CLASH-VLT: spectroscopic confirmation of a z=6.11 quintuply lensed galaxy in the Frontier Fields Cluster RXC J2248.7-4431
I. Balestra, E. Vanzella, P. Rosati, A. Monna, C. Grillo, M. Nonino, A. Mercurio, A. Biviano, L. Bradley, D. Coe, A. Fritz, M. Postman, S. Seitz, M. Scodeggio, P. Tozzi, W. Zheng, B. Ziegler, A. Zitrin, M. Annunziatella, M. Bartelmann, N. Benitez, T. Broadhurst, R. Bouwens, O. Czoske, M. Donahue, H. Ford, M. Girardi, L. Infante, S. Jouvel, D. Kelson, A. Koekemoer, U. Kuchner, D. Lemze, M. Lombardi, C. Maier, E. Medezinski, P. Melchior, M. Meneghetti, J. Merten, A. Molino, L. Moustakas, V. Presotto, R. Smit, K. Umetsu
aa r X i v : . [ a s t r o - ph . C O ] O c t Astronomy&Astrophysicsmanuscript no. RXJ2248_specz6obj c (cid:13)
ESO 2018August 13, 2018 L ETTER TO THE E DITOR
CLASH-VLT: spectroscopic confirmation of a z=6.11 quintuplylensed galaxy in the Frontier Fields Cluster RXC J2248.7-4431 ⋆ I. Balestra , , E. Vanzella , P. Rosati , A. Monna , C. Grillo , M. Nonino , A. Mercurio , A. Biviano , L. Bradley , D.Coe , A. Fritz , M. Postman , S. Seitz , , M. Scodeggio , P. Tozzi , W. Zheng , B. Ziegler , A. Zitrin , M.Annunziatella , , M. Bartelmann , N. Benitez , T. Broadhurst , R. Bouwens , O. Czoske , M. Donahue , H.Ford , M. Girardi , , L. Infante , S. Jouvel , D. Kelson , A. Koekemoer , U. Kuchner , D. Lemze , M.Lombardi , C. Maier , E. Medezinski , P. Melchior , M. Meneghetti , , J. Merten , A. Molino , L. Moustakas ,V. Presotto , R. Smit , and K. Umetsu (Affiliations can be found after the references) Received 2013/ Accepted 2013
ABSTRACT
We present VIsible Multi-Object Spectrograph (VIMOS) observations of a z ∼ z = . L ∗ , high- z galaxy has been recently discovered by Monna et al. (2013) usingdropout techniques with the 16-band HST photometry acquired as part of the Cluster Lensing And Supernova survey with Hubble(CLASH). Obtained as part of the CLASH-VLT survey, the VIMOS medium-resolution spectra of this source show a very faintcontinuum between ∼ ∼ α at z = . ± . EW = ±
10 Å, relatively well constrained thanks to the detection of the UV continuum, which is rarely achievedfor a sub- L ∗ galaxy at this redshift. After correcting for magnification, the star formation rate (SFR) estimated from the Ly α line isSFR(Ly α ) =
11 M ⊙ yr − and that estimated from the UV data is SFR(UV) = ⊙ yr − . We estimate that the effective radius of thesource is R e . . Σ SFR > ⊙ yr − kpc − and, using the Kennicutt-Schmidtrelation, a gas surface mass density Σ gas > M ⊙ pc − . Our results support the idea that this magnified, distant galaxy is a young andcompact object with luminosity 0 . L ∗ at z =
6, when the Universe was just 1 Gyr old, with a similar amount of mass in gas and stars.In the spirit of the Frontier Fields initiative, we also publish the redshifts of several multiply imaged sources and other backgroundobjects, which will help improving the strong-lensing model of this galaxy cluster.
Key words.
Gravitational lensing: strong – galaxies: high redshift.
