galactic extinction and reduc-tion artifacts using the method described in Ilbert et al. (2006).The resulting magnitudes reach 5 σ point -source complete-ness limits (i.e., σ m = .
2) of 26.8 / / / / u ∗ g ′ r ′ i ′ z ′ bands, respectively, su ffi cient to detect galaxies as faintas ∼ L ∗ at z = . L ∗ ). For further details on the properties of the CFHTLS-D1imaging and the reduction process, see the CFHTLS TERAPIXwebsite , Ilbert et al. (2006), and Bielby et al. (2012).As a compliment to the CFHTLS optical imaging, roughly75% of the CFHTLS-D1 field, including the entire area of inter-est for the present study, was imaged with WIRCam (Puget et al.2004) in the near infrared (NIR) J , H , and K s bands as part ofthe WIRCam Deep Survey (WIRDS; Bielby et al. 2012). Modelmagnitudes were drawn from the T0002 release of WIRDS data and corrected for Galactic extinction using the method describedin Bielby et al. (2012). The resulting magnitudes reach 5 σ pointsource completeness limits of 24.7, 24.6, and 24.5 in the J , H ,and K s bands, respectively, su ffi cient to detect galaxies as faintas ∼ L ∗ at z = .
3. For further details on the observation, re-duction, and characteristics of the WIRDS data see Bielby et al.(2012).Two generations of imaging with the
Spitzer
Space Tele-scope were taken on the CFHTLS-D1 field. Initially, a largeportion of the CFHTLS-D1 field was imaged at 3.6 / / / µ m from the Spitzer
InfraRed Array Camera (IRAC; Fazio etal. 2004) and at 24 µ m from the Multiband Imaging Photome-ter for Spitzer (MIPS; Rieke et al. 2004) as part of the
Spitzer
Wide-Area InfraRed Extragalactic survey (SWIRE; Lonsdaleet al. 2003). However, these data were too shallow to detecta large majority of the galaxies presented in this study. Addi-tional
Spitzer / IRAC data in the two non-cryogenic bands (3.6& 4.5 µ m) for the entirety of the field were obtained from theSpitzer Extragalactic Representative Volume Survey (SERVS;Mauduit et al. 2012). These data, which incorporated the SWIREdata when available, are moderately deeper, reaching 5 σ point-source completeness limits of 23.1 in both [3 .
6] and [4 . ∼ L ∗ cluster galaxy at z = .
3. Aperturemagnitudes measured within a radius of 1.9 ′′ , roughly equivalentto the full-width half-maximum (FWHM) point spread functions(PSFs) of the IRAC images in both bands, were drawn from theo ffi cial SERVS data catalog. These magnitudes were aperture- http: // terapix.iap.fr / rubrique.php?id_rubrique = http: // terapix.iap.fr / rubrique.php?id_rubrique = corrected by dividing the flux density as measured in the aper-ture by 0.736 and 0.716 in the 3.6 and 4.5 µ m channels, re-spectively , necessary for matching the model magnitudes of ourother optical and NIR (hereafter optical / NIR) imaging. For fur-ther details of the reduction of SERVS data for the CFHTLS-D1field, see Mauduit et al. 2012. The matching of SERVS sourcesto optical / NIR counterparts from our ground-based imaging wasperformed by using the known mapping of SWIRE sources (seeArnouts et al. 2007) when available and nearest-neighbor match-ing to the combined ground-based optical / NIR catalogs whenno SWIRE source was detected at the position of the SERVSsource. In total, 75.5% of all objects with spectroscopic datawere matched to a SERVS counterpart. Even for the highestredshifts probed by the VUDS / VVDS spectroscopy, z >
3, thisnumber remains high: a majority (62.5%) of galaxies with securespectroscopic redshifts above this limit are matched to a SERVScounterpart.The CFHTLS-D1 field has also been imaged at a variety ofother wavelengths with the Very Large Array (VLA), the Gi-ant Millimetre Radio Telescope (GMRT), the Spectral and Pho-tometric Imaging REceiver (SPIRE; Gri ffi n et al. 2010) aboardthe Herschel
Space Observatory (Pilbratt et al. 2010), and X-rayMulti-mirror Mission space telescope (
XMM-Newton ; Jansen etal. 2001). Since these data probe relatively shallowly in the var-ious luminosity functions at z = . Despite the high density and immense depth of the spectroscopiccoverage in the CFHTLS-D1 field, a majority of the objects inthe field that are detectable to the depth of our imaging data werenot targeted with spectroscopy. For these objects, informationcan only be obtained through fitting to their spectral energy dis-tributions (SEDs) in the observed-frame optical / NIR broadbandphotometry. To derive redshifts from the photometric data for un-targeted objects, as well as their associated physical parameters,e.g., stellar masses, mean luminosity-weighted stellar ages, andSFRs, we utilized the package Le Phare (Arnouts et al. 1999; Il-bert et al. 2006, 2009) in a method identical to the one describedin Lemaux et al. (2013). The process for deriving physical pa-rameters for galaxies that have been spectroscopically targetedwas similar to the one in Lemaux et al. (2013) with a few minormodifications that are described in Appendix A. For some anal-ysis, similar fitting was performed on VUDS rest-frame near-ultraviolet (NUV) spectra using the Galaxy Observed-SimulatedSED Interactive Program (GOSSIP; Franzetti et al. 2008). Thedetails of all synthetic model fitting to the photometric and spec-troscopic detail, as well as discussions of the e ff ect of variousassumptions made for these fitting processes, are discussed indetail in Appendix A. In Figure 2 we show a comparison of thephotometric redshifts derived from the CFHTLS and WIRDS(hereafter CFHTLS / WIRDS) imaging and their associated spec-tral redshifts for those galaxies with secure spectroscopic red-shifts. Of particular importance to this work is the true red-shift distribution of objects with z phot >
3, which are almost For further details see http: // irsa.ipac.caltech.edu / data / SPITZER / SERVS / docs / SERVS_DR1_v1.4.pdf http: // cfht.hawaii.edu / ∼ arnouts / LEPHARE / lephare.htmlArticle number, page 5 of 23 & Aproofs: manuscript no. AA / / CFHTLS-D1 VVDS+ORELSE+VUDS z pho t spec -0.50.00.5 ∆ z / ( + z s p ec ) Fig. 2.
Comparison of photometric redshifts as derived from eight-bandground-based optical / NIR imaging and spectroscopic redshifts for thoseobjects with secure spectroscopic redshifts (see §2.1). In the bottompanel ∆ z ≡ ( z spec − z phot ). Members of the Cl J0227-0421 protostructure(see §3.1) with secure spectroscopic redshifts are denoted in both pan-els by red diamonds, those with less secure spectroscopic redshifts areshown as blue Xs. For an discussion of the relevance of this comparisonfor this study and an explanation of the parabolic feature seen in thebottom panel see the text at the end of §2.3. always (82.7% of the time) at z spec >
3. The large majorityof cases where a galaxy is at z spec > α break was mistaken forthe Balmer / z phot ∼
3. An exploration into the role of environment inVUDS
We begin the exploration by briefly describing the search tech-nique used to find spectroscopic overdensities of galaxies in theVUDS survey. Though the search technique is broadly similarin all fields, we limit ourselves here to the search as performedon the CFHTLS-D1 field and defer the discussion of the searchin the two other fields for future work (though see Cucciati etal. 2014 for a discussion of the most significant overdensity inthe COSMOS field). The methodology used for the search, alongwith an involved discussion of the purity and completeness of theoverdensities found in all VUDS fields, will also be described ina future paper since here we are concerned with only the mostsignificant of the overdensities in the CFHTLS-D1 field.The search was performed as follows. All unique galaxieswith secure spectroscopic redshifts in the CFHTLS-D1 field (see§2.1) were combined into a single catalog, and this catalog wasused to generate density maps of secure spectroscopic objectsusing the methodology of Gutermuth et al. (2005). To be consid-ered a legitimate overdensity, referred to hereafter by the suf-
CFHLS-D1-VUDS Best Structure α J2000 [ ° ]-4.8-4.6-4.4-4.2 δ J [ ° ] Type-1 AGN 3.27 < z < 3.35 (flag=12,13,14)X-Ray AGN 3.27 < z < 3.35 (flag=12,13,14) z > 1.5 (flag=2,3,4)
Fig. 3.
Sky distribution of galaxies in the redshift range of the mostsignificantly detected spectroscopic protostructure in the CFHTLS-D1field (Cl J0227-0421). Galaxies with secure spectroscopic redshifts areplotted as red diamonds, those with less secure spectroscopic redshiftsare shown as blue Xs. Green stars denote those galaxies hosting a type-1 AGN and the cyan circle denotes the lone X-ray AGN host at theseredshifts. Plotted in the background are all galaxies in the CFHTLS-D1field with secure spectroscopic redshifts z spec > .
5. The dashed circledesignates 3 h − Mpc from the adopted center of the protostructure andis used in conjunction with the redshift range to define membership (see§3.1). The VIMOS footprint is clearly visible, and coverage gaps in theCFHTLS-D1 spectroscopy are apparent throughout the field and to thenorth of the protostructure. ficiently ambiguous term “protostructure”, we required sevenconcordant redshifts within a circle of radius 2 h − proper Mpcat the redshift of the source and a maximum distance betweengalaxies along the line of sight of 25 h − proper Mpc (equiva-lent to roughly ∆ v ∼ − − or ∆ z ∼ . − . . < z < . z ∼ .
3, is shown in Figure 3.Generated a posteriori were photometric redshift densitymaps of all galaxies within ∆ z phot ± . + z spec ) of the spec-troscopic redshift bounds of each protostructure using the samemethodology as used to create the spectral density maps. Whilewe did not require an overdensity of photometric redshift sourcesto consider a grouping of galaxies a protostructure, these mapsserved to lend credence to the overdensity seen in the spectro-scopic data and to more fully probe the large scale structure(LSS) of the galaxy overdensity. The latter is especially impor-tant because, as mentioned earlier, both LAEs and other star-forming galaxies at high redshift are highly clustered popula- Article number, page 6 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster
CFHLS-D1-VUDS Best Structure α J2000 [ ° ]-4.45-4.40-4.35-4.30-4.25 δ J [ ° ] spec < 3.35 z phot =z PS + σ z (1+z) z spec >1.5 M NUV -M r ′ ≥ NUV -M r ′ < 1.4 Type-1 AGNX-Ray AGN o f G a l a x i e s spec Σ G a l M p c - Fig. 4.
