The X-ray Zurich Environmental Study (X-ZENS). I. Chandra and XMM-Newton observations of AGNs in galaxies in nearby groups
J. D. Silverman, F. Miniati, A. Finoguenov, C. M. Carollo, A. Cibinel, S. J. Lilly, K. Schawinski
aa r X i v : . [ a s t r o - ph . C O ] O c t Submitted to The Astrophysical Journal
Preprint typeset using L A TEX style emulateapj v. 04/17/13
THE X-RAY ZURICH ENVIRONMENTAL STUDY (X-ZENS). I.
CHAN DRA
AND XMM-
N EW T ON
OBSERVATIONS OF AGNS IN GALAXIES IN NEARBY GROUPS
J. D. Silverman , F. Miniati , A. Finoguenov , C.M. Carollo , A. Cibinel , S. J. Lilly , K. Schawinski Submitted to The Astrophysical Journal
ABSTRACTWe describe X-ray observations with
Chandra and XMM-
N ewton of 18 M group ∼ − × M ⊙ , z ∼ .
05 galaxy groups from the
Zurich Environmental Study (ZENS) . The X-ray data aim at establishingthe frequency and properties, unaffected by host galaxy dilution and obscuration, of AGNs in centraland satellite galaxy members, also as a function of halo-centric distance. X-ray point-source detectionsare reported for 22 of the 177 galaxies targeted by the X-ray observations, down to a sensitivity levelof f X ∼ × − erg cm − s − in the broad 0.5-8.0 keV band, corresponding to a limiting luminosityof L . − ∼ × erg s − . With the majority of the X-ray sources attributed to AGNs of low-to-moderate levels ( L/L
Edd & − ), we discuss the detection rate in the context of the occupation ofAGNs to halos of this mass scale and redshift, and compare the structural and morphological propertiesbetween AGN-active and non-active galaxies of different rank and location within the group halos.We see a slight tendency for AGN hosts to have either relatively brighter/denser disks (or relativelyfainter/diffuse bulges) than non-active galaxies of similar mass. At galaxy mass scales < M ⊙ , halocentral galaxies appear to be a factor ∼ Keywords:
X-rays: galaxies, galaxies: Seyfert, galaxies: groups: general, quasars: general INTRODUCTION
There has been much progress in recent years in rec-ognizing the importance of both internal and externalprocesses in shaping the properties of galaxies across cos-mic time. Both dynamical instabilities and feedback ef-fects from either supernova or active galactic nuclei in-fluence the stellar mass growth of individual galaxies.With respect to external factors, there may be specificenvironments most conducive to the buildup of stellarmass, transformation of morphological type, and possi-bly black hole growth. Galaxy group-scale dark matterhalos with masses of order ∼ M ⊙ show a heightenedpopulation of bulge-dominated galaxies (Wilman et al.2009), known to have enhanced levels of AGN activity(Kauffmann et al. 2003; Pierce et al. 2007). This is pos-sibly a result of gravitational tides and dynamical fric-tion at their peak efficiency in accelerating galaxy evolu-tion - and thus plausibly the growth of the central super-massive black holes (SMBHs).Galaxy groups are indeed potentially sites where in-teractions and mergers occur on a cosmologically short [email protected] Kavli Institute for the Physics and Mathematics of the Uni-verse, Todai Institutes for Advanced Study, the University ofTokyo, Kashiwa, Japan 277-8583 (Kavli IPMU, WPI) Institute for Astronomy, ETH Z¨urich, CH-8093, Z¨urich,Switzerland. Department of Physics, University of Helsinki, GustafH¨allstr¨omin katu 2a, FI-00014 Helsinki, Finland CEA Saclay, DSM/Irfu/S´ervice d’Astrophysique, Orme desMerisiers, F-91191 Gif-sur-Yvette Cedex, France timescale (e.g., Barnes 1990). Mergers are a credible can-didate for triggering nuclear activity, given their abil-ity to generate large mass inflow rates to the nuclearregion thus fueling both starbursts (Sanders & Mirabel1996) and AGNs (e.g. Hopkins et al. 2008). In fact, en-hanced levels of AGN activity have been observed in closepairs of galaxies, both at low (Ellison et al. 2011) andhigh (Silverman et al. 2011) redshift, that are more com-mon in galaxy overdensities similar to the group scale(Lin et al. 2010; Kampczyk et al. 2013). Such a mech-anism within these environments may lead to the qui-escent black hole - bulge relation (Gebhardt et al. 2000;Ferrarese & Merritt 2000) that is seen locally and mayextend up to z ∼ Silverman et al. eral studies of AGN activity in nearby galaxy groupshave been undertaken, but the statistics with respectto AGNs and the selection of a well-defined parentgroup sample remains a challenge (e.g., Shen et al. 2007;Arnold et al. 2009). Recent large systematic redshift sur-veys of the local universe (2dfGRS: Colless et al. (2001);SDSS: Yang et al. (2007)), with their well-defined cata-logues of galaxy groups, offer the opportunity to con-duct large efforts to study z = 0 galaxies and theirSMBHs in these potential wells (e.g., Weinmann et al.2006; Sabater et al. 2013). Our own Zurich ENvironmen-tal Study (ZENS) focuses on multi-band optical data fora complete sample of galaxy groups extracted from the2dFGRS, that enables an accurate parameterization ofsub-structure in galaxies, including bulge-to-disk ratios,bar strengths, location and sizes of the star forming re-gions, strength of tidal features, etc (Carollo et al. 2013;Cibinel et al. 2013, 2012). Therefore, ZENS offers anoptimal sample to help clarify the physical drivers be-hind the coordinated growth of SMBH and their hosts ingalaxy groupsWe have initiated a follow-up study of the X-ray emis-sion from ZENS groups (X-ZENS). In this paper, wepresent the first-epoch X-ZENS observations of 18 groupsobserved with either Chandra or XMM-
N ewton , andprimarily describe the data analysis and the point-likeX-ray source population in these groups. X-ray emissionis a unique probe of low-luminosity, L X . − ergs − AGNs less affected by host galaxy dilution than op-tical tracers. In a companion paper (Miniati et al. inpreparation), we will describe the detection and prop-erties of the thermal diffuse intra-group medium (DIM)emission from the ZENS groups, a second key driver forour X-ZENS program.By providing a well-constrained sample of low-luminosity AGNs, the currently available X-ZENS datagive a first indication of (1) the occupation frequency ofAGNs in halos of mass M group ∼ − × M ⊙ , i.e.,intermediate-mass potentials within large-scale struc-ture, (2) whether AGNs show any preference to residein galaxies of any given rank, i.e., central or satellite, orat specific radial locations within these groups, and (3)compare the morphological and structural properties ofthe hosts of our AGNs to the overall ZENS galaxy pop-ulation. We highlight that this program is providing alocal benchmark for higher redshift studies in key sur-vey fields (e.g., COSMOS (Scoville et al. 2007), GOODS(Giavalisco et al. 2004), CANDELS (Grogin et al. 2011))that extend environmental studies of AGNs up to z ∼ H = 70 kms − Mpc − , Ω Λ = 0 .
7, Ω M = 0 .
3, and AB magnitudes. THE ZURICH ENVIRONMENTAL STUDY (ZENS) OFNEARBY GALAXY GROUPS
ZENS is a multi-wavelength study of galaxies in well-defined, optically-selected groups, primarily based onthe 2dFGRS (Colless et al. 2001, 2003). This largegalaxy spectroscopic redshift survey has produced nearly225,000 galaxies at 14.5 < b J < < Figure 1.
Relative distribution of (a) halo mass of the group,and (b) the filamentary large-scale structure (LSS) density. Thefull sample of 141 ZENS groups is shown by the dashed histogramwhile the filled histogram represents the 18 groups observed byeither
Chandra or XMM-
Newton . z > ∼ .
11. A friends-of-friends percolation algo-rithm was implemented to identify linked galaxies thatlikely share a common gravitational potential well; theresulting 2dFGRS Percolation-Inferred Galaxy Group(2PIGG) catalogue has 7000 groups (Eke et al. 2004a,b).The structurally-resolved analyses of ZENS are basedon deep, wide-field optical imaging, obtained at the ESO2.2m telescope equipped with the WFI camera, in the B and I passbands. The ZENS sample contains 141groups extracted from the 2PIGG catalog (Carollo et al.2013; Cibinel et al. 2013, 2012), for a total of 1630 galaxymembers. The ZENS sample was defined by selectingall groups within a narrow redshift range 0.05 < z < m ; num-ber of constituent members in the 2dFGRS spectro-scopic sample). This ensures a very uniform group se-lection criteria and eliminates any distance related bi-ases. The z ∼ M ∗ –2 to M ∗ +3,thereby covering all of the luminosity function of ”mas-sive” galaxies, (2) samples a range of group masses andlarge-scale environments, and (3) the ground-based res-olution is well-suited to determine structural properties -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 3
X-ray: 0.5 - 8 keV
50 kpc @ z =0.52 Op ti ca l: I- band Figure 2.