1. Introduction
Understanding the process of reionization of the intergalacticmedium in the early Universe and the nature of the first galax-ies responsible for that process are among the most importantgoals of modern cosmology (e.g. Robertson et al. 2010, and ref-erences therein). In recent years, great progress has been made inour ability to detect galaxies at z &
6. To the greatest extent, thishas been possible thanks to very deep observations with the Hub-ble Space Telescope (HST). The selection of optical dropoutsin deep HST fields has led to the identification of samples thatnow reach hundreds of candidate Lyman-break galaxies (LBG)at z ∼ − Cosmic Telescopes , massive lensing galaxy clus-ters allow the identification of faint, otherwise undetected, high- z galaxies. Despite the smaller volumes probed with gravita-tional lensing compared to those probed in deep fields, lensing Send offprint requests to : I. Balestra, e-mail: [email protected] ⋆ This work is based on data collected at ESO VLT (prog.ID 186.A-0798) and at NASA HST. has been successful in pushing the detection of distant galaxiesone step farther, as testified by the recent discoveries of several z ∼ −
11 candidates behind massive clusters (Bouwens et al.2012; Zheng et al. 2012; Coe et al. 2013). All of these high- z candidates have been identified by galaxy clusters observed aspart of the Cluster Lensing And Supernova survey with Hubble(CLASH; Postman et al. 2012) Multi-Cycle Treasury program.Ground-based spectroscopy of high-redshift sources in"blank" fields can be very challenging given that they are of-ten 27th magnitude or fainter. However, the flux magnificationsdue to gravitational lensing helps to bring these distant sourceswithin reach of ground-based spectroscopy. Lensed observa-tions reach significantly lower luminosities (e.g. Schenker et al.2012) than spectroscopic observations in deep fields (e.g.Vanzella et al. 2011).The most prominent spectral feature in the UV rest-framewavelengths probed by optical/NIR spectroscopy at z & α (Ly α ) emission line. The Ly α emission line is a reso-nant transition and suffers from radiative transfer effects, a prop-erty that further complicates the modeling of the escape frac-tion of Ly α photons from their host galaxies (Verhamme et al.2006; Dijkstra & Jeeson-Daniel 2013). The line itself is alsoan important diagnostic of the physical processes at work Article number, page 1 of 5&Aproofs: manuscript no. RXJ2248_specz6obj (Giavalisco et al. 1996; Vanzella et al. 2009), since its strengthand velocity profile depend on the instantaneous star formationrate, gas and dust content, metallicity, kinematics, and geometryof the interstellar medium (e.g., clumpy, anisotropic).The cluster RXC J2248.7-4431 (z=0.348) was observed aspart of the CLASH-VLT large spectroscopic program, which tar-gets 14 CLASH clusters in the southern sky. Five multiple im-ages of a young galaxy, identified as i -dropouts, have been re-cently discovered from the CLASH HST imaging of this clus-ter (see Monna et al. 2013, hereafter M13). Two of the bright-est images (ID2 and ID3) also have Spitzer detections, whichadditionally support the high- z nature of the source (see M13).The delensed UV luminosity of the magnified source, inferredfrom the strong-lensing analysis is L ∼ . L ∗ at z = β = − . ± .
25) and the inferred age( < ∼ M ⊙ ), and metallicity ( Z < . Z ⊙ ) areall consistent with that of a young galaxy (M13).In this letter, we report on our VIMOS/VLT observa-tions, which provide spectroscopic confirmation at z = . i -dropouts inRXC J2248.7-4431, and we infer some of the physical prop-erties of this young galaxy. In addition, we provide a list ofredshift measurements for several other strong-lensing featuresand magnified background sources targeted in this cluster by ourCLASH-VLT survey so far. Errors are quoted to the 1 σ confi-dence level, unless otherwise stated. We assume a cosmologywith Ω tot , Ω M , Ω Λ = . , . , . H =
70 km s − Mpc − .
2. VIMOS observations and data reduction
The cluster RXC J2248.7-4431 was observed in June-July 2013.The VIMOS data were acquired using four separate pointingswith one quadrant centered on the cluster core. Three of the fivemultiple images of the i -dropout candidate were targeted in fourmedium-resolution (MR) 1hr-pointings, providing us a total of4 hr of integration time on each of the three images. The threemultiple images targeted are displayed in Fig. 1. The identifi-cation numbers of the three targets are those used in M13. Themasks were designed with 1 ′′ -slits and the VIMOS spatial reso-lution is 0 . ′′ / pixel. The MR grism and the GG475 filter wereused. In this configuration the useable wavelength range is 4800-10 000 Å, the resolution is R =
580 (or ∼ . / pixel.Data reduction was performed using the Vimos InteractivePipeline Graphical Interface (VIPGI; Scodeggio et al. 2005),which uses standard automated procedures to compute bias sub-traction, flat-fielding, sky subtraction, and wavelength calibra-tion. The standard star EG-274 was used for flux calibration.The seeing varied from pointing to pointing. The best seeingconditions ( ∼ . − . ′′ ) were reached during pointing P1 andP3 (July 11 and 10), while they were poorer ( ∼ . − . ′′ ) dur-ing pointing P2 and P4 (July 6 and 9). As a consequence of thevarying seeing conditions the signal-to-noise ratio (S/N) of thespectra varied significantly in the different pointings.
3. Results
A strong emission line is detected at 8643Å in each of the single1hr-spectra (P1-P4) for all of the three multiple images targeted(ID2-ID4). If the emission line is identified to be Ly α , the red-shift of the source is z = . z phot ≃ .