Left:
Zoom-in of the sky distribution of all galaxies in the redshift range of the same protostructure that is shown in Figure 3 (Cl J0227-0421). This protostructure is also the most significantly detected protostructure in photometric redshift overdensity. The photometric density map,generated using the methodology of §3, is shown in the background, with the scale bar denoting the photometric redshift galaxy density. Asin Figure 3, the dashed circle designates 3 h − Mpc from the adopted center of the protostructure. Blue and red symbols show galaxies at theredshift of the protostructure di ff erentiated by their M NUV − M r ′ colors (where the delineation point was set roughly at the color of a 200 Myrold stellar population, see §4.1.2) and are logarithmically scaled (log ) by their stellar mass (see §4.1). Filled circles denote galaxies with securespectroscopic redshifts, while Xs denote those galaxies with less secure spectroscopic redshifts. Plotted in the background are all galaxies with z spec > .
5. Several massive, redder galaxies are observed in the bounds of the protostructure and extended filamentary structure can be seen to thewest-southwest of the central protostructure galaxy concentration.
Bottom right:
Spectroscopic redshift distribution of galaxies within 3 h − Mpcof the protostructure center. The shaded histogram displays only those galaxies with secure spectroscopic redshifts, while the histogram plottedwith a solid black line also includes those galaxies with less secure spectral redshifts. The redshift bounds defining membership are marked bymagenta dot-dashed lines. tions, and in a single VIMOS pointing, roughly only 20% ofobjects with photometric redshifts at z phot > , which was in turn used todefine a significance threshold for photometric redshift galaxyoverdensities. Shown in Figure 4 is an example of a density mapplotted for the protostructure in the CFHTLS-D1 field with thehighest significance in photometric redshift galaxy density. z ∼ . protostructure intheCFHTLS-D1field Of the 13 spectroscopically detected protostructures in theCFHTLS-D1 field using the search algorithm described above,one, a protostructure at z ∼ .
3, far exceeded the others both interms of the density of spectroscopic member galaxies and thedensity of potential photometric redshift members. As shown inFigure 5 and in Figures 3 and 4, this protostructure is detected More specifically, those photometric redshift source overdensitiesthat did not have a spectroscopic overdensity su ffi cient to fulfill the cri-teria set by the protostructure filter centered anywhere within 1 h − Mpcof the peak pixel of the photometric redshift overdensity. extremely significantly both in the number of spectroscopicallyconfirmed member galaxies, δ g al = . ± .
8, and in its over-density of sources with photometric redshifts consistent with theprotostructure redshift, σ S Ex , LS S = . R pro j < h − proper Mpc, in order tobe as inclusive as possible while still probing a reasonably smallvolume, we allowed the defined transverse extent of the proto-structure increase to the (projected) radius at which the galaxydensity fell to ∼
50% of the density calculated with the nominalfilter size.For the protostructure that is the subject of this paper, re-ferred to hereafter as Cl J0227-0421 , the projected radius atwhich the galaxy density fell to this value was found to be R pro j < h − proper Mpc. This distance is still easily spanned by z = ∼ − , could not reachthe core of the protostructure by z =
0. This radial cut is used While a prefix designating this protostructure a cluster may seempresumptuous, the reason for this is formally quantified in §3.1.1.Article number, page 7 of 23 & Aproofs: manuscript no. AA / / CFHTLS-D1 VUDS+VVDS gal in Filter0200400600800 o f t r i a l s δ Protostruct. =10.5 ± σ Protostruct. =13.5
Filter: ∆χ =16 h Mpc, R proj <3.0 h
Mpc2.5 < z initial < 3.5 µ =1.7 σ =1.3 CFHTLS-D1 Photo-z Significance σ SEx o f s pu r i ou s d e t . σ SEx, z=3.3 LSS ( σ z=3.3 LSS - µ σ )/ σ σ =7.98 µ σ =7.25 σ σ =3.19 Fig. 5.
Left:
Spectroscopic overdensity of Cl J0227-0421. Plotted in black is the histogram of the number of galaxies with secure spectroscopicredshifts falling within a filter of the dimensions listed ( χ refers to proper distance) for 1000 observations of random locations and central redshifts(2 . < z spec < .
5) across the CFHTLS-D1 field avoiding gaps in spectroscopic coverage. The solid green line shows the best-fit Poisson distribu-tion with the numbers to the left denoting the best-fit parameters. The number of members of Cl J0227-0421 with secure spectroscopic redshiftswithin the filter bounds is plotted as a red vertical dashed line. The Cl J0227-0421 galaxy overdensity, δ g al ≡ ( N g al , Proto − Struct . − µ ) /µ , is shown tothe left of the vertical dashed line along with the formal significance of the spectroscopic overdensity, σ Proto − Struct . ≡ ( N g al , Proto − Struct . − µ ) /σ . Right:
Photometric redshift galaxy overdensity of Cl J0227-0421. The black histogram shows the Source Extractor (SEx) significance distribution ofspurious density peaks in the CFHTLS-D1 field (see §3). The solid red line shows the best-fit Gaussian distribution to the significance distributionof the spurious peaks and the associated best-fit parameters are shown above this line. The horizontal dashed blue line denotes the SEx significanceof the photometric redshift galaxy overdensity in Cl J0227-0421, while the number directly to the left of the line gives the formal significance ofthe overdensity that accounts for spurious density peaks. for all subsequent analysis with one exception mentioned later,though we note that all results for this protostructure, includ-ing the magnitude of the spectroscopic overdensity, are largelyinsensitive to the specific choice of the size of the dimensionsprobed for R pro j < h − proper Mpc and ∆ χ < h − properMpc. We also tested for e ff ects on δ g al as a result of non-uniformspectral sampling and found no di ff erence in the calculated valueif the “field” search described in Figure 5 was instead limited tothe area over which the protostructure extended (i.e., the sameVIMOS quadrant).The spatial center of Cl J0227-0421 was calculated in amethod similar to the one described in Ascaso et al. (2014) for allgalaxies within 3 . < z < .
35 and R pro j < h − Mpc, but withthe peak of the photometric redshift source density map servingas the initial guess as the center. Unit weighting was chosen overluminosity weighting owing to significant contamination fromAGN activity of the brightest galaxy in the protostructure (see§4.1.1) in both the K s and the IRAC bands. Regardless, the cen-ters calculated from K s -band luminosity-weighted average or aunit-weighted average of members within R pro j < h − Mpcare shifted negligibly from the adopted center ( ∼ ′′ or ∼ z = . ff ecton our results. In Figure 4 a spectroscopic redshift histogram isplotted of all galaxies with 2 . < z spec < . R pro j < h − of the number-weighted center. Both the unit-weighted spec-troscopic center and the photometric member density center aregiven in Table 1, along with the number of members within theadopted bounds of Cl J0227-0421 and their median redshift. Intotal, 19 members with secure spectroscopic redshifts are foundwithin Cl J0227-0421 (referred to hereafter as “spectral mem-bers”), with another six galaxies having spectroscopic redshiftsconsistent with that of the protostructure but with a lower reli- ability (i.e., flags = α . In Ta-ble 2, available through CDS, we give the identification number,right ascension, declination, spectroscopic and photometric red-shift, apparent and absolute magnitude, stellar mass, and SFR ofeach of the spectral members and questionable spectral membersof Cl J0227-0421.With the relatively large number of spectral members af-forded by the VUDS and VVDS spectroscopy, an attempt canbe made to calculate the dynamics of the members of Cl J0227-0421. Because the member galaxies of this protostructure havehad little time to interact, it is likely that the dynamics will departappreciably from the near-virialized dynamics observed in mem-bers of lower redshift structures. In addition, the estimated dy-namics of cluster and group members that have had much moretime to mature have been found to vary considerably with spec-troscopic sampling, both in the number of members and the rep-resentative number of sampled galaxies of various types, mak-ing any estimate here highly uncertain. With this warning, theline-of-sight velocity dispersion (referred to simply as the ve-locity dispersion hereafter), σ v , was calculated using the methodof Rumbaugh et al. (2013) for the 14 spectral members within R pro j < h − Mpc. A smaller radial cut was used here to probegalaxies that have had a greater chance to interact with each otherand the protostructure potential, though σ v does not vary within Article number, page 8 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster
Fig. 6.
Mosaic of the one-dimensional rest-frame VIMOS spectra of ten spectral members of Cl J0227-0421. The black line in each panel is theflux density spectrum, and the dashed magenta line is the formal uncertainty spectrum (see Le Fèvre et al. 2014 and references therein for detailson the generation of the uncertainty spectrum). Important spectral features are marked. The spectrum of the proto-BCG, a type-1, and X-ray AGNhost is shown in the 3rd panel from the top on the left. The spectra of the two other type-1 AGN hosts are shown in the top and 4th from thetop panel on the left. The first four galaxies plotted in the left panel were observed as part of the VVDS-Deep sample and, as such, do not haveobserved spectra blueward of λ rest ∼ < Fig. 7.
Mosaic of the one-dimensional rest-frame VIMOS spectra remaining nine spectral members of Cl J0227-0421. Also plotted in the bottomright hand panel (ID = α , though this galaxy does not enter any of our analysis and is presented here and elsewhere only forillustrative purposes. The meanings of all lines are the same as in Figure 6. Article number, page 9 of 23 & Aproofs: manuscript no. AA / / Table 1.
General properties of Cl J0227-0421
Spectral-number-weighted center [ α J , δ J ] = [02:27:00.6, -04:21:20.2]Photo- z density map center [ α J , δ J ] = [02:26:55.2, -04:20:45.6]Number of spectral members 19 (6) a Median redshift ˜ z = . δ g al = . ± . σ Proto − S truct . = . z overdensity σ S Ex , LS S = . b Galaxy velocity dispersion σ v = ±
343 km s − Notes. ( a ) The first number refers to all spectroscopically confirmed members with R proj < h − Mpc and 3 . < z spec < .
35. The number inparentheses refers to tentative members with less reliable redshift measurements (see §2.1). ( b ) This number refers to the formal significance of thedetection after accounting for spurious density peaks (see §3.1).