SDSS J142808.89-023124.8. Optical (I-band; left panel) and
Chandra
X-ray ( right panel) images of the central region of2PIGG-n1381. The X-ray image has been smoothed with a Gaussian (FWHM=2 pixels). Galaxy group members are marked in bothimages by red circles ( r = 20 ′′ ) with the central galaxy being the furthest to the left. The optical and X-ray source associated with SDSSJ142808.89-023124.8 is shown in green. This object is clearly an AGN based on the broad emission component to the H α emission linesitting at its base. of galaxies such as bars, bulges, disks, tidal features andcolor gradients.For all galaxies in ZENS, we know the mass of thegroup halo in which they reside, their halo-centric (pro-jected) distance, and their large-scale structure density;they are furthermore classified as centrals or satelliteswithin their host group halos. The group mass (M group )is determined by the sum of the optical luminosity ofeach member (with a weighting scheme chosen to com-pensate for the survey incompleteness of the 2dFGRS)and assumed light-to-mass conversion factor. The large-scale over-density ( δ LSS ) is measured by using the groupsthemselves as point-sources and measuring the distanceto the 5th nearest neighbor; for this exercise, groups areweighted by their halo mass, restricted to be within aredshift interval δz = ± .
01 and required to be abovea given optical luminosity. Groups are further identifiedas relaxed or unrelaxed depending on whether a clearcentral galaxy is identified by considering both stellarmass, projected halo-centric distance, and relative veloc-ity within the group. We refer the reader to Carollo et al.(2013) for full details on the ZENS sample and derivedphysical properties. X-RAY OBSERVATIONS: DATA ACQUISITION,ANALYSIS AND SOURCE DETECTION
We extract from our ZENS survey a sample of 18galaxy groups (Table 1) for which we have acquired X-ray observations with either
Chandra or XMM-
N ewton to primarily assess their AGN content by their X-rayemission and the presence of the thermal emission fromthe diffuse intragroup medium (DIM). The point sourcesensitivity of
Chandra is ideal to detect AGNs to low lu-minosities ( L X ∼ × erg s − ) with short exposuretimes (i.e., 10 ksec). While the potential to detect ex-tended emission from the DIM with Chandra exists, thehigher collecting area in the soft-band of XMM-
N ewton can provide more accurate measurements of spectral pa-rameters (e.g., temperature) for the X-ray bright cases(Miniati et al. in preparation). The groups for
Chandra followup are chosen to ensure that we sample the widerange in galactic composition by selecting those with (1)a number of spectroscopically-identified members ≥
7, (2)a halo mass M group in the range 1 . − . × M ⊙ , and(3) a projected radii on the sky less then 9 ′ ( ˆ R . . Chandra /ACIS-IFOV (17 ′ × ′ ). Those observed by XMM- N ewton aresimilarly selected (see Miniati et al. in preparation for
Silverman et al. details). Figure 1 shows the region of parameter space inboth group mass (M
Group ) and large-scale density ( δ LSS )covered by our group sample with X-ray observations, incomparison with the full ZENS sample.
Chandra
We carried out
Chandra /ACIS-I observations of 12ZENS galaxy groups in Cycle 11 (PI J. Silverman; pro-posal z ∼ .
05 down to a limiting luminosityof L (0 . − ∼ × erg s − for detections withat least 4 counts in the broad energy band 0.5-8 keV.The 16 . ′ × . ′ field-of-view (FOV) of CCDs Chandra were chosen to maximizethe number of galaxies that fall within the ACIS-I FOV.This resulted in offsets as given in Table 2 from the groupcenters listed in Table 1. We also tried to avoid havinggalaxies falling within or near chip gaps; this was ac-complished by adjusting the pointing location once theplanned observation date was set thus the roll angle wasknown. In other cases, this was needlessly achieved bysplitting the observation into smaller time intervals andapplying small offsets ( < ′ ) to the aim point. In Table 2,we provide details on the individual exposures.We use CIAO tools (Version 4.3 and CALDB version4.4.6) to perform the data analysis. In cases where mul-tiple exposures were taken, we use the task merge all tocombine the individual frames that generates a summedevent file (level 2). Source counts are measured in threeenergy bands (Broad (B): 0.5-8.0 keV, Soft (S): 0.5-2keV, Hard (H): 2-8 keV) by placing circular aperturesat the location of galaxies within our groups. The ex-traction radius for each galaxy is set to include closeto 100% of the flux while the background contributionis estimated in an annulus centered on each individ-ual galaxy with an area greater than the source regionand free of any other nearby X-ray sources. We con-sider a positive detection as any source with greaterthan or equal to 4 net counts in any of the three en-ergy bands. This is the same significance threshold em-ployed by the Bootes survey (Kenter et al. 2005) that hasdemonstrated that there is a low false-positive detectionrate even at these low count levels. We use the CIAOtool ’aprates’ to estimate count rates and associated 1 σ errors. Exposure maps, required for conversion of countrates to physical fluxes (i.e., photons cm − s − ) , aregenerated in the three bands that correct for off-axis ef-fects such as vignetting. Due to the fact that merge all does not properly deal with individual frames having apointing offset, an exposure map was created for eachindividual obs id ( mkinstmap , mkexpmap ), reprojected( reproject image ) to a common astrometric frame andthen coadded ( dm merge ). These broad-band maps arebased on a spectral weighting scheme that assumes apower-law spectral energy distribution with a photon in-dex of 1.7. Broad-band fluxes are then determined bymultiplying the photon flux by the mean energy of apowerlaw distribution of photons with the spectral indexas given above. Errors on the total detected counts in Figure 3.
Specific star formation rate versus stellar mass forgalaxies in ZENS groups with X-ray coverage. Color denotes threespectral types (quiescent - red, moderately star-forming - green,and strongly star forming - blue). Galaxies firmly identified asquenched, but with a large error in the sSFR obtained throughtemplate-fits to their SEDs, are all placed at a value of -14. Galax-ies with detected X-ray emission are identified with a large blackcircle; they cover the range of spectral types. Open squares high-light ZENS galaxies with signs of AGN activity (either X-ray oroptical). the broad band allow an assessment of flux measurementerrors while neglecting uncertainty of the assumed spec-tral model. In Table 3, the measurements are providedfor the individual
Chandra detections. The magnitudeof the error on the measured counts can be used to esti-mate uncertainties on flux and luminosity. We note thatthere will be an additional error associated with the con-version of counts to energy units since we do not havefull knowledge of the spectral shape of the individual de-tections due to the small number of counts.We further run wavdetect on each combined image ina given energy band. This enables us to both validatethe significance of the source detections with apertureestimates, as mentioned above, and to construct imagesdevoid of X-ray point sources, required for the detectionof diffuse emission (see Miniati et al. in preparation fordetails). A significance threshold of 5 × − was setto detect sources with low count statistics ( N ∼ − , , , × . ′′ thatare identical to those used by the Chandra survey of theBootes field (Kenter et al. 2005).
XMM-
N ewton
We also acquired X-ray imaging of 9 ZENS groupswith XMM-
N ewton (PI F. Miniati; proposal -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 5parable to the
Chandra observations reported above.This is due to the fact that the XMM observations weretaken with the aim of providing sufficient count statis-tics to accurately measure the physical properties of theDIM. While four additional groups have some archival X-ray coverage with XMM-
N ewton , they are highly offsetfrom the center position of our groups and miss most ofthe galaxy members. One of them (2PIGG n1377) suf-fers from flaring and is not useful even though a singlebright X-ray source is present that is associated with agalaxy (2PIGG n1377 18) belonging to the group. Outof these seven groups, only one (2PIGG s1571) is in com-mon with the
Chandra sample. In fact, this groupclearly has diffuse emission detected with both instru-ments as presented in Miniati et al. (in preparation).The XMM-
N ewton observation further aids in the de-tection of a X-ray point source associated with the cen-tral galaxy in 2PIGG s1571 that was uncertain from the
Chandra observation. This highlights the complemen-tarity of
Chandra and XMM.For point-source detection, we use the 0.5-2 keVand 2-7.5 keV bands while masking those energy in-tervals impacted by strong instrumental effects, as inFinoguenov et al. (2007). A detailed modeling of theunresolved background, foreground and detector back-ground has been undertaken, following the prescriptionof Bielby et al. (2010). We use 4 ′′ pixels and run thewavelet image reconstruction on 8 ′′ and 16 ′′ scales, witha 4 σ detection threshold. We produce catalogs, based onthe detections in two energy bands. Physical flux unitsare obtained by converting the count rate (adjusted toaccount the flux losses within the 15” flux extraction cir-cle due to XMM PSF) using PIMMS with a power lawphoton index of Γ = 1 .