9, see M13) and with the strong-lensing
Fig. 1.
HST color-composite image. The red circles (ID1-5) show thepositions of the quintuply lensed dropout candidate at z =
6, labeled asin M13. The three VIMOS slits (yellow boxes) were placed on ID2, ID3,and ID4 in four repeated 1hr-pointings. ID1 was not targeted because itis too close to a bright cluster-member galaxy.
Fig. 2.
Spatial profiles along the direction of the slit for the spectra ofID3, derived by collapsing counts in the dispersion direction between9050 Å and 9280 Å for each pointing separately. P1 is plotted in blue ,P3 in red , and P2 and P4 in black . The continuum is clearly detectedin P1 ( ∼ σ ) and P3 ( ∼ σ ). Spectra are generally at slightly differentpixel positions in different pointings. model predictions ( z lensing ≃ .
0, see M13). For the brightest im-age (ID3), in the two pointings with better seeing (P1 and P3) afaint continuum is also detected (at about 5 σ ) between ∼ ∼ α equivalent width ( EW >
50 Å) show very weak or even lackUV stellar and interstellar absoption lines (e.g., Vanzella et al.
Article number, page 2 of 5. Balestra et al.: CLASH-VLT: Spectroscopic confirmation of z=6.11 lensed galaxy
Fig. 3.
VIMOS 1D spectrum of ID3 (2h-exposure, pointing P1+P3; black line ) and of the sky rescaled to arbitrary units ( green line ). The lowerinset shows the sum of the 2D spectra (2 × α is shown in the central inset. The vertical blue dotted linemarks the position of the peak of the line, which shows a clearly asymmetric profile. In the 2D spectrum, the green segments mark the position ofthe peak of the flux and that of the 20% flux level. σ upper limit on the 0 . −
10 keV rest-frame luminos-ity is only 5 × erg s − (obtained by stacking Chandra countsat the position of the three brightest magnified images), a low-luminosity AGN cannot be excluded with the current shallowX-ray.To obtain a more precise measurement of the continuum fluxand, hence, a more reliable estimate of the EW of the Ly α , weused only the highest S/N spectrum (ID3 2hr-stacked spectrum).The integrated flux of the line is 1 . × − erg s − cm − andthe continuum has a flux of 2 . × − erg s − cm − Å − in thewavelength range ∼ − EW = ±
10Å for the Ly α , where the error is estimatedfrom the statistical error on the flux of the continuum.The line profile is asymmetric with a more pronounced redwing (see inset of Fig. 3). The asymmetry of the line is typicallyobserved at these redshifts and in this case is mainly due to theintergalactic medium, which at z ∼ α ) and stellarultraviolet (UV 1600Å) fluxes, without correction for dust atten-uation. After correcting for the magnification factor ( µ = α ) =
11 M ⊙ yr − and SFR(UV) = ⊙ yr − , re- spectively. The intrinsic SFR(Ly α ) can be even higher (possiblydouble) since the IGM most probably attenuates ∼
50% of theline flux. Interestingly, the SFR(Ly α ) is higher, or at the mostsimilar to that derived from the UV continuum. As discussed inVerhamme et al. (2008), this is indicative of very low or negligi-ble dust attenuation coupled with very young stellar populationsand recent onset of star formation activity (consistent with re-cent results by M13). Given the stellar mass derived from SEDfitting ( ∼ × M ⊙ , see M13), the specific star formation rateis sSFR(Ly α ) =
55 Gyr − (or sSFR(UV) =
15 Gyr − ).To estimate the size of the source we measured the effectiveradii of ID2, ID3, and ID4 from the HST images and the mag-nification factors obtained from the lensing model by M13. Weestimate that the effective radius of the source is R e . . Σ S FR > ⊙ yr − kpc − and we can use the Kennicutt-Schmidt relation to derive the gassurface mass density Σ gas > M ⊙ pc − . This indicates that thestellar-over-gas mass ratio is close to unity, in line with theoreti-cal expectations for small mass ( . M ⊙ ) and low SFR objectsat these redshifts (see Calura et al. 2008). As part of our CLASH-VLT survey, we targeted several multi-ply imaged sources and candidate high- z objects. In Table 1, wereport the spectroscopic redshifts obtained so far for this clusteralso with the low-resolution blue grism. We confirm 10 of the14 multiple image systems used by M13. These measurementsconfirm the high accuracy of the photometric redshifts and addi-tionally support the strong-lensing model ( z lens ≃ z spec ). Article number, page 3 of 5&Aproofs: manuscript no. RXJ2248_specz6obj
Table 1.
Redshifts of 10 multiple image systems and other backgroundobjects measured in the core of RXC J2248.7-4431 to date.