CFHTLS-D1 Best Structure -4000 -2000 0 2000 4000 ∆ v LOS [km s -1 ]01234 o f G a l a x i e s Protostruct. = 3.293 σ v = 995 ±
343 km/s
Fig. 8. Di ff erential velocity distribution of the all spectral members ofCl J0227-0421. The median redshift of the secure members is shown inthe top right corner of the plot. Also shown in the top right corner is thevalue of the best-fit line of sight (LOS) velocity dispersion ( σ v , see §3.1for details). The resulting Gaussian function generated by the best-fit σ v is overplotted on the di ff erential velocity histogram (solid black line)along with those functions generated from σ v ± σ σ v . The high degreeof skewness of the di ff erential velocity distribution of member galaxiescan be clearly seen. the errors if we instead chose to use all members within R pro j < h − Mpc.Four di ff erent methods were used to calculate the velocitydispersion, identical to those of Rumbaugh et al. (2013), witherrors estimated through jackknifing. The velocity dispersion es-timated by the f-pseudosigma method, a method that performsadequately in probing the true distribution of sparsely samplednon-Gaussian distributions (see Beers et al. 1990), was adoptedas the best-fit velocity dispersion. Our results are not heavily re-liant on this choice because all other methods had values con- sistent with this values within 1 σ of their (large) formal errors.The di ff erential velocity distribution of member galaxies, plot-ted in Figure 8, is highly non-Gaussian, with a skewness of 1.06,probably a result of the (relatively) small number of membergalaxies with secure spectroscopic redshifts and the high redshiftof the protostructure. The f-pseudosigma galaxy velocity disper-sion, which is the value used throughout the remainder of thispaper, was calculated to be σ v = ±
343 km s − . The corre-sponding virial radius at the redshift of Cl J0227-0421, a quan-tity used extensively in the next section, was calculated usingthe methodology of Lemaux et al. (2012) to be R v ir = . ± h − Mpc. While we adopt this value of the virial radius for theremainder of the paper, we do not require it for any of our anal-ysis to have a physical meaning outside of a distance from thecenter of the protostructure, which represents some density con-trast to which we can scale global quantities. Indeed, it has beensuggested that a high percentage of the mass of a structure at agiven redshift lies not within the virial radius at the redshift of asource, but rather in the virial radius as estimated from the crit-ical density evaluated at z = R v ir , z = = . ± h − Mpc, is far more well matchedto our protostructure filter and the criterion used to define mem-bership throughout this paper. The choice of adopting R v ir at theredshift of the protostucture for use in our analysis was governedsimply by convention and convenience, and another value, suchas R or R v ir , z = , could have been chosen with no e ff ect on ourresults. At lower redshift ( z < ∼
1) strong correlations are observed be-tween the properties of cluster and group galaxies and the totalmass of the structure in which they reside. The maturity of thedynamical evolution of a host structure or a perturbing event,such as a cluster-cluster merger, can also govern the properties ofits galaxy content to some degree (e.g., Ma et al. 2010; Lemauxet al. 2012; Rumbaugh et al. 2012; Stroe et al. 2014, though seealso De Propris et al. 2013). However, averaged over many struc-tures, the halo mass has been found to be intimately linked to thefraction of blue, star-forming, and starbursting member galaxies,the properties of the brightest cluster and group galaxies, andthe shape of member galaxy luminosity / stellar mass functions.While still di ffi cult to measure and correctly calibrate, halo massproxies at these redshifts are relatively numerous. The dynamicsof large numbers of spectroscopically confirmed members galax-ies, weak or strong gravitational lensing, and measurements ofthe properties of the hot intracluster medium, either through Article number, page 10 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster
Bremsstrahlung emission or via the inverse-Compton scatteringof cosmic microwave background photons have all been usede ff ectively at z < ∼ ff ectiveness (in di ff erent ways) as the redshift of theobserved structure increases, and indeed, few halo mass mea-surements, calculated via these methods, exist for structures withredshifts in excess of z > ∼ . z ∼ .
3, estimating the halo mass of ClJ0227-0421 is daunting. Because of the large uncertainties andlarge number of assumptions that are required of any particularmethod, in this section we attempt four di ff erent methods of esti-mating or constraining the halo mass Cl J0227-0421. In this sec-tion we briefly describe the methods used and the halo mass andassociated uncertainties that result from each line of reasoning.For further details on the framework, assumptions, and detailsof each method see Appendix B. While the results of this exer-cise can be used to test the standard Λ CDM concordance modelof cosmology, as done in numerous other works investigatinghigh-redshift structures (e.g., Foley et al. 2011; Gonzalez et al.2012; Bayliss et al. 2013), the goal here is to simply provide agreater context for Cl J0227-0421 with which to compare otherhigh-redshift protostructures and to provide a backdrop for thepreliminary investigation of galaxy evolution that follows.We begin by calculating the halo mass of Cl J0227-0421from the dynamics of the spectral members. The calculation wasperformed in a method identical to Lemaux et al. (2012), thoughthe impact of adopting assumptions valid at z ∼ z ∼ . M d y n , v ir = . ± . × h − M ⊙ . Given that such high massappears to already be in place at such high redshift, it is inter-esting to consider what the potential evolution of the halo of ClJ0227-0421 would be to the present day. Adopting the formal-ism of McBride et al. (2009) and Fakhouri & Ma (2010) basedon results from the Millennium and Millennium-II simulations,the mean halo growth rate as a function of redshift and halo massis defined as h ˙ Mi mean = . M ⊙ yr − M z M ⊙ ! . (1 + . z ) × q Ω m , (1 + z ) + Ω Λ , (1)where M z is the halo mass of the protostructure at the red-shift of interest. Using the dynamical mass calculated above,integrating this formula from z = z = . z = .
29 yields M d y n , v ir , z = = . ± . × h − M ⊙ . Errors are determinedfrom those in the velocity dispersion. The halo mass estimatedfrom this calculation is enormous, enough to rival the most mas-sive galaxy clusters observed in the local universe (e.g., Pi ff arettiet al. 2011; Wang et al. 2014). However, the number of assump-tions, their associated uncertainties, and the formal errors com-ing from the velocity dispersion calculation are also enormous.In addition, the above formula is meant to be applied to a singlehalo, whereas the dynamical mass estimate above may make useof galaxies that populate several di ff erent halos, a subtlety thatwe have, with the current data, no power to constrain. If it is in-deed the case that the galaxies used to estimate the dynamicalmass of Cl J0227-0421 populate multiple halos, the z = Table 3.
Halo mass estimates of Cl J0227-0421
Method M v ir , z = . a M v ir , z = a Dynamics 3 . ± . × . ± . × Stellar mass 1 . ± . × . ± . × X-ray < . ± . × —Galaxy density — 3 . + . − . × Notes. ( a ) In units of h − M ⊙ evaluated at the virial radius of Cl J0227-0421 ( R v ir = . h − Mpc) estimated here will necessarily be an upper limit, though howconstraining this limit depends on the multiplicity, mass ratio,and the proximity of the constituent subhalos. Regardless, suchan e ff ect is unlikely to be greater than the formal uncertaintiesin the evolved halo mass. It is su ffi cient to say, then, that the dy-namical mass estimate places Cl J0227-0421 as a progenitor ofa cluster within similar to or exceeding the mass of the Comacluster ( M d y n ∼ − × M ⊙ ; Kent & Gunn 1996; Colless &Dunn 1996).A second approach is to use the stellar content of the pro-tostructure as a proxy for the total mass. An estimate from thismethod is, however, likely to be a lower limit due to some frac-tion, perhaps considerable at these redshifts (see, e.g., Capak etal. 2011), of the baryonic content of member galaxies residingin unprocessed gas. Briefly, the calculation takes the form ofsumming up the total stellar mass content in all members of theprotostructure within a certain radius, accounting for the missednumber of members, and using the resulting total stellar massof the members to estimate the total halo mass based on knowncorrelations. For more details, see Appendix B. The resultinghalo mass is scaled to a common radius with that of all othermethods (where we chose the virial radius for convenience) us-ing a Navarro-Frenk-White (NFW, Navarro et al. 1996) profileas described in Appendix B. This halo mass estimate from thismethod was M Σ M s , v ir = . ± . × h − M ⊙ , consistentwithin approximately 1 σ with the dynamical halo mass estimate.This halo mass was evolved to z = M Σ M s , v ir z = = . ± . × h − M ⊙ .As mentioned previously, the CFHTLS-D1 field was imagedwith XMM-Newton / EPIC to a depth of 10.6 ks in the proximityof Cl J0227-0421 While this depth is not su ffi cient to signif-icantly detect X-ray emission from any nascent ICM that mayexist in the protostructure, we determined an upper limit on thisemission of f X , [0 . −
2] keV < . ± . × − ergs s − cm − by the method described in Appendix B. This flux limit wasconverted into an observed-frame luminosity value at the red-shift of Cl J0227-0421 and k -corrected with the Chandra Inter-active Analysis of Observations package (CIAO; Fruscione et al.2006) to the rest-frame [0.1-2.4] keV band using a Raymond-Smith thermal plasma model (Raymond & Smith 1977) with atemperature of 2 keV and an abundance of 0.3 Z ⊙ (though usingmodels of di ff ering temperatures or abundances gives consistentresults within ∼ r and was trans-formed to a mass limit at the virial radius using the methodsdescribed in Appendix B. The resulting hydrostatic halo mass Another pointing of XMM was centered to the northeast of the pro-tostructure to a depth of 24 ks, but the large o ff -axis angle of the proto-structure in this pointing resulted in an X-ray flux limit that was similarto that of the 10.6 ks exposure. Article number, page 11 of 23 & Aproofs: manuscript no. AA / / limit is M X , v ir < . ± . × h − M ⊙ . Because this valueis a limit, we do not attempt to evolve it to the present day.The final halo mass calculation is based on translating thespectroscopic overdensity into a halo mass using a relationshipbetween the clustering of galaxies and their underlying dark mat-ter distribution. This methodology relies heavily on the one pre-sented in Chiang et al. (2013) and Steidel et al. (1998), and themanifestation of this methodology that was adapted for this workis presented in Cucciati et al. (2014). As such, we only mentionthose aspects relevant for this calculation on Cl J0227-0421 andrefer interested readers to those studies. The galaxy overdensity, δ g al , calculated in §3.1 was calculated again using a box filterwith half-height dimensions of R e = . z ∼ .
3, yielding δ g al = . ± . M s > M ⊙ was im-posed on both the spectral members, and the field and a galaxybias, b = .