7, and no correction for intrinsicabsorption. We then cross-matched our X-ray catalog tothe positions of the ZENS galaxies. X-ray luminosities inthe broad-band 0.5-8.0 keV are used throughout. In Ta-ble 4, the measurements are provided for XMM-
N ewton point-source detections. As for the
Chandra sources, theerrors on flux and luminosity can be propagated from theerror on count rate. In total, we have X-ray coverage of177 unique galaxies that fall within either the
Chandra or XMM footprints. X-RAY EMISSION FROM ZENS GALAXIES
Out of the 177 galaxies that are confirmed members ofthe 18 groups observed by
Chandra and XMM-
N ewton ,we detect X-ray emission from 22 galaxies with at least 4counts in one of the three
Chandra
X-ray bands (broad:0.5-8 keV, soft: 0.5-2.0 keV or hard: 2-8 keV) or above aS/N of 4 in either the soft (0.5-2 keV) or hard (2-7.5 keV)bands with XMM-
N ewton . Pertaining to the
Chandra observations, the on-axis detection limit for our sampleis & × − erg cm − s − that corresponds to a limit-ing X-ray luminosity of ∼ × erg s − at the redshift z ∼ .
05 of our group sample. The flux limits of theXMM-
N ewton observations are comparable as demon-strated by their typical luminosity limit of a ∼ × erg s − . All X-ray source detections have fluxes between10 − and 10 − erg cm − s − that correspond to a lu-minosity range above the limiting value given above anda maximum luminosity of 7 × erg s − . The opti-cal properties of these individual X-ray detections areprovided in Tables 5 and 6. All optical properties are re- Figure 4.
X-ray luminosity for point-like sources split into star-forming ( a ) and quiescent ( b ) galaxies. Black points mark indi-vidual detections ( Chandra : filled circles; XMM-
Newton : filledsquares) while arrows indicate upper limits. Slanted lines denotethe region where normal galaxies lie either star-forming or quies-cent with a 1 σ interval indicated by the dashed lines. Open squareshighlight ZENS galaxies with signs of AGN activity (either X-rayor optical). ported as given in the ZENS catalog (Carollo et al. 2013;Cibinel et al. 2013, 2012).Before proceeding to discuss the X-ray source popula-tion, we highlight the most luminous X-ray source in oursample with L . − . = 6 . × erg s − with 80X-ray counts in the broad band observation of 2PIGG-n1381 (Figure 2) with Chandra . The optical counter-part SDSS J142808.89-023124.8 (Ahn et al. 2013) is anemission-line galaxy at z = 0 . v = 60 km s − where ∆ v is the line-of-sightvelocity offset from the group center) and distance fromthe bright central galaxy (∆ r proj = 57 kpc). The stel-lar mass ( M galaxy = 6 . × M ⊙ ) and specific starformation rate (sSFR = SFR/M galaxy = 1 . × − yr − ) of its host galaxy, taken from the ZENS data cata-log (Carollo et al. 2013), place it within the moderatelystar-forming population (see next section). The opticalemission line ratios are indicative of a composite nature Silverman et al.
Chandra/ACIS-I (0.5-8.0 keV) 9.2’’ (10 kpc @ z=0.0556) ESO / W FI O ptical (I-band) Figure 5. top left panel : Chandra
X-ray image in the broad band (0.5-8 keV) binned by a factor of 2 for a pixel scale of 0.98 ′′ . top middle panel Optical I-band image that ismatched in scale to the X-ray. In both panels, the ellipse represents the region, as determined by ’wavdetect’, that encompasses ≈
90% of theX-ray source counts that likely originates from the nucleus of this galaxy. top right panel
Voronoi-tessellated color map (see Cibinel et al.2012).
Bottom
Optical spectrum from SDSS with narrow emission lines (
F W HM ∼
200 km s − ) as labelled. (AGN+star forming); we infer that the X-ray emissionis predominantly due to an AGN, given that it exceedsby three orders of magnitude the typical X-ray emissionfrom a normal star-forming galaxy (see details below).The presence of a weak broad component to the H α linein the SDSS spectra provides strong further evidence foran underlying AGN. There may possibly be also evi-dence for an offset of the broad from the narrow com-ponent of H α , that may indicate an underlying double-peaked profile (e.g., Strateva et al. 2003) typically seenin low-luminosity AGNs (Ho et al. 2000) (or possibly amore exotic event such as a gravitational recoil, (e.g.,Civano et al. 2010)). This object exemplifies the impor-tance of X-ray selection to cleanly identify galaxies har-boring AGN of such low luminosity. AGN versus normal galaxies as the origin of X-rayemission
The level of X-ray emission in ZENS galaxies is border-ing on the overlap between accretion onto a black hole,stellar remnants and diffuse hot gas. In addition to AGN,point-like X-ray emission may be attributed to the cu-mulative contribution of low-mass and high-mass X-raybinaries. There is additionally room for a component ofthermal emission in early-type galaxies. The high-massX-ray binary population is likely to contribute signifi-cantly only to those galaxies with high star formationrates. There are many studies (e.g., Ranalli et al. 2003;O’Sullivan et al. 2003; Lehmer et al. 2010; Boroson et al. 2011) of X-ray emission from normal galaxies (i.e., with-out AGN) that enable us to determine whether the levelof X-ray emission is characteristically higher than thatexpected for galaxies of a given luminosity, stellar massand star formation rate. While we are mainly concernedwith nearby galaxies, such relations between X-ray emis-sion and galaxy type appear to be universal and extendover a wide range of redshift (Lehmer et al. 2007, 2008).We first investigate the nature of the X-ray emission bysplitting the galaxy samples into either star-forming orquiescent as classified by the presence of prominent emis-sion lines in the optical spectra combined with NUV-NIRcolors (Cibinel et al. 2012). The diversity of galaxy spec-tral type within the 18 X-ZENS groups can be seen inFigure 3 where we plot the sSFR as a function of theirstellar mass M galaxy as done in Figure 8 of Cibinel et al.(2012). It is worth pointing out that the ZENS sam-ple does have incompleteness at low masses that cannotbe neglected; the lack of galaxies with M galaxy ∼ M ⊙ and sSFR between ∼ − − − yr − is clearlyevident. In the figure, we indicate with an open circlethose galaxies that are detected in X-rays. There is awide spread in the distribution of sSFR at a constantstellar mass for galaxies with X-ray emission, with X-ray sources associated with galaxies at the extreme ends,i.e., either strongly star-forming or quiescent. Withinthe limited statistics, the relative numbers of quiescentversus star-forming galaxies that emit X-rays reflects the -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 7
Figure 6.
Emission-line ratio diagram: [OIII] λ β vs.[NII] λ α . Black points indicate emission-line galaxies fromSDSS, and red points show the location of our X-ray ZENS sam-ple. The curves are the division set between star-forming galaxiesand AGN as established by Kauffmann et al. (2003) (dashed) andKewley et al. (2006) (solid). We place galaxies in our sample withonly constraints on the ratio of [NII] λ α at a fixed value of[OIII] λ β =-0.8 relative number density of the underlying galaxy pop-ulation. While the hosts span a wide range in massfrom 10 M ⊙ to above 10 M ⊙ , there is a preference formassive galaxies that one would expect given the well-established increase of AGN activity as a function of stel-lar mass (e.g., Kauffmann et al. 2003; Silverman et al.2009b; Haggard et al. 2010). The significance of a de-pendence of AGN activity on the stellar mass of theirhosts is further addressed in Section 5.3.In Figure 4, the X-ray luminosity is shown as a func-tion of star-formation rate for galaxies classified as eitherhighly or moderately forming stars (panel a ), and of B luminosity for the quenched population (panel b ). Foreach ZENS galaxy without an X-ray detection, we indi-cate an upper limit on the broad-band X-ray luminosity.For those galaxies covered by Chandra , this is equivalentto our limit on a source detection of four counts at theposition on the detector thus accounting for variations inthe effective area as a function of off-axis angle. The up-per limits on the XMM-
N ewton non-detections are basedon a flux level at 2 σ above the background. For refer-ence, we provide the typical scaling relations for normalgalaxies (Ranalli et al. 2003; O’Sullivan et al. 2003) be-tween these quantities plus an interval of ± σ to indicatethe typical spread.With respect to 111 star-forming galaxies (Fig. 4 a )falling within the Chandra and XMM-
N ewton coverage,we find that 5 out of 10 galaxies with X-ray detectionsare above the relation for normal, star-forming galax-ies, by at least 2 σ deviation, thus likely indicative of anAGN contribution effectively boosting the X-ray lumi-nosity above our detection limits. From the distributionof SFRs of the X-ray non-detections, it is clear that themajority of the galaxies in ZENS have SFRs low enoughthat if we do detect an X-ray source, it is most probablydue to an AGN. We have detected with XM M − N ewton
Figure 7.