RA Dec z spec z phot z lens (1) (2) (3) (4) (5)22:48:43.45 -44:32:04.6 6.110 5 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . − . + . − . − Notes. (1-2) J2000 coordinates, (3) spectroscopic redshift, (4-5) photometric andlensing-model predicted redshift (and IDs) from M13.
4. Discussion
We presented VIMOS/VLT observations providing a spectro-scopic confirmation at z = .
110 for a galaxy quintuply imagedby the Frontier Fields cluster RXC J2248.7-4431.The VIMOS spectra clearly show a strong Ly α and a rela-tively faint, but significantly detected (5 σ ), continuum. This is arare case where the detection of the continuum can be achievedin galaxies with delensed magnitudes of ∼
27. Remarkably, thiswas achieved in only 1 hr of exposure. Our results, together withthose recently presented by M13, suggest that this magnified,distant galaxy is a young ( <
300 Myr, ∼ M ⊙ , and Z < . Z ⊙ )and compact ( . . L ∗ at z =
6, with asimilar amount of mass in gas and stars. We can rule out the pres-ence of a strong AGN because of the absence of N V emissionline in the VIMOS spectra and because of the Chandra upperlimit on the X-ray luminosity ( < × erg s − ).While this paper was finalized, Boone et al. (2013) very re-cently reported the detection of a submillimeter source withAPEX/LABOCA at 870 µ m, which they tentatively associatedwith the z ∼ z ∼ A v ∼ .
5) can be reconciled with the ex-tremely steep UV spectral slope derived from CLASH photom-etry ( β ∼ − .
9, see M13) and the prominent Ly α emission, EW ≃ z = .
110 source as a young and compact object with low dustcontent in an early phase of evolution, when the Universe wasjust 1 Gyr old. High spatial-resolution submillimiter observa-tions with ALMA will be extremely useful to reliably constrain the SFR and the molecular gas content in this magnified high- z system. Acknowledgements.
We thank the anonymous referee for the valuable commentsand suggestions. We acknowledge partial support by the DFG Cluster of Excel-lence Origin Structure of the Universe. AZ is supported by contract research“Internationale Spitzenforschung II/2-6” of the Baden-Württemberg Stiftung.
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Article number, page 4 of 5. Balestra et al.: CLASH-VLT: Spectroscopic confirmation of z=6.11 lensed galaxy INAF - Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11,I-34131, Trieste, Italy INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello16 I-80131 Napoli, Italy INAF - Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy Dipartimento di Fisica e Scienze della Terra, Università di Ferrara,Via Saragat 1, I-44122 Ferrara, Italy University Observatory Munich, Scheinerstrasse 1, D-81679München, Germany Dark Cosmology Centre, Niels Bohr Institute, University of Copen-hagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark Space Telescope Science Institute, 3700 San Martin Drive, Balti-more, MD 21218, USA INAF/IASF-Milano, via Bassini 15, 20133 Milano, Italy INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125Firenze, Italy Max-Planck-Institut für extraterrestrische Physik, Postfach 1312,Giessenbachstr., D-85741 Garching, Germany Department of Physics and Astronomy, The Johns Hopkins Univer-sity, 3400 North Charles Street, Baltimore, MD 21218, USA University of Vienna, Department of Astrophysics, Türkenschanzstr.17, 1180 Wien, Austria Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg,Germany Dipartimento di Fisica, Università degli Studi di Trieste, Via Tiepolo11, I-34143 Trieste, Italy Instituto de Astrofísica de Andalucía (CSIC), C/Camino Bajo deHuétor 24, Granada 18008, Spain Department of Theoretical Physics, University of the Basque Coun-try, P. O. Box 644, 48080 Bilbao, Spain Leiden Observatory, Leiden University, P. O. Box 9513,2300 RALeiden, The Netherlands Department of Physics and Astronomy, Michigan State University,East Lansing, MI 48824, USA Observatories of the Carnegie Institution of Washington, Pasadena,CA 91 101, USA INFN - Bologna, Via Ranzani 1, I-40127 Bologna, Italy Dipartimento di Fisica, Università degli Studi di Milano, via Celoria16, I-20133 Milan, Italy Department of Physics and Astronomy, The Johns Hopkins Univer-sity, 3400 North Charles Street, Baltimore, MD 21218, USA Institut de Ciències de l’Espai (IEEC-CSIC), E-08193 Bellaterra(Barcelona), Spain Departamento de Astronomia y Astrofisica, Pontificia UniversidadCatolica de Chile, V. Mackenna 4860, Santiago 22, Chile Department of Physics, The Ohio State University, Columbus, OH,USA Jet Propulsion Laboratory, California Institute of Technology, 4800Oak Grove Dr, Pasadena, CA 91109, USA27