38, was adopted based on a linear interpolation of bi-ases at di ff erent redshifts presented for an identical stellar masscut in Chiang et al. (2013).At this point a long overdue matter of nomenclature needs tobe mentioned regarding the designation of Cl J0227-0421. Hav-ing now calculated δ g al for an equivalent sample as presentedin Chiang et al. (2013), we can directly compare this value tothe simulated protostructures from Chiang et al. (2013) to esti-mate the probability that Cl J0227-0421 will evolve to a clus-ter by z =
0. Even the 1 σ lower bound of δ g al calculated forCl J0227-0421 exceeds the threshold at which a protostructurewill always evolve into a cluster as determined for an identi-cal filter size at an identical redshift in the Millennium simula-tions. In this way we justify the designation of Cl J0227-0421as a cluster in the process of formation thus allowing us to re-fer to it as a protocluster for the remainder of the paper. Fromthe calculated δ g al and adopted bias factor the halo mass of ClJ0227-0421, evolved to z = M δ m , v ir , z = = . + . − . × h − M ⊙ . The constraints on thehalo mass of the Cl J0227-0421 protocluster placed by all fourmethodologies are summarized in Table 3.Though extremely large uncertainties exist both formally andin the assumptions made to derive the values given in Table 3,and perhaps because of this, the high degree of concordancebetween the values derived from four methods is astonishing.While the exact value of the halo mass of Cl J0227-0421 canonly be constrained, at best, within a factor of ∼
3, the valuesgiven in Table 3, along with the high value of δ g al presented inboth this section and §3, paint the picture that Cl J0227-0421 isa protocluster with a large amount of mass already assembledvery early in the history of the universe and that it is destinedto descend into a cluster whose mass will rival or exceed theComa cluster. With this global picture in mind, we proceed tomake a preliminary investigation into the properties of the galax-ies housed within this emerging cluster.
4. The Effect of environment in Cl J0227-0421
The Cl J0227-0421 protocluster is characterized by 19 confirmedmembers, six additional potential spectroscopic members, and ahigh density of spectroscopic sampling over the entire spatialand redshift extent of the protocluster. Despite this, the numberof member galaxies remains small relative to samples at lowerredshifts where questions about galaxy evolution still abound.Complicating matters, the dominant environmental process or processes appears to depend non-trivially on the particular struc-ture or structures being observed, the spectroscopic sampling ofmember galaxies, and the data available, especially the presenceor absence of multi-wavelength data (see, e.g., the review inOemler et al. 2009). With a sample size of one, we can onlyhope to provide an initial and cursory glance at the e ff ect of en-vironmental processes (or lack thereof) on galaxy evolution inthe high-redshift universe by studying the galaxy population ofCl J0227-0421.Compounding the di ffi culty of this study is the high redshiftof the protocluster. At the redshift of Cl J0227-0421 the band-passes of our ground-based optical / NIR imaging, as well as ouroptical spectral coverage, have been pushed far to the blue inthe rest frame. As a result, the spectral and photometric diagnos-tics typically employed for galaxy evolution studies are either ofquestionable accuracy or impossible with the current data. Whilethe accuracy and possible limitations or biases to the SED-fittingprocess are mentioned in Appendix A, we stress here that thetesting of the SED-fitting process, as well as understanding theproper methods to extract relevant parameters and their associ-ated uncertainties from the rest-frame, near-ultraviolet (NUV)spectra, is still an ongoing investigation in VUDS. With thesewarnings, we begin a preliminary investigation into the e ff ect ofenvironment in the early universe, deferring more complex anal-ysis to future work with the full VUDS sample. − magnitudeand color − stellar-massproperties Plotted in the left panel of Figure 9 is the observed-frame z ′ − K s color − magnitude diagram (CMD) of the spectral members of ClJ0227-0421. These two bands were chosen because they bracketthe Balmer / K s band was preferred over either the [3.6] or the [4.5] magni-tude because the WIRDS imaging is marginally deeper than thatof SERVS. Also plotted in the lefthand panel of Figure 9 are allgalaxies with secure spectroscopic redshifts from 2 . < z < . ∼
500 galax-ies, referred to hereafter as “field” galaxies, was chosen to repre-sent a control sample for the spectral members of Cl J0227-0421at roughly the same epoch . One of the most striking features ofthe observed-frame CMD is that, while the protocluster galaxiesare found in an extremely small volume relative to the full fieldgalaxy sample (see §4.1.2), the galaxies in the two samples es-sentially span the same region of color − magnitude phase space.While a large number of the spectral members lie at rather ordi-nary magnitudes and colors with respect to the field population,several galaxies exist within the protocluster bounds that are ex-tremely bright and exhibit (typically) redder observed-frame col-ors. As can be seen in the righthand panel of Figure 9, where thefractional observed-frame K s luminosity distribution of bothsamples is plotted, not only does Cl J0227-0421 contain sev-eral bright galaxies, but such galaxies also make a greater con-tribution to the overall population than similar galaxies in thefield. This di ff erence is considerable, since the fraction of proto-cluster member galaxies with log( L K s ) > ∼ . By multiwavelength data we mean here and throughout the paper anyimaging data blueward or redward of the typical optical / NIR imagingavailable for most environmental studies. The field redshift window represents a ∼ ±
300 Myr window roughlycentered on the cosmic time measured at the protocluster redshift This luminosity was calculated using the K -band luminosity ofthe Sun (http: // / ∼ cnaw / sun.html) k -corrected in theobserved-frame to the CFHT WIRCam K s filter.Article number, page 12 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster CFHTLS-D1 VUDS+VVDS CMD
20 21 22 23 24 25K s -0.50.00.51.01.52.02.5 z ′ - K s t=70 Myr130 Myr190 Myr270 Myr350 Myr400 Myr Z O • O • O • Protocluster Mem. (flag=2,3,4)Protocluster Mem. (flag=1,9)Field 2.9 < z < 3.7 (flag=2,3,4)
CFHTLS-D1 VUDS+VVDS K s, obs L O • -1 )0.00.10.20.3 F r ac ti on a l o f G a l a x i e s Field Galaxies 2.9 Fig. 9. Left: Observed-frame CFHTLS / WIRDS color-magnitude diagram (CMD). Members of Cl J0227-0421 with secure spectroscopic redshiftsare shown as red diamonds, those with less secure spectroscopic redshifts are shown as blue Xs. The meanings of the green star and cyan circle arethe same as those in Figure 3. All galaxies in the entire CFHTLS-D1 field with secure spectroscopic redshifts 2 . < z spec < . σ point-source completeness limits of the CFHTLS / WIRDS imaging. Model tracks for a z = . L ∗ galaxy withthree di ff erent stellar metallicities are overplotted (see §4.1.2). Several galaxies which are extremely bright in the K s band, including two of thefour brightest galaxies in the entire spectroscopic sample over this redshift range, are members of the protocluster. Right: Fractional distribution ofobserved-frame K s -band luminosities in the protocluster and field samples. Only those protocluster members with secure spectroscopic redshiftsand only those galaxies lying outside of the blue shaded region in the left panel are plotted. Though a large fraction of the protocluster membergalaxies have relatively normal K s -band luminosities with respect to the general field population, the percentage of bright (log( L K s ) ∼ > . that of the field (33.0% and 16.8%, respectively). The proper-ties of these bright, and typically redder protocluster galaxies,were foreshadowed in Figure 4 and will be discussed extensivelythroughout this section. The one galaxy in this population that is an exception is thebrightest galaxy in the protocluster, referred to hereafter as the“proto-BCG”. This galaxy is extremely bright in the K s band( K s = . z ′ − K s = . . < z spec < . σ depth of our VLA data at z ∼ . P ν, . GHz , σ < . − ) , neither this object nor any other protocluster member isdetected in the radio. This limit is much lower than the typicaloutput of high-redshift, radio-loud quasars (log( P ν, . GHz ) > ∼ 27W Hz − ), precluding the possibility that this galaxy containsan analogous phenomenon to those used in other large surveysas signposts for overdense environments (e.g., Wylezalek et al.2013, 2014).The rest-frame NUV spectrum of the proto-BCG does, how-ever, contain several high-ionization emission features whoseFWHMs are several 1000 km s − , attesting to the presence ofan active central engine. The proto-BCG is also the only spec-tral member to be detected in the XMM-Newton imaging, andit has a rest-frame, full-band luminosity of L X , [0 . − , keV] = k − corrections for X-ray and radio point sources were calculated fol-lowing the methods described in Lemaux et al. (2013). The reported luminosity is corrected for Galactic absorption, see Chi-appetti et al. (2005) . ± . × ergs s − , placing the AGN in the proto-BCGwell within the QSO regime (e.g., George et al. 2000). Given theimmense energy output of this AGN it is possible that it is ei-ther a progenitor or a descendant of the high-power, radio-loudquasars found in other overdense environments. The host galaxyis also the only spectral member to be even moderately detectedin the Herschel / SPIRE imaging. The formal significance of thedetection is 2.5 σ , which falls below the formal limit requiredfor a secure detection. However, the proto-BCG is also detected,significantly, at 24 µ m , giving us some additional confidence thatthe SPIRE detection is legitimate. Tentatively assuming this de-tection is real, the total infrared luminosity of the proto-BCG im-plies that it is forming stars at a rate of S FR proto − BCG = ± M ⊙ yr − . The proto-BCG is also located at a large (projected)distance from the protocluster center (1.1 h − proper Mpc), aproperty that is typical in lower redshift clusters still undergoingformation (e.g., Katayama et al. 2003; Fassbender et al. 2011;Zitrin et al. 2012; Lidman et al. 2013). It appears that the proto-BCG of Cl J0227-0421 is still very much in the process of evolv-ing. We now turn back to the bright galaxies in the protocluster thatare observed at redder colors. At lower redshift, massive clusters,like the one that Cl J0227-0421 is predicted to evolve into, showmarked increases in the abundances of bright and massive red-sequence galaxies (RSGs) relative to less dense environments(e.g., Ball et al. 2008; Wetzel et al. 2012). The origin of suchgalaxies is the subject of much debate, since it is unclear howearly and through which processes such galaxies built up con-sider stellar masses and eventually quenched. In this respect, thepresence of several bright galaxies already within the bounds ofthe protocluster at z ∼ . Article number, page 13 of 23 & Aproofs: manuscript no. AA / / CFHTLS-D1 VUDS+VVDS CMD t=70 Myr130 Myr190 Myr270 Myr350 Myr400 Myr Z O • O • O • Field Galaxies 2.9 Left: Rest-frame M r ′ / M NUV − M r ′ CMD of all galaxies in the entire CFHTLS-D1 field with secure redshifts 2 . < z spec < . M s ) > 9. The stellar mass cut is imposed here in an attempt to mitigate any induced di ff erential bias between the field and protoclustermembers (see §4.1.2). The meanings of the symbols are identical to those of Figure 9 as is the meaning of the light blue-shaded region and thegalaxy model tracks. Here and in the right panel, color and absolute magnitude histograms, normalized such that the maximum value is unity,are shown for each population. While most protocluster members have relatively typical colors and magnitudes with respect to the field, thereexists a sub-dominant population of extremely bright (and typically redder) protocluster galaxies. The median SFRs, as derived from the SEDfitting process, of the two samples is shown in the upper right hand corner. Right: Color-stellar-mass (CSMD) of the same galaxy populationsshown in the left panel. The meanings of all symbols are the same. The bimodality observed in the CMD remains in the CSMD, with severalprotocluster galaxies having with extreme stellar masses log( M s ) ∼ > . 