Optical spectra from 2dFGRS of four galaxies thatlikely host an AGN based on their having both X-ray emissionand optical line ratios indicative of such nuclear activity. Exam-ples are shown of galaxies classified in ZENS as either star-forming(2PIGG n1606 3, 2PIGG s1783 7) or quiescent (2PIGG n1572 13,2PIGG 1520 4). Dotted lines mark the location of the emissionlines H β , [OIII] λ α and [NII] λ X-ray emission from three galaxies falling within the re-gion ( ± σ ) of a normal star-forming galaxy; based ontheir optical emission-line properties (see below), all ofthese are indeed likely to have a contribution to the X-rayemission from an AGN.We highlight the fourth such galaxy (2PIGG-n1598 5)falling within the locus of normal star-forming galaxies.It has the highest SFR of all ZENS galaxies observed by Chandra . With a sSFR 2 . × − yr − at its stellarmass of M galaxy = 5 . × M ⊙ , it has been iden-tified as the central galaxy (with evidence for a strongbar) within its group. We can make use of the highresolution imaging of Chandra and quality optical WFI-ESO imaging from ZENS to determine if the X-rays arecoming from the nuclear region or the overall extent ofthe galaxy. In Figure 5, we show the X-ray and opti-cal images of 2PIGG-n1598 5. It is clear that the X-rayemission is co-spatial with the nucleus. The emissionline ratios of [NII]/H α and [OIII]/ Hβ , seen in an SDSSspectrum (Fig. 5 bottom ), place this object right on theborder between AGNs and star-forming galaxies as de-fined by Kewley et al. (2006), although, the [SII]/H α ra-tio might be in principle more typical of HII regions. We Silverman et al. can also use the H α emission (detected within the SDSSfiber that covers only the very central region of the galaxydue to the fiber diameter of 3 ′′ ) to make a more accurateassessment on the upper limit to the level of star forma-tion in the nuclear region cospatial with the X-ray emis-sion. Based on a H α luminosity of 8 . × erg s − , weestimate the star formation rate of the central region ofthe galaxy to be around 0.07 M ⊙ yr − (Kennicutt 1998),substantially lower than that of the entire galaxy andnot likely to produce the amount of X-rays observed by Chandra . Therefore, we conclude that a low-luminosityAGN with L X = 8 × erg s − is powering the X-rayemission seen in 2PIGG-n1598 5.Out of the 66 quiescent galaxies observed in X-rays,there are 12 detections with 10 of them falling with aregion of the L X − L B plane spanned by normal galax-ies (see Figure 4 b ). These galaxies need not necessarilyhave an AGN to explain their level of X-ray emissionas was primarily the case for the star-forming popula-tion. As done above, we can measure the spatial ex-tent of the X-ray emission, based on Chandra imag-ing and determine whether it is extended beyond thatexpected based on the point spread function (PSF) atthe given position on the detector since the size of thePSF is a strong function of the off-axis angle. Based onthe three objects having
Chandra imaging, we find thattwo (2PIGG n1347 3 and 2PIGG n1671 4) are clearlyextended while 2PIGG s1752 4 has a spatial count dis-tribution similar to that expected due to an unresolvedpoint source. Therefore, we are able to classify the latteras an AGN.As an independent test to further solidify the pres-ence of an AGN, we make use of the optical emissionline properties of ZENS galaxies using the 2dFGRS spec-tra. We specifically measure the emission line ratios[NII] λ α , and [OIII] λ β if both lines are de-tected. In Figure 6, we show the distribution of theirratios as compared to emission-line galaxies from SDSS.For many galaxies in our sample, the H α and [NII] emis-sion lines are clearly present while the H β and [OIII]lines are very faint, primarily for the quiescent galaxypopulation. In Figure 7, we provide examples of the2dFGRS spectra of galaxies, classified in ZENS as eitherstar-forming or quiescent, that have X-ray emission plac-ing them within the locus of normal galaxies (Figure 4),and optical line ratios typical of an AGN. A limitation isthat we cannot separate Seyferts and LINERS, where thelatter have been heavily debated in the literature as towhether their line ratios are indicative of AGN photoion-ization or that due to post-AGB stars (Sarzi et al. 2010;Yan & Blanton 2012). A reasonable working hypothesis,mainly pertaining to quenched galaxies, is that a high[NII]/H α ratio is an indication of an AGN, irrespectiveof the H β and [OIII] line strengths. In Figure 6, we findthat 10 of the 13 galaxies for which we could measurea [NII]/H α emission-line ratio are unlikely to be typicalstar-forming galaxies, given their location relative to thedemarcation line of Kauffmann et al. (2003). The opticalspectra of the remaining 9 galaxies with X-ray detectionshave either very weak or non-existent H α or [NII] λ Chandra res-olution (otherwise, if diffuse, it is attributed to thermalemission). Third, we consider the optical emission lineratios to aid in the discrimination between photoioniza-tion indicative of an AGN and UV emission from youngstars. Taking these together, we find that the 16 of22 X-ray sources are associated with emission from anAGN. Those that are not AGN are mostly quiescentgalaxies with either noticeable thermal emission or un-resolved stellar remnants. Our detection rate for AGNsis consistent with the steeply rising faint end of the localAGN X-ray luminosity function (Sazonov & Revnivtsev2004; Ueda et al. 2011), for which our sample spans L X ∼ . − . × erg s − , and with the area coverageof the X-ZENS observations.It is important to recognize that the X-ray emissionmay be of a composite nature with AGN, stellar or ther-mal processes all making some contribution to the totalX-ray emission. This may provide a boost in the numberof X-ray detections as expected from normal galaxies,complicate the selection of AGN, and hamper the deter-mination of a pure AGN luminosity used to determine abolometric quantity (Section 5.4). With the limited sam-ple in hand, there is little that can be done to quantifysufficiently such effects. A larger sample will allow usto determine at what luminosities does such dilution ofthe X-ray emission become problematic. There may evenbe X-ray data in the archive of local galaxies where thiscan be addressed more effectively than with the ZENSsample.Finally note that we detect X-rays from two (satel-lite) galaxies with low stellar masses M galaxy ∼ M ⊙ (see Figure 3). If the X-rays are attributed toblack hole accretion, the black holes are likely to be oflow mass M BH ∼ M ⊙ with Eddington rates be-low 10 − (see Figure 13, obtained assuming a typicalbulge-to-disk ratio of ∼ . M galaxy < × M ⊙ in the Chandra Deep FieldSouth survey that emit X-rays which are likely attributedto AGN activity (with M BH ∼ a f ew × M ⊙ ). Suchlow black holes mass and accretion rate regimes are stilllargely unexplored (see Reines et al. 2013, for a recent ef-fort using optical selection); this shows the potential foropening them to detailed studies of X-ZENS-like surveysextended however to larger group samples. THE AGN CONTENT OF THE X-ZENS GROUPS
While the main focus of this first X-ZENS paper is todescribe our current X-ray observations and point-sourcedetections, we briefly carry out a preliminary analysis ofthe demographics of our AGN sample with respect to thelarger ZENS database and in comparison with the field.Specifically, we discuss below whether AGNs in galaxygroups show any preference for any given type of galaxyhosts and environments, i.e., galaxies of different bulge-to-total ratio, central or satellite rank within the halos,and at small or large halo-centric distances.
Structural properties of ZENS galaxies hostingAGNs -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 9
Figure 8.
Right : Bulge-to-total ratio versus galaxy stellar mass of ZENS galaxies (black points; elliptical galaxies are placed at B/T=1).Median B/T ratios in bins of stellar mass are indicated by the green crosses.
Left : For galaxies with both a bulge and a disk component,we plot the disk scale length h versus the half-light radius of the bulge r / ,Bulge , both in units of kpc. Both panels show with a solid linethe best-fit linear relation for the whole galaxy population, with the AGNs marked as larger red circles. We first determine whether galaxies in groups with X-ray luminosities above our detection threshold are pref-erentially hosted by any particular type of galaxy host.The left panel of Figure 8 shows, as a function of galaxystellar mass, the bulge-to-total ratio
B/T , when avail-able, as derived from double-component fits to the bulgeand disk light distributions. These were described with,respectively, a general S´ersic profile (S´ersic 1968) and anexponential profile (see Cibinel et al. 2013). The wholeZENS galaxy sample is reported with black points, andin red are highlighted galaxies with an X-ray detectedAGN . Galaxies with a purely elliptical morphology areplaced at a bulge-to-total value of 1. For reference, weshow the median ratio B/T for galaxies with double com-ponent fits in bins of stellar mass (10 < M < . M ⊙ ;∆ log M = 0 . M ⊙ ). For galaxies which have both abulge and a disk component, the right panel of the samefigure shows the disk scale-length h plotted versus thehalf-light radius of the bulge, r / ,Bulge . Interestingly,the AGN hosts appear to be on average slightly ‘under-bulged’ relative to the galaxy population at similar stellarmasses (i.e., the AGN hosts systematically lie in the bot-tom half of the B/T versus galaxy stellar mass relation);on the other hand, the bulges of AGN hosts appear to besimilarly ‘embedded’ within their surrounding disks thanthe global galaxy population at similar mass scales (i.e.,they are distributed evenly around the best fit to the h vs r / ,Bulge relation. Possible explanations are that, ata given mass, the hosts of AGNs have either relativelybrighter/denser disks or relatively fainter/diffuse bulgesthan their non-active relatives. Halo occupation distribution
We measure the halo occupation distribution (HOD) ofAGNs in the X-ZENS groups. With 16 AGNs distributedin 18 groups, the mean number of AGNs with L X > erg s − per group is 0 . ± .
22 (with the error based onPoisson number counts). We compare the number of From this analysis we exclude SDSSJ142808.89-023124.8,which is not included in the 2DF sample, to avoid introducingbiases in our assessment.
AGNs detected in our groups to those expected based onthe X-ray luminosity function (XLF) of the global AGNpopulation (Ueda et al. 2011), to establish whether anover-density of AGNs is seen within group-sized poten-tials. We restrict the analysis to AGNs found in the
Chandra observations of the 12 groups, since we imple-mented an identical X-ray count threshold as in the ob-servations of the Bootes survey (Kenter et al. 2005), andcan thus make use of the well-established sky area curveas a function of X-ray flux sensitivity. We find a surfacedensity of 9.2 AGNs per square degree considering X-raydetections within 8.5 ′ of the aim point. We then integratethe XLF over the narrow redshift range 0 . < z < . . − ergs − . Based on the XLF, we would expect to detect 3.1AGNs per square degree with redshifts and luminositieswithin these ranges. The enhancement of AGNs of a fac-tor ≈ M R < −
20 is ∼ ± . . ± . ∼ − < M i < −
20) iscompatible with estimates of the AGN fraction of galax-ies in the field (Haggard et al. 2010); such a narrow se-lection of absolute magnitude is required to compare totheir results (i.e., sample 2 in their study).0
Silverman et al.
Figure 9.
Mean number of AGNs per galaxy group (i.e., halo occupation) in the 13 < log M group <
14 mass range as a function ofredshift. The X-ZENS data point is shown in red; in black are the measurements from the COSMOS study of Allevato et al. (2012), scaledto match the X-ray luminosities of our sample. The solid line indicates a best-fit relation (dotted lines at ± σ ) with an evolution rate ∝ (1 + z ) . , i.e., similar to the evolution of the global AGN population. We then determine how the AGN HOD of X-ZENS groups compares to higher redshift measurements.Allevato et al. (2012) provide the AGN distribution inX-ray selected groups in COSMOS up to z ∼
1. De-spite the different selection criteria for the COSMOS andX-ZENS samples, the halo mass ranges are very sim-ilar thus allowing us to make such a comparison andlook for evolutionary trends. For this exercise, we as-sume that the shape of the 0.5-8 keV XLF does notchange with redshift, and that the faint-end slope is astrict power law below L X ∼ erg s − . The first as-sumption is supported by observational evidence up to z ∼ σ errors: < N AGN > = 0 . +0 . − . × (1 + z ) . ± . (1) Even considering the uncertainties, we find that thereis very good agreement between the two samples as in-dicated by the redshift evolution of the AGN HOD,shown by the solid curve (Fig. 9). Furthemore, therate of evolution is practically identical to the globalX-ray luminosity function of Ueda et al. (2003) whichhas evolution parameterized as (1 + z ) . , which is alsowell reproduced by other determinations for the AGNpopulation (Silverman et al. 2008; Ebrero et al. 2009;Aird et al. 2010). We conclude that the rate of declinewith redshift of AGN activity in galaxy groups is similarto that of the global AGN population. The origin of thissimilarity may rest on either a predominance of AGN ac-tivity in halos at the M group ∼ − M ⊙ mass scale,as supported by clustering analyses of AGNs (e.g., Por-ciani & Norberg 2006), or on the independence of AGNevolution on halo mass. AGN hosts: Centrals, inner satellites or outersatellites?
Next, we investigate whether AGN activity is typi-cal of central or satellite galaxies within groups, andwhether it is enhanced in the cores or outskirts of thetypical ∼ M ⊙ galaxy groups that we probe in X-ZENS. There is evidence in a number of other stud-ies (e.g., Ruderman & Ebeling 2005; Martini et al. 2007;Martel et al. 2007; Fassbender et al. 2012) that AGNshow a preference for the inner regions of massive clus-ters, at halo masses larger than the X-ZENS values, andthat AGNs may possibly be associated with the brightest -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 11
Figure 10.
Fraction of all galaxies split into two mass bins andpresented as central (left) and satellite (right) galaxies separately(open histogram). A mass range of 10 . M ⊙ < M galaxy < . M ⊙ is chosen where both the centrals and satellite popu-lations are well-represented in ZENS. The fraction of galaxies ineach bin that host an AGN is indicated by hatched histogramsand associated error bars. All histograms are normalized to thetotal number of galaxies (within that rank bin) in the mass range10 . M ⊙ < M galaxy < . M ⊙ . The AGN fractions withineach mass and rank bin are given as percentages. Figure 11.
Halo-centric distance R of all ZENS galaxies, irre-spective of having X-ray coverage, in units of ˆ R (small open cir-cles), versus galaxy stellar mass. Galaxies hosting X-ray detectedAGNs are plotted as large stars. The color coding is illustrative ofthe bulge-to-total ratio (B/T). Open black stars have no discerniblemeasurement of B/T. cluster members. There are however other studies whichreport opposite results, i.e., even possible AGN excessesin the outskirts of galaxy clusters (e.g., Gilmour et al.2009; Pentericci et al. 2013). At the lower halo massscales of our sample, cosmological simulations predictthat AGN activity should be more closely tied to cen-tral galaxies as opposed to satellites (Richardson et al.2013).We quantify the fraction of galaxies, split as centraland satellites, that host an AGN over the total massrange 10 . ≥ M galaxy ≤ .
6. We find that 5 of 14 cen-tral galaxies host an AGN (0 . +0 . − . ), and only 4 outof 30 satellite galaxies host an AGN (0 . +0 . − . ). Thus,although there is a hint for an enhancement by a fac- Figure 12.
Stellar mass distribution of galaxies with X-ray ob-servations (black histogram) and those that host AGN (red his-togram). Both distributions have a peak set to unity. AGNs areclearly preferential to more massive galaxies. tor of ∼ < M ⊙ , there is a clear effect forcentrals hosting AGNs about four times more frequentlythan satellites of similar mass., as shown in Figure 10.In Figure 11 we furthermore explore whether there isany differential effect with halo-centric distance in thefrequency of AGNs in satellites. The figure shows halo-centric distance R in units of the group characteristicradius ˆ R (see Carollo et al. 2013, for an explicit def-inition of this parameter) as a function of stellar mass.The majority of the detected AGNs (11/15), as shownby the star symbols, are within R/ ˆ R < .