8. These galaxies comprise some of the most massive galaxies in the entirespectroscopic sample in the range 2 . < z spec < . 7. Though several field galaxies exhibit similar colors and stellar masses, the volume used todefine the field sample is ∼ 250 larger than that used to define the bounds of the protocluster. The proto-BCG, marked by the circumscribed cyancircle, likely has its stellar mass estimate contaminated by the presence of its powerful AGN, though the other two type-1 AGN hosts likely do not. length coverage of the optical / NIR imaging employed here, it isfar from certain that a direct connection can be drawn betweenthe brightness of a galaxy in the observed-frame K s band and themassive RSGs observed in lower redshift clusters.To understand this connection it is necessary to appeal toour SED fitting process. Plotted in Figure 10 is the rest-frame M NUV − M r ′ CMD and color − stellar mass (CSMD) for both theCl J0227-0421 spectral members and the field galaxy sample.Overplotted here and in Figure 9 are colors and magnitudes de-rived from BC03 stellar synthesis models generated by EZGal .These models were normalized to a lower redshift ( z ∼ . L ∗ cluster galaxy in the observed-frame F W band (De Propriset al. 2013) and generated for a variety of di ff erent formationepochs and at a variety of di ff erent metallicities. As a roughcheck, we note that the galaxy properties generated by thesemodels show broad agreement in the observed-frame K bandwith L ∗ galaxies at similar redshifts ( z ∼ − 4) observed in pho-tometric surveys and in simulations (e.g., Cirasuolo et al. 2010,Henriques et al. 2012, Muzzin et al. 2013).While e ff ects of dust can be significant in both the CMDsand the CSMD (see, e.g., Lemaux et al. 2013) and, indeed, havebeen invoked as the primary culprit for the origins of incipientprotocluster red sequences observed at high redshift (Overzier etal. 2009), the comparisons that will be made here are di ff erential.As such, it is only necessary for our purposes that the dust prop-erties of the protocluster members not di ff er, on average, fromthose in the field at the same redshifts. In an attempt to ensurethat this assumption holds, a stellar mass cut of M s > M ⊙ is imposed on all galaxies plotted in Figure 10, which, as men-tioned in Appendix A, is the rough limit to which the VUDSspectroscopic sample should be representative at these redshifts.This cut was made for two reasons, both of which are predi-cated on the possibility of a relationship between stellar massand SFR suggested by a variety of observations at a variety of http: // / ezgal / model epochs (e.g., Brinchmann et al. 2004; Daddi et al. 2007; Elbaz etal. 2007; Noeske et al. 2007; Santini et al. 2009; Gonz´alez et al.2011; Koyama et al. 2013). Since there is a known relationshipbetween the SFR and the dust content of a galaxy, making thiscut ensures that, to the best of our ability, the two samples havethe same average dust content. The second reason is to ensurefair comparisons between the SFRs of the protocluster membersand the field population discussed later in this section.The large span in the protocluster galaxy properties observedpreviously in Figure 9 now appears as a bimodality in both pan-els of Figure 10. There is a clear population of lower mass, lowerluminosity, blue galaxies in the protocluster bounds that sharethese properties with the bulk of the field sample. There remain,however, several spectral members that are more luminous, moremassive, and redder than the overall population. These threegalaxies, which we refer to hereafter as “proto-RSGs”, have col-ors that are consistent with the last major star-formation eventending ∼ 300 Myr in the past, i.e., z f ∼ . 75. This formationepoch is consistent with the formation epoch derived for mas-sive RSGs in lower redshift ( z ∼ − 2) clusters (e.g., Rettura etal. 2010; Raichoor et al. 2011; Hilton et al. 2012; Lemaux et al.2012; Zeimann et al. 2012). The amount of stellar mass alreadyin place for these galaxies at this redshift is immense consideringthe short period of time that they have had to form their stellarcontent. These stellar masses approach those of z ∼ ∼ z ∼ . < z < . 0) and inslightly less massive structures ( σ v = − 550 km s − ). Thisresult is also broadly consistent with overdensities of bright or Article number, page 14 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster λ obs ( µ m)0.1 1.0 λ rest ( µ m)-2-10123 l og ( f ν µ Jy - ) m AB ID : 20465339 z spec = 3.2852 M s = 7.96E+10 M O • Age = 3.21E+08 yr Fig. 11. Left: Postage stamp of the X-ray and type-1 AGN host proto-BCG of Cl J0227-0421. Blue, green, and red channels are assigned tothe i ′ K s [4 . 5] bands, respectively, and logarithmic scaling is used for thebrightness. Image dimensions are 25 ′′ ( ∼ 200 kpc at z ∼ . 3) on eachside. The bright object to the north-northwest of the proto-BCG wasnot targeted by spectroscopy, but has a well-fit photometric redshift of z phot = . ± . Right: Observed-frame optical / NIR spectral energydistribution (SED) of the proto-BCG with the best-fit galaxy templateoverplotted in red. The proto-BCG has a power-law continuum bothin the UV and in the IR, suggesting the physical parameter estimatescoming from the SED fitting process are contaminated by the presenceof the AGN. The best-fit stellar mass and luminosity-weighted stellarage are shown in the upper left corner of the plot. λ obs ( µ m)0.1 1.0 λ rest ( µ m)-2-10123 l og ( f ν µ Jy - ) m AB ID : 20461765 z spec = 3.2831 M s = 1.60E+11 M O • Age = 1.02E+09 yr Fig. 12. Left: Postage stamp of the most massive galaxy in ClJ0227-0421 generated in an identical manner to that of Figure 11. Right: Observed-frame optical / NIR SED of this galaxy plotted againstthe best-fit galaxy template. As before, the best-fit stellar mass andluminosity-weighted stellar age are shown in the upper left corner ofthe plot. While this galaxy is also host to a type-1 AGN, the propertiesof its SED are much di ff erent than that of the proto-BCG and the SEDis generally well fit by the galaxy template. There is some discrepancyin the IR portion of the SED, but an identical stellar mass is recovered(within the random errors) if IRAC bands are excluded from the fit. massive red galaxies among the populations of z ∼ z ∼ λ obs ( µ m)0.1 1.0 λ rest ( µ m)-2-10123 l og ( f ν µ Jy - ) m AB ID : 20467962 z spec = 3.3247 M s = 7.13E+10 M O • Age = 1.02E+09 yr Fig. 13. Left: Postage stamp of the third brightest and fourth most mas-sive galaxy in Cl J0227-0421 generated in an identical manner to thatof Figure 11. Right: Observed-frame optical / NIR SED of this galaxyplotted against the best-fit galaxy template. Best-fit stellar mass andluminosity-weighted stellar age are shown in the upper left corner ofthe plot. While this galaxy is also host to a type-1 AGN, the same testwas performed on this galaxy as was performed on the galaxy shownin Figure 12 and negligible di ff erences in the derived stellar mass werefound. λ obs ( µ m)0.1 1.0 λ rest ( µ m)-2-10123 l og ( f ν µ Jy - ) m AB ID : 520339360 z spec = 3.3315 M s = 1.10E+11 M O • Age = 8.06E+08 yr Fig. 14. Left: Postage stamp of the fourth brightest and second mostmassive galaxy in Cl J0227-0421 generated in an identical mannerto Figure 11. Right: Observed-frame optical / NIR SED of this galaxyplotted against the best-fit galaxy template. Best-fit stellar mass andluminosity-weighted stellar age are shown in the upper left corner ofthe plot. This galaxy is not host to a type-1 AGN. Though the best-fit galaxy template appears significantly di ff erent from the other twomassive, redder protocluster galaxies plotted in Figures 12 and 13, thedominant stellar population in this galaxy appears relatively old. taminated by the presence of broadband emission of the AGN.For the proto-BCG the estimate of the physical parameters isquite clearly contaminated by the presence of the AGN becausethe SED exhibits a power-law continuum both in the ultravioletand the observed-frame NIR. (This galaxy is also detected in thereddest of the IRAC bands in the SWIRE imaging.) Thus, the lu-minosity and color of its stellar content, as well as the estimatedstellar mass cannot be considered to be reliable. However, in theremaining two cases, the galaxy template is well-matched to theobserved SED of the AGN hosts. These fits are shown in Fig-ures 12 and 13, along with i ′ K s [4 . 5] RGB postage stamp of eachAGN host. Also plotted in Figure 14 are the SED fit and postagestamp of the one proto-RSG not host to a type-1 AGN.For the two proto-RSGs host to a type-1 AGN, only a fewminor discrepancies with the galaxy template are apparent, mostnotably in the IRAC bands. The concern here is that a dust-obscured AGN (AGNs that are generally not typically associated Article number, page 15 of 23 & Aproofs: manuscript no. AA / / with type-1 broadline AGN) would be contributing appreciablyto the rest-frame NIR luminosity, thus leading to an erroneouslyhigh stellar mass measurement. The contamination from such anAGN, however, decreases precipitously below rest-frame wave-lengths of λ rest < − µ m (e.g., Sajina et al. 2012).To test for the presence of contamination, we measured thestellar mass eliminating bands near this limit (i.e., the two IRACbands) and measured the stellar mass again and found negligibledi ff erences ( ∼ . ffi cient to say that there is no reason to believe with the currentdata that the properties of these galaxies presented in this sectionare a ff ected by the presence of their AGN. In future work, hy-brid galaxy / AGN templates will be used to investigate the e ff ectof modeling on the derived physical parameters of such galaxiesfurther (as in, e.g., Salvato et al. 2009). Though there is muchuncertainty in this process, we note that the fits of the two non-proto-BCG type-1 AGN hosts yield luminosity-weighted stellarages that are ∼ M s ) > . 