5. Thismay however be possibly explained by the number ofgalaxy and galaxy mass variations with halo-centric dis-tance (including differences between centrals and satel-lites). Indeed, a 2-D K-S test returns no significant differ-ence between the radial distribution of AGNs and galax-ies at stellar masses above 10 . M ⊙ . Also, a 2-D KStest on the stellar mass distribution of galaxies targetedin X-rays and those hosting AGNs shows that these dis-tributions are dissimilar at the 3.3 σ level, as shown inFigure 12: AGNs are indeed more prevalent in massivegalaxies ( log M galaxy & . Black hole masses and Eddington ratios
Finally we assess in what part of parameter space ourAGN fall in terms of their likely black hole masses andEddington rates. To do so, we use the local scaling re-lation between black hole mass and the stellar mass ofits host galaxy (H¨aring & Rix 2004). Bolometric lumi-nosities are derived by applying correction factor of 20 tothe broad-band X-ray luminosity as applicable to X-rayselected AGNs in COSMOS (Lusso et al. 2012), with acaveat that it is an area of debate whether this factoris appropriate for low luminosity AGNs. In Figure 13,we find that the majority of our sample falls well belowthe Eddington rate with
L/L
Edd ∼ − . This is 2-3 or-2 Silverman et al.
Figure 13.
Eddington ratio versus black hole mass as derivedfrom the stellar mass of the host galaxies and an application ofknown scaling relation (H¨aring & Rix 2004). Colors representwhether a galaxy has been quenched (red) or not (blue) with adefinition set as having an sSFR above or below 10 − yr − . Cen-tral and satellite galaxies are indicated by filled and open symbols.Lines of constant bolometric luminosity are provided including theeffective limit of our survey at L bol = 10 erg s − shaded in grey. ders of magnitude below more luminous quasars such asthose in SDSS (e.g., Shen et al. 2011; Kelly & Shen 2013)and AGNs in COSMOS (Trump et al. 2009; Lusso et al.2012), i.e., much lower than these latter species, evenassuming a large correction factor to the normalizationvalue used above. Disentangling in Figure 13 quenchedgalaxies (with sSFR < − yr − ) from star forminggalaxies, and centrals from satellites, it is clear thatthe X-ZENS AGNs with the lowest Eddington ratios( < − ) are hosted at the high mass end of the galaxypopulation. Already our small samples shows that such‘starved’ AGNs can occur both in central and satellitegalaxies (albeit with a possible preference for centrals),and in quenched as well as star forming hosts (albeit witha preference for quenched systems). SUMMARY
We have presented the X-ray observations of 18 of our z ∼ .
05 ZENS galaxy groups. The observations weretaken using both
Chandra and XMM-
N ewton . This pa-per has focused on the data acquisition, analysis andpoint-source catalog. With
Chandra exposures of 10ksec each, we reach a depth sufficient to detect AGNsdown to L . − & × erg s − at z ∼ .
05. OurXMM-
N ewton data reaches comparable depths due tothe requirement to significantly detect diffuse emissionfrom the DIM as presented in a companion X-ZENS pa-per (Miniati et al. in preparation). In total, we detectX-ray emission from 22 out of 177 galaxies targeted inX-rays.We distinguish the origin of the X-ray emission as dueto either an AGN, or thermal emission or from stellarremnants. We find that X-ray emission seen in stronglystar-forming galaxies, unresolved in all cases, is likely due to an AGN in 9 out of 10 cases, even in galaxies oflow mass that may harbor low mass black holes. X-rayemission in quiescent galaxies is seen to have a signifi-cant contribution from hot diffuse gas and/or a cumu-lative signal from stellar remnants (i.e., low mass X-raybinaries) based on the X-ray emission at expected levelscomparable to normal (non-AGN) galaxies, and in somecases, spatially extended as compared to that expecteddue to PSF variations across the focal plane. Of the qui-escent galaxies, we associate 7 out of 12 galaxies withAGN activity. In total, we find that 16 of the 22 X-raysources are likely due to accretion onto a SMBH, downto very low Eddington rates of ∼ − .While our primary aim here is to provide details on ourX-ray program, we have began to address some of the sci-entific questions on AGN activity in groups at this massscale. We have measured the halo occupation density ofAGNs, and found a lower occupation fraction relative togroups of similar mass up to z ∼
1. The observed declinewith decreasing redshift is entirely consistent with theknown evolution of the global AGN population, and sug-gests either that AGNs do inhabit preferentially such in-termediate group-sized halos, or that the growth of blackholes in groups at this mass scale proceeds at a similarrate than in other environments.As also seen in other studies, AGNs tend to be hostedby massive galaxies. At a given galaxy mass, the galaxieswhich host an AGN may have either relatively brighter,and thus possibly denser disks, or relatively fainter, andthus possibly more diffuse bulges, than galaxies which donot host an AGN. At galaxy masses < M ⊙ , AGNsappear four times more often in central than in satellitegalaxies of similar mass, an effect which explains whyAGNs are preferentially found in the cores of groups,without any detectable trend in the frequency of AGNsin satellite galaxies at different halo-centric distances.X-ZENS provides a low-redshift benchmark for com-parisons with X-ray surveys of groups at higher redshifts,and a low-mass benchmark for comparisons with X-raysurveys of massive clusters – two of the main scientificmotivations of our program.This work was supported by World Premier Inter-national Research Center Initiative (WPI Initiative),MEXT, Japan. AC acknowledges financial support fromthe Swiss National Science Foundation (SNF). KS grate-fully acknowledges support from Swiss National ScienceFoundation Grant PP00P2 138979/1REFERENCES Ahn C. P., Alexandroff R., Allende Prieto C., et al., 2013, ArXive-printsAird J., Nandra K., Laird E. S., et al., 2010, MNRAS, 401, 2531Allevato V., Finoguenov A., Hasinger G., et al., 2012, ApJ, 758,47Arnold T. J., Martini P., Mulchaey J. S., Berti A., Jeltema T. E.,2009, ApJ, 707, 1691Barnes J. E., 1990, Nature, 344, 379Bielby R. M., Finoguenov A., Tanaka M., et al., 2010, A&A, 523,A66Boroson B., Kim D.-W., Fabbiano G., 2011, ApJ, 729, 12Carollo C. M., Cibinel A., Lilly S. J., et al., 2013, ApJ, 776, 71Cibinel A., Carollo C. M., Lilly S. J., et al., 2012, ArXiv e-printsCibinel A., Carollo C. M., Lilly S. J., et al., 2013, ApJ, 776, 72Civano F., Elvis M., Lanzuisi G., et al., 2010, ApJ, 717, 209 -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 13