8) and red ( M NUV − M r ′ > M NUV − M r ′ color limitwas adopted to roughly di ff erentiate those galaxies whose dom-inant stellar population has an age in excess of ∼ 200 Myr forall models plotted in Figure 10 to those with dominate youngergenerations of stars. This criterion is not, however, su ffi cient todesignate such populations as “passive” (see, e.g., Ilbert et al.2013, Arnouts et al. 2013), but is only su ffi cient to ensure thatthe last major star-formation event of these galaxies was several100 Myr in the past.We stress here that the quantities relating to this populationthat follow were derived extremely roughly, and it will be nec-essary to refine this estimate once the full spectroscopic selec-tion function and composite survey geometry has been quanti-fied. Again, however, we are saved here by a relative compari-son between the protocluster members and the field, which wereselected in the same way from the same surveys. The space den-sity of proto-RSGs in the field was found to be ρ pRSG , field = . ± . × − h − Mpc − , where errors were calculated fromPoisson statistics.Despite the crudeness of the calculation, or, rather, given itscrudeness, this number is remarkably similar to the space den-sity found for massive galaxies at similar redshifts in wide-fieldphotometric surveys (Ilbert et al. 2013; Muzzin et al. 2013). Incontrast, the space density of proto-RSGs in the bounds of ClJ0227-0421 is much higher: ρ pRSG , pcl = . ± . × − h − Mpc − . This di ff erence is in large part due the volumes used todefine the two samples, the volume spanned by the field samplebeing ∼ 250 times larger than that of the spectral members ofCl J0227-0421. The quotient of the two densities yields a proto-RSG density contrast of δ pRSG = . ± . 2. In other words,sampling a volume equal to that of the protocluster in a ran-dom part of the field would typically result in 0.12 proto-RSGs,or roughly one proto-RSG in every eight protocluster-sized vol-umes. Instead, three such galaxies are found within the proto-cluster volume. This is perhaps unfair, however, as there are alsomore total galaxies within the protocluster bounds as attestedto by the high value of δ g al found for Cl J0227-0421. The proto-RSG overdensity holds, however, if we instead consider the frac-tion of such galaxies in the two environments, since their fractionamong the mass-limited protocluster member sample is nearlytriple that of the field (20% and 6.9%, respectively). While theuncertainties on these quantities are extremely large owing to thesmall number of spectral members, the observed overdensity ofproto-RSGs within the confines of Cl J0227-0421 appears to bereal. This line of thought will continue to be expanded with thefull VUDS sample. We now move from considering the brightest and most massivegalaxies in the protocluster to the spectral member population asa whole. At this point in the analysis of VUDS data, few met-rics exist, and even fewer have been extensively tested, whichwould allow us to attempt to separate the general member pop-ulation from the field sample as a whole. One of these metrics,stellar mass, has already been discussed. However, when con-sidering the spectral member sample as a whole, the distributionof stellar masses of the bulk of the member population do notappear, by eye, to di ff er appreciably from the field population asa whole. This statement is quantified by a Kolmogorov-Smirnov(KS) test, which reveals no significant di ff erence between thestellar mass distributions of the two samples. The second metricis the SFR of those galaxies that comprise the two populations asderived from the SED fitting. As discussed in §2.3, the SFRs ofa given galaxy are subject to a high level of uncertainty comingfrom the choice of models. Additionally, while it has been shownthat, at lower redshift ( z ∼ 1) with a great deal of care, SFRs de-rived from SED fitting can exhibit enough precision with respectto more traditional star-formation proxies (Mostek et al. 2012),SED-fit SFRs have not been tested enough at higher redshift.Therefore, we rely only on di ff erential ensemble properties whenattempting to gain any insight from these SFRs. The distributionof SFRs between the two samples formally shows no significantdi ff erence when compared by a KS test. However, as illustratedthroughout the section with regard to stellar mass, a null resultwith a KS test does not necessarily preclude the possibility thatthe properties of the two populations do not di ff er in some way.With a small number of galaxies in the spectral member sam-ple, we focus instead on the median SFRs of the two samples Article number, page 16 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster because this quantity does not su ff er the same limitations as aKS test or a sample mean for small samples with respect to theextreme ends of the distribution. Significance intervals for themedian SFR of each sample were calculated by bootstrapping.Shown in the lefthand panel of Figure 10 are the median SFRsfor the field and protocluster member samples. Again, however,the small number of galaxies in the latter sample confounds in-terpretation. The median level of star formation activity amongthe protocluster galaxies appears suppressed relative to the fieldby roughly a factor of two (i.e., S FR med ∼ 30 vs. ∼ M ⊙ yr − , respectively) , but the large errors mean that this result issignificant only at the ∼ σ level. Moving beyond the formalstatistical errors, the di ff erence in SFRs between the two sam-ples remains significant at ≥ σ (with the same directionality) ifother SFHs are employed or if the mean is adopted rather thanthe median. Still, however tempting, we cannot make any defi-nite claims with the current data and wait instead to incorporatethe large sample of galaxies in overdense environments in theearly universe a ff orded by the full VUDS sample to see whetherthis trend gains significance.The final tool that we have at our disposal is the rest-frameNUV spectra of both the members of Cl J0227-0421 and fieldgalaxies at the same epoch. A preliminary and simple methodof implementing this tool is to count the number of galaxies ex-hibiting emission lines (in this case Ly α ) vs. those that do not.Such an exercise can perhaps give information about the con-stituent galaxy populations of the two samples since LAEs havebeen shown to di ff er appreciably in their SFRs, luminosities, andin the ages of their dominant stellar population with respect toLyman break galaxies that do not emit in Ly α (e.g., Shapley etal. 2003; Lai et al. 2007; Pentericci et al. 2007, 2009; Korneiet al. 2010). However, no clear excess in LAE fraction is ob-served for the spectral members of Cl J0227-0421. Among themembers that have spectra with su ffi cient wavelength coverage,33 ± 15% of the members exhibited rest-frame Ly α equivalentwidths (EWs) in excess of 25Å, a fraction consistent with the av-erage fraction among a volume-limited sample of VUDS galax-ies at these redshifts. For further details on the measurementsof Ly α EWs and the average LAE fraction, as measured fromVUDS, over a large redshift baseline, see Cassata et al. (2014).A slightly more complicated method of implementing thistool is through the combination of spectra to create a high signal-to-noise ratio (S / N) spectrum that in some way represents the av-erage properties of the constituent galaxies, a process we refer tohereafter as “coadding” (with the resulting product referred to asa “coadded spectrum”). After removing all type-1 AGN hosts,the galaxies comprising the two samples were coadded sepa-rately, loosely following the methodology outlined in Lemaux etal. (2013). Here, however, we chose not to use weighting basedon the formal uncertainty spectra generated for VUDS by theVIMOS pipeline (see Le Le Fèvre et al. 2014 and referencestherein for details), because the properties of these spectra havenot been extensively tested to date. The e ff ect of not weightingeach input flux density by its uncertainty (or variance) is to ap-preciably add noise to the continua of the coadded spectra inthe regions of under- or oversubtracted airglow lines. The addednoise in the resulting coadded spectra in turn leads to increaseduncertainty in physical parameters derived from the spectra. Thisproblem is compounded for small samples and samples that spanlimited redshift intervals. Though we attempted to mitigate this At this stage of the analysis, the tentative Herschel / SPIRE detectionof the proto-BCG is not incorporated due to its ambiguous nature andthe lack of matched Herschel / SPIRE data for the full field sample. e ff ect in some way by also generating coadded spectra based onthe median flux density of input spectra at each pixel, we restrictourselves here to a broad qualitative comparison of the coaddedspectra, turning only very briefly to a quantitative comparison.Plotted in Figure 15 is the mean (hereafter “average”) andmedian coadded spectra of members of Cl J0227-0421 not hostto a type-1 AGN against the backdrop of those of the field sam-ple. Over the region where the airglow contamination is minimal( λ rest < α ab-sorption trough, and many ISM absorption features are of com-parable relative strength in the two sets of coadded spectra. Onebroad di ff erence appears to be the slope of the NUV continuum,which appears steeper in the both the average protocluster galaxyand in the median coadded spectrum of protocluster membersrelative to the field. While highly subject to both the amount, ge-ometry, and composition of dust in a galaxy, the IMF, and stellarmetallicity (e.g., Castellano et al. 2012; Wilkens et al. 2012), thisquantity, typically referred to as the β − slope, can be linked to theSFRs, stellar masses, and mean luminosity-weighted stellar agesof galaxies.While several recent measurements of this quantity inhigh-redshift galaxies have been made with photometry (e.g.,Bouwens et al. 2012; Dunlop et al. 2012; Finkelstein et al. 2012;Jiang et al. 2013; Hathi et al. 2013; Castellano et al. 2014), sim-ilar measurements with spectroscopy have not, to date, been at-tempted for a large population of such galaxies. The relationshipof the β − slope with galaxy properties is an area of active inves-tigation with VUDS (Hathi et al. 2014), and these results willbe used in the future, along with the full VUDS sample to inter-pret and contextualize di ff erences in the β − slope as a function ofenvironment. Another possible di ff erence between the averagespectral properties of the two populations lies in the strength ofthe emission features, with protocluster galaxies having, on av-erage, slightly stronger Ly α , HeII, and CIII] emission. Since, asnoted above, the two populations have comparable fractions ofLAEs, this possibly suggests a connection between environmentand Ly α escape fraction, as well as possible AGN activity. How-ever, with the limited sample presented here, it is not possible tomake definitive claims, and we instead wait for the inclusion ofthe full sample of VUDS overdensities to explore this thoughtrigorously.In an attempt to make a slightly more quantitative compari-son, both coadded spectra were fit using GOSSIP to BC03 syn-thetic spectral models that spanned an identical parameter spaceto those used for the SED fitting process. Because the method ofcoadding spectra destroys absolute flux calibration through nor-malization, without a more complicated implementation of simi-larly averaged photometry for each sample, only relative quanti-ties, i.e., stellar age, metallicity, and extinction, can be used fromthese fits. Though large uncertainties exist in the fitting process,the best-fit models to the two average coadded spectra yield pa-rameters that also show a high degree of similarity. The averagefield and protocluster galaxies have identical metallicities andluminosity-weighted stellar ages and stellar extinctions that dif-fer only by one resolution element (300 Myr and 400 Myr and E ( B − V ) = . E ( B − V ) = . Article number, page 17 of 23 & Aproofs: manuscript no. AA / / Average VUDS/VVDS-UD Galaxy f[ A r b i t r a r y U n i t s ] SiIIOI+SiII*CIISiIV+OIV] CIV HeII NiII AlIII CIII] Protocluster MembersField Galaxies Protocluster