Colless M., Dalton G., Maddox S., et al., 2001, MNRAS, 328,1039Colless M., Peterson B. A., Jackson C., et al., 2003, preprint(astro-ph/0306581)Dickey J. M., Lockman F. J., 1990, ARA&A, 28, 215Ebrero J., Carrera F. J., Page M. J., et al., 2009, A&A, 493, 55Eke V. R., Baugh C. M., Cole S., et al., 2004a, MNRAS, 348, 866Eke V. R., Frenk C. S., Baugh C. M., et al., 2004b, MNRAS, 355,769Ellison S. L., Patton D. R., Mendel J. T., Scudder J. M., 2011,MNRAS, 418, 2043Fassbender R., ˇSuhada R., Nastasi A., 2012, Advances inAstronomy, 2012Ferrarese L., Merritt D., 2000, ApJ, 539, L9Finoguenov A., Guzzo L., Hasinger G., et al., 2007, ApJS, 172,182Gebhardt K., Bender R., Bower G., et al., 2000, ApJ, 539, L13Georgakakis A., Gerke B. F., Nandra K., et al., 2008, MNRAS,391, 183Giavalisco M., Ferguson H. C., Koekemoer A. M., et al., 2004,ApJ, 600, L93Gilmour R., Best P., Almaini O., 2009, MNRAS, 392, 1509Grogin N. A., Kocevski D. D., Faber S. M., et al., 2011, ApJS,197, 35Haggard D., Green P. J., Anderson S. F., et al., 2010, ApJ, 723,1447H¨aring N., Rix H.-W., 2004, ApJ, 604, L89Hasinger G., Miyaji T., Schmidt M., 2005, A&A, 441, 417Ho L. C., Rudnick G., Rix H.-W., et al., 2000, ApJ, 541, 120Hopkins P. F., Hernquist L., Cox T. J., Kereˇs D., 2008, ApJS,175, 356Jahnke K., Bongiorno A., Brusa M., et al., 2009, ApJ, 706, L215Kampczyk P., Lilly S. J., de Ravel L., et al., 2013, ApJ, 762, 43Kauffmann G., Heckman T. M., Tremonti C., et al., 2003,MNRAS, 346, 1055Kelly B. C., Shen Y., 2013, ApJ, 764, 45Kennicutt Jr. R. C., 1998, ApJ, 498, 541Kenter A., Murray S. S., Forman W. R., et al., 2005, ApJS, 161, 9Kewley L. J., Groves B., Kauffmann G., Heckman T., 2006,MNRAS, 372, 961Knobel C., Lilly S. J., Kovaˇc K., et al., 2013, ApJ, 769, 24Lehmer B. D., Alexander D. M., Bauer F. E., et al., 2010, ApJ,724, 559Lehmer B. D., Brandt W. N., Alexander D. M., et al., 2007, ApJ,657, 681Lehmer B. D., Brandt W. N., Alexander D. M., et al., 2008, ApJ,681, 1163Lin L., Cooper M. C., Jian H.-Y., et al., 2010, ApJ, 718, 1158Lusso E., Comastri A., Simmons B. D., et al., 2012, MNRAS,425, 623Martel A. R., Menanteau F., Tozzi P., Ford H. C., Infante L.,2007, ApJS, 168, 19Martini P., Kelson D. D., Kim E., Mulchaey J. S., Athey A. A.,2006, ApJ, 644, 116 Martini P., Mulchaey J. S., Kelson D. D., 2007, ApJ, 664, 761Merloni A., Rudnick G., Di Matteo T., 2004, MNRAS, 354, L37O’Sullivan E., Ponman T. J., Collins R. S., 2003, MNRAS, 340,1375Peng Y.-j., Lilly S. J., Renzini A., Carollo M., 2012, ApJ, 757, 4Pentericci L., Castellano M., Menci N., et al., 2013, A&A, 552,A111Pierce C. M., Lotz J. M., Laird E. S., et al., 2007, ApJ, 660, L19Ranalli P., Comastri A., Setti G., 2003, A&A, 399, 39Reines A. E., Greene J. E., Geha M., 2013, ApJ, 775, 116Richardson J., Chatterjee S., Zheng Z., Myers A. D., Hickox R.,2013, ApJ, 774, 143Ruderman J. T., Ebeling H., 2005, ApJ, 623, L81Sabater J., Best P. N., Argudo-Fern´andez M., 2013, MNRAS,430, 638Sanders D. B., Mirabel I. F., 1996, ARA&A, 34, 749Sarzi M., Shields J. C., Schawinski K., et al., 2010, MNRAS, 402,2187Sazonov S. Y., Revnivtsev M. G., 2004, A&A, 423, 469Schramm M., Silverman J. D., Greene J. E., et al., 2013, ApJ,773, 150Scoville N., Aussel H., Brusa M., et al., 2007, ApJS, 172, 1S´ersic J. L., 1968, Atlas de galaxias australesShen Y., Mulchaey J. S., Raychaudhury S., Rasmussen J.,Ponman T. J., 2007, ApJ, 654, L115Shen Y., Richards G. T., Strauss M. A., et al., 2011, ApJS, 194,45Shin M.-S., Ostriker J. P., Ciotti L., 2012, ApJ, 745, 13Silverman J. D., Green P. J., Barkhouse W. A., et al., 2008, ApJ,679, 118Silverman J. D., Kampczyk P., Jahnke K., et al., 2011, ApJ, 743,2Silverman J. D., Kovaˇc K., Knobel C., et al., 2009a, ApJ, 695, 171Silverman J. D., Lamareille F., Maier C., et al., 2009b, ApJ, 696,396Smolˇci´c V., Finoguenov A., Zamorani G., et al., 2011, MNRAS,416, L31Strateva I. V., Strauss M. A., Hao L., et al., 2003, AJ, 126, 1720Tanaka M., Finoguenov A., Lilly S. J., et al., 2012, PASJ, 64, 22Trump J. R., Impey C. D., Kelly B. C., et al., 2009, ApJ, 700, 49Ueda Y., Akiyama M., Ohta K., Miyaji T., 2003, ApJ, 598, 886Ueda Y., Hiroi K., Isobe N., et al., 2011, PASJ, 63, 937van den Bosch F. C., Aquino D., Yang X., et al., 2008, MNRAS,387, 79Weinmann S. M., van den Bosch F. C., Yang X., Mo H. J.,Croton D. J., Moore B., 2006, MNRAS, 372, 1161Wilman D. J., Oemler Jr. A., Mulchaey J. S., McGee S. L.,Balogh M. L., Bower R. G., 2009, ApJ, 692, 298Yan R., Blanton M. R., 2012, ApJ, 747, 61Yang X., Mo H. J., van den Bosch F. C., Pasquali A., Li C.,Barden M., 2007, ApJ, 671, 153 Silverman et al.
Table 1
ZENS Galaxy groups with X-ray observationsName RA a DEC a Redshift a N H b N m c ˆ R M group c (J2000) (J2000) (Mpc) ( × M ⊙ ) Chandra − s1571 02:37:04.33 -25:23:34.3 0.0568 1.96 10 0.501 1.522PIGG − n1610 09:53:38.23 -05:08:21.4 0.0562 3.85 10 0.495 1.452PIGG − n1702 09:54:30.67 -04:06:03.3 0.0574 3.62 9 0.573 2.262PIGG − n1347 09:59:44.62 -05:16:52.6 0.0521 3.67 10 0.534 2.902PIGG − n1480 10:15:31.91 -05:37:06.9 0.0537 4.50 13 0.574 2.262PIGG − n1320 10:17:55.04 -01:22:53.4 0.0508 4.19 10 0.631 3.002PIGG − n1441 11:18:10.68 -04:27:36.1 0.0531 4.34 15 0.658 3.412PIGG − n1381 14:28:12.53 -02:31:12.4 0.0522 3.98 10 0.468 1.222PIGG − n1598 14:35:54.08 -01:16:42.7 0.0560 3.68 9 0.606 2.672PIGG − n1746 14:40:20.07 -03:45:56.2 0.0585 5.64 9 0.516 1.652PIGG − s1752 22:21:10.68 -26:00:24.6 0.0577 1.65 11 0.775 5.602PIGG − s1671 22:24:00.14 -30:00:17.9 0.0567 1.11 10 0.618 2.83XMM2PIGG − s1520 00:02:01.79 -34:52:55.5 0.0543 1.09 9 0.505 1.552PIGG − s1571 see above2PIGG − s1783 22:17:26.33 -36:59:48.1 0.0583 1.18 8 0.741 4.902PIGG − n1606 10:38:49.84 01:48:24.7 0.0561 3.76 7 0.505 1.552PIGG − s1614 22:25:15.88 -25:23:15.4 0.0568 1.72 18 0.746 4.982PIGG − s1471 23:45:01.81 -26:37:26.8 0.0528 1.59 15 0.689 3.912PIGG − n1572 14:25:33.40 -01:30:00.4 0.0550 3.57 19 0.733 4.72 a As reported in Eke et al. (2004a). b Galactic neutral hydrogen column (Dickey & Lockman 1990); units of 10 cm − . a Number of galaxy members as reported in Eke et al. (2004a). c Characteristic group radius and total halo mass as given in Carollo et al. (2013).
Table 2
Chandra observation logTarget RA a DEC a Observation Exposure OBSoffset ( ′ ) offset( ′ ) date (GMT) time (ks) ID2PIGG-s1571 +1.86 -2.02 19 Oct 2010 10.06 116132PIGG-n1610 -0.45 -1.73 22 Jan 2010 2.62 11617-0.44 -1.74 22 Jan 2010 2.47 11618-0.43 -1.27 22 Jan 2010 2.47 11619-0.43 -1.27 22 Jan 2010 2.47 116202PIGG-n1702 +1.92 -1.51 22 Jan 2010 2.47 11621+1.91 -1.51 22 Jan 2010 2.47 11622+1.91 -1.51 22 Jan 2010 2.47 11623+1.92 -1.51 22 Jan 2010 2.47 116242PIGG-n1347 +2.19 -4.89 02 Feb 2010 5.17 11625+3.49 -4.94 08 Feb 2010 5.12 116272PIGG-n1480 +1.74 +3.46 03 Feb 2010 4.79 11629+1.93 +4.87 03 Feb 2010 5.18 116312PIGG-n1320 -0.84 -1.31 28 Jan 2010 2.64 11633-0.84 -1.31 28 Jan 2010 2.47 11634-0.84 -1.31 28 Jan 2010 2.47 11635-0.84 -1.31 28 Jan 2010 2.47 116362PIGG-n1441 -2.73 +2.28 03 Feb 2010 4.80 11637-2.89 +1.53 20 Apr 2010 5.17 116392PIGG-n1381 -3.44 +1.42 07 May 2010 5.06 11641-4.64 +0.24 24 Dec 2009 2.63 11643-4.63 +0.29 18 Dec 2009 2.47 116442PIGG-n1598 -1.54 -5.32 07 May 2010 9.79 116452PIGG-n1746 -2.35 +4.78 08 May 2010 7.63 11649-2.39 +7.34 18 Dec 2009 2.55 116522PIGG-s1752 -0.80 +0.37 09 Sept 2009 5.17 11653-0.65 -1.80 10 Sept 2009 5.18 116552PIGG-s1671 -3.23 +0.99 22 July 2010 2.56 11657-2.98 +0.32 14 Sept 2009 2.67 11658-3.11 -0.56 19 Sept 2009 2.68 11659-3.02 -0.54 16 Sept 2009 2.68 11660 a Position of the
Chandra aim-point given as an offset from thegroup centers provided in Table 1. -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 15
Table 3
Chandra photometry of ZENS galaxiesName RA (X-ray) DEC (X-ray) Off-axis Counts f X a L X b Class(J2000) (J2000) angle ( ′ ) B S Hs1571 10 02:36:55.95 -25:28:39.4 4.99 6 . +3 . − . . +2 . − . . +2 . − . -14.2 40.7 AGNs1671 4 22:23:54.06 -30:00:46.3 2.41 10 . +3 . − . . +3 . − . . +2 . − . -13.9 40.9 Galaxys1752 4 22:21:10.59 -26:00:24.6 1.40 11 . +3 . − . . +3 . − . . +2 . − . -13.9 41.0 AGNn1347 3 c . +4 . − . . +3 . − . . +3 . − . -13.8 41.0 Galaxyn1381 3 14:28:06.04 -02:31:27.32 2.86 4 . +3 . − . . +2 . − . . +2 . − . -14.4 40.6 AGNn1598 5 14:36:16.89 -01:23:13.65 8.15 9 . +3 . − . . +3 . − . . +2 . − . -13.9 40.9 AGNn1702 3 d . +2 . − . . +1 . − . . +2 . − . -14.2 40.5 AGNn1381 add 14:28:08.90 -02:31:24.68 3.18 80 . +9 . − . . +4 . − . . +8 . − . -13.0 41.8 AGN a The log of the flux in units erg cm − s − in the 0.5-8.0 keV band. b The log of the luminosity in units erg s − in the 0.5-8.0 keV band. c X-ray emission is slightly extended and X-ray source is not found by wavdetect; given positionindicates peak X-ray emission d No position is given since source is not detected by wavdetect with a significance above our threshold.The optical position was used for count extraction.
Table 4
XMM-
Newton photometry of ZENS galaxiesName RA (X-ray) DEC (X-ray) Count rate a Count rate a Flux b L X c Class(J2000) (J2000) S H B Bs1520 4 00:02:01.6 -34:52:56 6.8 ± < . ± < . ± ± ± < . ± ± ± ± ± < . ± < . ± < . ± ± ± ± . ± < . ± < . ± < . a units 10 − s − in either the observed soft (S: 0.5-2.0 keV) or hard (H: 2.0-7.5 keV)band b The log of the flux in units erg cm − s − in the 0.5-8.0 keV band (see text for details) c The log luminosity in units erg s − in the 0.5-8.0 keV band (see text for details) Silverman et al.
Table 5
Optical properties of X-ray sources associated with ZENS galaxies I.Name RA (opt) DEC (opt) Redshift B a I a K s b M galaxy c SF R
Morph.(J2000) (J2000) ( × M ⊙ ) (M ⊙ yr − ) type d s1571 10 02:36:55.96 -25:28:39.9 0.0575 19.2 18.2 18.2 0.077 1.7 5s1671 4 22:23:54.12 -30:00:47.7 0.0568 15.5 14.1 13.4 14.8 0.03 0s1752 4 22:21:10.69 -26:00:24.8 0.0569 15.3 13.8 13.1 31.3 0.09 1n1347 3 09:59:44.18 -05:22:01.2 0.0526 14.8 13.3 12.5 28.2 0.08 0n1381 3 14:28:06.07 -02:31:24.5 0.0522 17.3 16.6 16.1 0.75 0.86 4n1598 5 14:36:16.84 -01:23:13.3 0.0556 15.6 14.5 13.8 5.92 15.2 3n1702 3 09:54:57.08 -04:05:14.6 0.0574 19.0 18.1 — 0.15 0.22 4s1520 4 00:30:27.00 -34:52:55.7 0.0548 16.2 14.8 14.1 5.56 0.04 1s1571 8 02:37:04.34 -25:23:34.5 0.0570 15.8 14.4 13.6 16.7 0.13 0s1783 2 22:17:26.34 -36:59:48.3 0.0583 16.5 15.0 14.0 12.5 0.12 0s1783 3 22:17:20.86 -36:58:23.0 0.0588 17.1 15.6 14.8 3.73 0.31 3s1783 4 22:17:19.53 -36:56:45.3 0.0586 16.6 15.2 14.7 5.93 5.93e-4 3s1783 7 22:17:06.24 -36:56:51.3 0.0587 15.0 13.7 13.1 21.7 8.56 3n1606 3 10:38:45.39 +01:46:43.4 0.0566 16.3 15.2 14.6 3.05 6.45 3n1606 5 10:38:41.52 +01:43:28.1 0.0555 16.5 15.8 15.0 0.75 2.94 3s1614 10 22:25:19.02 -25:24:26.70 0.0586 16.1 15.0 14.3 2.39 17.56 2s1471 4 23:45:43.81 -26:43:10.0 0.0515 15.8 15.0 14.6 2.27 4.12 4s1471 5 23:45:34.24 -26:43:36.6 0.0514 17.1 15.8 15.4 2.40 4.7e-3 2s1471 11 23:45:05.72 -26:40:48.1 0.0522 15.8 14.3 — 20.2 2.02e-3 3s1471 12 23:45:01.82 -26:37:27.0 0.0520 15.9 14.4 13.7 23.40 1.67 2n1572 13 14:25:55.11 -01:28:18.1 0.0550 16.6 15.2 14.8 9.53 < − g sdss i sdss K s a n1381 add e a Petrosian apparent magnitude; rest-frame b c Stellar mass d ZENS morphological type: 0=elliptical, 1=S0, 2=Bulge-dominated spiral; 3=Intermediate spiral;4=Disk-dominated spiral; 5=irregular e SDSS J142808.89-023124.8 -ZENS: X-ray point sources in ZENS groups at z ∼ .
05 17
Table 6
Optical properties of X-ray sources associated with ZENS galaxies II.Name Central a R/ ˆ R n sersic c n sersic c r total / r bulge / B/T c,f,g h: disk scaleflag (total) (bulge) (kpc) (kpc) length (kpc) c,g s1571 10 0 0.749 · · · · · · · · · · · · · · · s1671 4 1 0 3.92 · · · · · · · · · · · · s1752 4 1 0 3.34 2 . +0 . − . . +0 . − . . +0 . − . . +0 . − . n1347 3 1 0 5.64 · · · · · · · · · · · · n1381 3 0 0.241 1.10 2 . +0 . − . . +0 . − . . +0 . − . . +0 . − . n1598 5 1 0 · · · . +0 . − . . +0 . − . . +0 . − . . +0 . − . n1702 3 0 0.801 1.49 5 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1520 4 1 0 6.33 3 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1571 8 1 0 6.82 · · · · · · · · · · · · s1783 2 0 0.465 3.01 · · · · · · · · · · · · s1783 3 0 0.308 9.28 2 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1783 4 0 0.248 2.64 4 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1783 7 1 0 3.86 4 . +0 . − . . +0 . − . . +0 . − . . +0 . − . n1606 3 0 0.278 2.09 9 . +0 . − . . +0 . − . . +0 . − . . +0 . − . n1606 5 0 0.736 2.15 1 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1614 10 0 0.253 3.02 · · · · · · · · · · · · s1471 4 0 1.104 1.96 4 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1471 5 0 0.955 3.15 3 . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1471 11 0 0.348 · · · . +0 . − . . +0 . − . . +0 . − . . +0 . − . s1471 12 1 0 3.46 2 . +0 . − . . +0 . − . . +0 . − . . +0 . − . n1572 13 1 0 4.47 · · · · · · · · · · · · a Central galaxy =1 and satellite=0 b Distance of the galaxy to the group center in units of ˆ R as defined in Carollo et al. (2013). A value ofzero denotes a central galaxy c based on the I-band imaging d Half-light radius of the total galaxy light profile. e Half-light radius of the bulge component of galaxy light profile. f Ratio of the bulge-to-total luminosity. g Formal errors based on GIM2D; larger systematic errors are discussed in Cibinel et al. (2013)
Table 7
AGN statistics in ZENS galaxiesType Mass range . +0 . − . . +0 . − . . +0 . − . Satellite 10.4-11.6 30 4 0 . +0 . − . . +0 . − . . +0 . − ..