MembersField Galaxies Left: Rest-frame mean “coadded” VUDS spectra of all spectral members of Cl J0227-0421 with log( M s ) > ff erences do exist; the average protocluster galaxy appears to have slightly stronger emission features(Ly α , HeII, and CIII]) and a steeper rest-frame NUV continuum slope. Right: Median “coadded” VUDS spectra of the same two samples shownin the left panel. The major di ff erence between the median spectra and the mean spectra plotted in the left panel is the enormous decrease in thestrength of the Ly α line, which is a reflection of the fact that most galaxies do not emit Ly α at these redshifts. In the median spectrum, the Ly α emission is roughly equivalent among the field galaxies and the protocluster members, owing to the similar fractional contribution of LAEs toboth samples (see text). The di ff erences in the median and mean spectra suggest that, though the galaxies in the protocluster emit Ly α roughly asfrequently as the general field population, those galaxies which do emit Ly α in the protocluster are doing so more profusely. the suppression of star-formation activity within the protoclus-ter boundaries, as well as di ff erences in the average β − slopeand average emission line strengths, all other ensemble quanti-ties showed broad concordance between the two samples. Therewas a significant di ff erence, however, between the two sampleswhen a specific population was isolated, with a marked excess ofmassive, red galaxies, observed in the protocluster environment.Among these galaxies, we also observed tentative evidence of in-creased AGN activity, including in the extremely bright and blueproto-BCG. With the large amount of total mass already assem-bled in Cl J0227-0421 at this redshift (see §3.1.1), these resultshint at a picture where we are witnessing the birth of quench-ing within the protocluster environment, with massive galaxiesbeginning to transition to the red sequence setting the stage forfuture environmental influences on the bluer, lower mass mem-ber galaxies. While this picture will continue to be focused andcontextualized with the full sample of VUDS protoclusters, thisfirst look into the heart of an emerging massive cluster at highredshift has provided several enticing clues to what may eventu-ally form. 5. Summary and conclusions In this paper we have described a systematic search for over-densities at high redshift ( z > 2) in the CFHTLS-D1 field usingnewly obtained VUDS spectroscopic data in conjunction withthe wealth of other imaging and spectroscopic data available forthis field. We then described the discovery and characterizationof the most significant of these overdensities, the Cl J0227-0421protocluster at z ∼ . 3. Here we briefly outline the main conclu-sions of this study. • With 19 confirmed spectroscopic members and six potentialspectroscopic members, Cl J0227-0421 is significantly over-dense relative to the field at these redshifts. Using a largefield coeval population from VUDS and VVDS along withthe 19 confirmed spectroscopic members, we estimated thesignificance of the spectroscopic overdensity of Cl J0227- 0421 to be σ = . δ g al = . ± . σ = . • Four di ff erent methods were used to estimate or place lim-its on the halo mass of Cl J0227-0421 at z ∼ . z = M z ∼ . ∼ × M ⊙ ) and will evolve into a clus-ter with a halo mass rivaling or exceeding that of Coma( M z = ∼ × M ⊙ ). • The properties of the spectroscopic member galaxies of ClJ0227-0421 were investigated. In the brightest protoclustergalaxy, we found evidence of a powerful active galactic nu-clei, as well as tentative evidence of vigorous star formationactivity ( ∼ M ⊙ yr − ). Within the protocluster environ-ment, a significant excess of brighter, redder, and more mas-sive galaxies appeared relative to a similarly selected fieldpopulation at similar redshifts. This excess was quantifiedboth absolutely, δ pRSG = . ± . 2, and relatively, with afractional excess of such galaxies within the protocluster ofaround three. Based on comparisons with models, the lastmajor star-formation event in these galaxies was estimatedto be in excess of 300 Myr prior to z ∼ . 3, indicating thatwe may be witnessing the onset of environmentally-drivenquenching processes. • The remaining protocluster members had properties thatwere broadly similar to those of field galaxies. While wefound weak evidence of suppression of the star formationrates among the general protocluster member population andsubtle di ff erences between the stacked spectra of the twopopulations, these di ff erences were not significant enough tobe conclusive. Article number, page 18 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster Despite the massive nature of Cl J0227-0421, the relativelysmall number of protocluster members statistically limited theconclusions that could be drawn. Still, the results of severallines of analysis presented in this paper were tantalizingly sug-gestive of the e ff ect of environment at z ∼ . 3. These linesof analysis will be continued with the ∼ 40 overdensities foundwithin the entire VUDS sample to search for definitive signsof environmentally-driven evolution and transformation in thehigh-redshift universe. Acknowledgements. We thank ESO sta ff for their continuous support for theVUDS survey, particularly the Paranal sta ff conducting the observations and Ma-rina Rejkuba and the ESO user support group in Garching. This work is sup-ported by funding from the European Research Council Advanced Grant ERC-2010-AdG-268107-EARLY and by INAF Grants PRIN 2010, PRIN 2012 andPICS 2013. AC, OC, MT and VS acknowledge the grant MIUR PRIN 2010–2011. DM gratefully acknowledges LAM hospitality during the initial phasesof the project. B.C.L. gratefully acknowledges the kindness, support, and saltymu ffi ns of Debora Pelliccia provided, even at the most unreasonable of hours,throughout the course of this work. This work is based on data products madeavailable at the CESAM data center, Laboratoire d’Astrophysique de Marseille.This work partly uses observations obtained with MegaPrime / MegaCam, a jointproject of CFHT and CEA / DAPNIA, at the Canada-France-Hawaii Telescope(CFHT) , which is operated by the National Research Council (NRC) of Canada,the Institut National des Sciences de l’Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii. 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Redshifts derived in this manner are known, even up tothe highest redshifts of our sample, to be relatively invariantunder the inclusion of Spitzer imaging (see, e.g., Bradaˇc et al.2014; Ryan et al. 2014) for datasets with broadband filters thatprobe both the Lyman-limit / Ly α break and the Balmer / z > 2, the redshift range of interest for this study. No sta-tistically significant di ff erence in the normalized absolute me-dian deviation, σ ∆ z / (1 + z s ) , (NMAD; Hoaglin et al. 1983), or thecatastrophic outlier rate (i.e., | z p − z s | / (1 + z s ) > . 15, see Il-bert et al. 2013) was found between photometric redshifts de-termined with and without SERVS data included. A comparisonof the photometric redshifts derived from the CFHTLS / WIRDSphotometry and spectroscopic redshifts of all objects targetedin the CFHTLS-D1 field that have a secure spectroscopic red-shift yielded a catastrophic outlier rate of 9.7%, a rate consider-ably higher than the VVDS data alone (see Lemaux et al. 2013).However, once catastrophic outliers were rejected, the NMADwas σ ∆ z / (1 + z s ) = . Article number, page 20 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster references therein. The SERVS data, previously unused, was in-corporated for this instance of the SED fitting as the physicalparameters derived from the SED fitting are heavily used in thisstudy and are known to be sensitive to the inclusion of IRACdata (see discussion in Bradaˇc et al. 2014). Two other changesrelative to the version of the fitting presented in Lemaux et al.(2013) were made at this point. The first was to use best-fitmodel, i.e., the combination of template and physical param-eters that minimized the χ with respect to photometric data,rather than the median of the probability distribution function(PDF). This choice was made because roughly 15% of our spec-troscopic sample were significantly detected in an insu ffi cientnumber of photometric bands to satisfactorily calculate a PDF.For the 85% of the sample for which a comparison could bemade, no systematic o ff set was observed between the best-fit andmedian stellar masses, luminosity-weighted stellar ages, and starformation rates (SFRs), with a negligibly small scatter betweenthe two estimators of 0.04 dex for all three parameters. The sec-ond change made in this version of the SED fitting was the use ofMAG_AUTO measurements instead of the scaled aperture mag-nitude measurements used in the Lemaux et al. (2013). Thoughthe former are known to be more susceptible to blending, we pre-ferred these measurements as they showed greater consistencywith the aperture-corrected SERVS magnitudes.Because it has been suggested that high-redshift galaxieshave star formation histories (SFHs), which deviate considerablyfrom the simple exponentially decaying tau model (e.g., Maras-ton et al. 2010; Reddy et al. 2012; Schaerer et al 2013; Buat et al.2014; though see also, e.g., Ryan et al. 2014; Sklias et al. 2014),the e ff ect of changing the SFH was tested by rerunning SEDfitting using Bruzual & Charlot (2003; hereafter BC03) delayedtau models with an identical initial mass function (IMF; Chabrier2003) and an identical range of extinctions, taus, and metallici-ties. For the two physical parameters that are of paramount in-terest for this study, stellar mass and SFR, only a small system-atic o ff set of 0.05 dex between the best-fit parameters of the twodi ff erent SFHs was observed, with the delayed tau model yield-ing slightly higher SFRs and slightly lower stellar masses. Ther.m.s. scatter between the two sets of parameters measured withthe two di ff erent SFHs was also small: 0.04 dex for both pa-rameters. Because all of the comparisons that are made in thispaper are internal, it would have no e ff ect on our results if theSFHs of galaxies in our sample were globally mischaracterizedto the same level. However, since we made comparisons betweengalaxies in di ff erent environments, it is possible that the SFHs ofgalaxies depends on environment (e.g., Kau ff mann et al. 2004),which would lead to a di ff erential bias in the physical parame-ters. It is therefore comforting that, at least for these two SFHs,the di ff erences between the physical parameters derived for thetwo sets of models is negligibly small. Because of its consistencywith the previous SED fitting and to ease comparisons with thevast majority of other studies, we decided to adopt those param-eters derived from the exponentially decaying tau model. Withthese sets of models, the typical (random) uncertainty in the stel-lar mass and SFRs of galaxies in the range of interest for thisstudy (i.e., 9 < log( M s ) < 12, 2 . < z spec < . 7) coming fromthe SED fitting process were 0.16 and 0.10 dex, respectively.Spectra were fit using the GOSSIP software, a package cre-ated to to fit the spectro-photometric emission of a galaxy with aset of synthetic models with a library builder that allows for theconstruction of various resolution BC03 and Maraston (2005,2011) models. For this study, we fit only exponentially decay-ing BC03 models with the same assumptions and those span-ning the same parameter space as those adopted for the photo- metric SED fitting described above. Among the many improve-ments that have been recently implemented on GOSSIP, one ofthe most important targets for the redshift range of VUDS is re-lated to the treatment of the intergalactic medium (IGM) extinc-tion. While a typical assumption is to employ the IGM modelof Madau (1995), which produces, for a given redshift, a sin-gle IGM extinction curve, GOSSIP is able to choose up to fivedi ff erent IGM curves along various sight lines, which provides amore realistic determination of the resultant physical parameters.Both GOSSIP and the improvements that have been made to itfor general use with the VUDS survey is explained in Thomas etal. (2014). Appendix B: Details of the halo mass estimates ofCl J0227-0421 The initial methodology used to determine an estimate on thehalo mass of Cl J0227-0421 utilized the information providedby the dynamics of the member galaxies. The implicit assump-tion in this method is that the protocluster is in a virialized state,an assumption that almost certainly does not hold at this redshiftgiven the limited time member galaxies have had to interact withthe potential. The high degree of skewness observed in the dif-ferential velocity distribution of the spectral members quoted in§3.1 attests to the failure of this assumption. In the case of astructure in the initial stages of its collapse, the measured ve-locity dispersion will potentially decrease relative to the virialvalue owing to galaxies appearing compressed along the redshiftdimension (e.g., Steidel et al. 1998). At later stages, however,the measured velocity dispersion will be an overestimate of thevirial value as galaxies that have fallen from long distances beginto make their first passes through the protocluster core. Given theyoung age of the universe at z ∼ . 3, the former is the strongerof the two possibilities. However, with no knowledge of the trueevolutionary stage of the dynamics of the spectral members ofCl J0227-0421, we remained doubtful about this point, and sim-ply calculated the dynamical mass with the knowledge that thisquantity can be a lower or an upper limit. The virial dynamicalmass was calculated via M d y n , v ir = √ σ v . G H ( z ) (B.1)where G is Newton’s gravitational constant and H ( z ) is the valueof the Hubble parameter at the redshift of interest. This formulais used directly to calculate the dynamical mass at the virial ra-dius reported in Table 3.The calculation relating the stellar mass of members of theprotocluster to the total mass was done in the following manner.Loosely following the methodology of Strazzullo et al. (2013),we adopt the relationship between halo mass and the stellar massof members within r , the radius at which the mean density is200 times that of the critical density, calibrated using data fromthe Sloan Digital Sky Survey (SDSS) by Andreon (2012). Todetermine the amount of stellar content in Cl J0227-0421, wesummed the stellar masses of all spectral members with stellarmasses in excess of 10 M ⊙ , chosen as it is roughly the turnoverin number counts of all VUDS galaxies with secure spectro-scopic redshifts from 2 . < z < . 7, the redshift bounds usedto define our field sample in §4.1. The large projected radiusover which we sum the stellar mass of spectral members (i.e., R pro j < h − ) was motivated by the high likelihood of suchgalaxies becoming as virialized members by z ∼ Article number, page 21 of 23 & Aproofs: manuscript no. AA / / al. 2013, Zemp 2013), the redshift at which the Andreon (2012)relation was calibrated. Since the virial radius is typically de-fined to be smaller than r (Biviano et al. 2006; Poggianti et al.2009), such galaxies should be accounted for in this relation.A large number of the objects within the protocluster boundsare, however, not sampled spectroscopically or do not have a se-cure spectroscopic redshift. To account for this lack of sampling,we calculated the probability of being a true member by com-paring the spectroscopic and photometric redshifts of those ob-jects with secure spectroscopic redshifts. Two probabilities werecalculated, one for objects with photometric redshifts consistentwith the redshift of the protocluster and those that were not. Thecorrection to the composite stellar mass is then Σ M s , corr ( M s > M ⊙ ) = Σ M s , uncorr P ( z p , mem | z s , mem ) N p , mem + P ( z p , nm | z s , mem ) N p , nm + N s , mem N s , mem ! (B.2)where N p , mem is the number of photometric redshift membersthat went untargeted or that have questionable spectroscopic red-shifts within the bounds of the protocluster, and N p , nm is theequivalent quantity for photometric redshift non-members.The two probabilities, defined as the likelihood of being aspectral member in the event that an object is a photometric red-shift member or non-member, were determined to be 31.0% and 0.4%, resulting in a correction factor of 5.5. In this calcula-tion the assumption is made that the untargeted members havean identical stellar mass distribution to the spectral members,which is reasonable given that the VUDS sample should be rep-resentative of galaxy populations at these redshifts bounded bythis stellar mass limit. The quantity in Equation B.2 was furthercorrected for galaxies between 10 < M s < M ⊙ by inte-grating the stellar mass functions (multiplied by M s ) derived byIlbert et al. (2013) for galaxies at 3 < z < 4. The lower limitof this correction is set by the rough stellar mass completenesslimit of SDSS at the redshift of those clusters used for calibration(Panter et al. 2007). The resultant total corrected stellar mass is3.22 + . − . × h − M ⊙ , where the errors were determined fromthe SED fitting process. It is interesting to note that this cor-rected total stellar mass rivals the total stellar mass contained inred-sequence galaxies in massive, z ∼ ∼ r corre-sponding to the composite corrected stellar mass calculated fromthe members of Cl J0227-0421 is M Σ M s , = . ± . × h − M ⊙ at z ∼ . 3. However, to compare this value fairly to theprevious estimate it is necessary to correct the halo mass to thatat the virial radius. This correction was done by modeling thehalo mass profile as an NFW profile with a concentration at thevirial radius of c v ir = . ff y et al. 2008). A correction factor Though this number looks at first glance to be inconsistent with theclaim in §3 that a significant overdensity of z phot members is likely to bereal, the threshold of significance was determined empirically througha comparison of photometric redshift source and spectral overdensitiesand thus accounts for this impurity. of c NFW = . ± . 48 was determined by the ratio of the totalmass contained within r v ir to that contained within r as de-termined by the velocity dispersion. This correction factor wasused to derive the final z = . z = . ′ ) of the protocluster center, further compoundingthe di ffi culty of the measurement because the shot noise fromthis source is the overwhelming source of background noise insome parts of the adopted apertures. After masking, we inte-grated the count rate in concentric annuli and subsequently cor-rected for vignetting with the uncertainties derived through Pois-son statistics. The measurement yields the total (summing threeEPIC detectors: MOS1, MOS2, PN) count rate into the [0.5-2]keV band within physical aperture corresponding to 0.5 Mpc at z ∼ . × − ergs cm − counts − (Adami et al.2011), resulting in the limit quoted in §3.1.1. All random errorsquoted for this method are Poissonian. This flux limit was con-verted into a rest-frame luminosity using the method described in§3.1.1, which resulted in a luminosity limit of L X , [0 . − . , rest < . ± . × ergs s − . The relationship between the X-rayluminosity in the rest-frame [0.1-2.4] keV, as measured at r and the hydrostatic equilibrium mass is given as (Arnaud et al.2010, Pi ff aretti et al. 2011) h ( z ) − / L X , , [0 . − . 4] keV , rest ! = c M X , × M ⊙ ! α (B.3)where h ( z ) = H ( z ) / H , log( c ) = . ± . α = . ± . 12. Just as the member dynamics are unlikely to be governedby the virial theorem, the protocluster ICM, if it exists, is almostcertainly not going to be in hydrostatic equilibrium, and thusthis method only serves to crudely place limits on the halo mass.Using the above formula and correcting to the virial radius withan NFW as with the previous method results in the hydrostatichalo mass limit given in Table 3.The galaxy overdensity, δ g al , calculated in §3.1.1 within the“e ff ective” radius of Cl J0227-0421 was transformed into thematter overdensity, δ m , via (Steidel et al. 1998):1 + b δ m = C (1 + δ g al ) (B.4)and C = + f − f (1 + δ m ) / (B.5)where C is defined as factor to correct the observed volume forredshift space distortions such that C = V apparent / V true , where V apparent is the measured volume and V true is the volume aftercorrection. This factor, discussed extensively in Steidel et al.(1998), has a complicated dependence on both the magnitude The flux limit is actually measured at roughly r , but the correc-tion between the luminosity at this radius and the luminosity at r isnegligible (see Pi ff aretti et al. 2011).Article number, page 22 of 23. C. Lemaux et al.: VUDS discovery of a high-redshift protocluster and the directionality of the velocities of the member galaxies.As in Cucciati et al. (2014), we made the assumption here thatthe structure is under collapse, such that C < f in Equation B.5 can beapproximated as Ω / m ( z ), the matter density relative to critical ofthe universe at redshift z (for further details see Lahav et al. 1991;Padmanabhan 1993; Steidel et al. 1998). Solving the above sys-tem of equations results in a correction factor of C = . + . − . and a matter overdensity of δ m = . + . − . . The latter value canbe translated into a z = M δ m , tot , z = = C e (1 + δ m , e ) Ω m , ρ crit , V e (B.6)where V e is the e ff ective volume, i.e., (2 R e ) , where R e is mea-sured in comoving Mpc, and C e is an additional correction factorto account for mass outside of the e ff ective radius. In Chiang etal. (2013), C e was found to be 2.5; i.e., 40% of the mass wascontained within a box defined by R e , and that is the value weadopt here. In principle, under the assumptions made here it isnecessary to decrease the observed R e along the line-of-sight dimension (i.e., a smaller redshift window) by a fac-tor C to match the simulated volume that is absent of distor-tions due to peculiar velocities. However, the value of δ g al for ClJ0227-0421 is essentially invariant with respect to the redshiftwindow chosen (for ∆ z spec < . 08) and, given the large uncer-tainty in the value of C , both in the formal error and the numberof assumptions, we make no attempt to correct our observed red-shift window for this e ff ect. An additional uncertainty in this cal-culation relates to the scatter of δ m and M δ m , tot , z = of simulatedclusters. Because of the relatively small number of z =0 clustersin the analysis of Chiang et al. (2013) at or exceeding the totalmass of the predicted descendant of Cl J0227-0421, this scatteris highly uncertain. Because of this uncertainty, and because theinclusion of this scatter in the formal error on the total mass cal-culated from this method does not change the interpretation ofour results, we chose to ignore it. The total halo mass derivedin Equation B.6 is then converted to a halo mass at the virial ra-dius using the method defined in the previous mass calculations,resulting in the value given in Table 3.