Multi-wavelength observations of a rich galaxy cluster at z ~ 1: the HST/ACS colour-magnitude diagram

Abstract

XMMU J1229+0151 is a rich galaxy cluster with redshift z=0.975, that was serendipitously detected in X-rays within the scope of the XMM-Newton Distant Cluster Project. HST/ACS observations in the i775 and z850 passbands, as well as VLT/FORS2 spectroscopy were further obtained, in addition to follow-up Near-Infrared (NIR) imaging in J- and Ks-bands with NTT/SOFI. We investigate the photometric, structural and spectral properties of the early-type galaxies in the high-redshift cluster XMMU J1229+0151. Source detection and aperture photometry are performed in the optical and NIR imaging. Galaxy morphology is inspected visually and by means of Sersic profile fitting to the 21 spectroscopically confirmed cluster members in the ACS field of view. The i775-z850 colour-magnitude relation (CMR) is derived with a method based on galaxy magnitudes obtained by fitting the surface brightness of the galaxies with Sersic models. The i775-z850 CMR of the spectroscopic members shows a very tight red-sequence with a zero point of 0.86+-0.04 mag and intrinsic scatter equal to 0.039 mag. The CMR obtained with the galaxy models has similar parameters. Stellar masses and formation ages of the cluster galaxies are derived by fitting the observed spectral energy distributions (SED) with models based on Bruzual & Charlot 2003. We obtain a star formation weighted age of 4.3 Gyr for a median mass of 7.4e10 Msun. Instead of an unambiguous brightest cluster galaxy (BCG), we find three bright galaxies with a similar z850 magnitude, which are, in addition, the most massive cluster members, with ~ 2e11 Msun. Our results strengthen the current evidence for a lack of significant evolution of the scatter and slope of the red-sequence out to z~1.

Full text

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arXiv:0903.3853v1 [astro-ph.CO] 23 Mar 2009
Astronomy & Astrophysics manuscript no. 1229˙ref
March 23, 2009
c ? ESO 2009
Multi-wavelength observations⋆of a rich galaxy cluster at z ∼ 1:
the HST/ACS colour-magnitude diagram
J. S. Santos,1,2P. Rosati,3R. Gobat,3C. Lidman,4K. Dawson,5S. Perlmutter,5H. B¨ ohringer,2I. Balestra,2
C.R. Mullis,6R. Fassbender,2J. Kohnert,7G. Lamer,7A. Rettura,8C. Rit´ e,3A. Schwope7
1INAF-Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
2Max-Planck-Institut f¨ ur extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: jsantos@oats.inaf.it
3European Southern Observatory, Karl Schwarzschild Strasse 2, Garching bei Muenchen, D-85748, Germany
4European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
5E.O. Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720
6Wachovia Corporation, NC6740, 100 N. Main Street, Winston-Salem, NC 27101
7Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
8Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD21218, USA
Received ... ; accepted ...
ABSTRACT
Context. XMMU J1229+0151 is a rich galaxy cluster with redshift z=0.975, that was serendipitously detected in X-rays within the
scope of the XMM-Newton Distant Cluster Project. HST/ACS observations in the i775and z850passbands, as well as VLT/FORS2
spectroscopy were further obtained, in addition to follow-up Near-Infrared (NIR) imaging in J- and Ks-bands with NTT/SOFI.
Aims. We investigate the photometric, structural and spectral properties of the early-type galaxies in the high-redshift cluster XMMU
J1229+0151.
Methods. Source detection and aperture photometry are performed in the optical and NIR imaging. Galaxy morphology is inspected
visually and by means of Sersic profile fitting to the 21 spectroscopically confirmed cluster members in the ACS field of view.
The i775-z850colour-magnitude relation (CMR) is derived with a method based on galaxy magnitudes obtained by fitting the surface
brightness of the galaxies with Sersic models. Stellar masses and formation ages of the cluster galaxies are derived by fitting the
observed spectral energy distributions (SED) with models based on Bruzual & Charlot 2003. Star formation histories of the early-type
galaxies are constrained through the analysis of the stacked spectrophotometric data.
Results. The structural Sersic index n obtained with the model fitting is in agreement with the visual morphological classification of
the confirmed members, indicating a clear predominance of elliptical galaxies (15/21). The i775-z850colour-magnitude relation of the
spectroscopic members shows a very tight red-sequence with a zero point of 0.86±0.04 mag and intrinsic scatter equal to 0.039 mag.
The CMR obtained with the galaxy models has similar parameters. By fitting both the spectra and SED of the early-type population
we obtain a star formation weighted age of 4.3 Gyr for a median mass of 7.4 ×1010M⊙. Instead of an unambiguous brightest cluster
galaxy (BCG), we find three bright galaxies with a similar z850magnitude, which are, in addition, the most massive cluster members,
with ∼2 ×1011M⊙. Our results strengthen the current evidence for a lack of significant evolution of the scatter and slope of the
red-sequence out to z ∼ 1.
Key words. galaxies: clusters: individual: XMMU J1229+0151 - galaxies: high-redshift
1. Introduction
Distant (z∼1) galaxy clusters are unique astrophysical labora-
tories particularly suited to witness and study galaxy formation
and evolution.
Detailed studies of the properties of galaxies in large sam-
ples of high-redshift clusters are required to distinguish the
two main galaxy formation scenarios, which have been un-
der discussion for more than 30 years. In the monolithic
picture (Eggen et al. (1962); Larson (1974)), massive galax-
ies are expected to be formed early from a single progen-
⋆Based on observations carried out using the Advanced Camera
for Surveys at the Hubble Space Telescope under Program ID 10496;
the Very Large Telescope at the ESO Paranal Observatory under
Program IDs 176.A-0589(A), 276.A-5034(A) and the New Technology
Telescope at the ESO La Silla Observatory under Program ID 078.A-
0265(B)
itor. In contrast, the hierarchical scenario (Toomre (1977);
White & Rees (1978)) predicts that elliptical galaxies should
form later, through mergers. The behavior of early-type galax-
ies (ETGs), which are found to comprise both the most mas-
sive and oldest systems, is the main cause for this debate.
Indeed, it is now established that the star formation histo-
ries of ellipticals are mass-dependent from both observational
(Thomas et al. (2005), van der Wel et al. (2005)) and theoretical
studies (e.g. De Lucia et al. (2004), Menci et al. (2008)), such
that low mass galaxies have more extended star formation his-
tories than massive ones. This implies that the less massive
galaxies have a lower formation redshift than the more massive
systems, whose star formation histories are predicted to peak
at z∼5 (De Lucia et al. (2006)). This scenario is commonly re-
ferred to as “downsizing” (Cowie et al. (1996)). Supporting this
picture, there is strong observational evidence for the bulk of the

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2 J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 1
stars in massive ellipticals to be already formed at redshift > 2
(van Dokkum (2005), Holden et al. (2005)).
Thecolour-magnitude relation
(CMR,
Sandage & Visvanathan (1978))
law used to assess the evolution of galaxy populations. The
CMR of local clusters shows the existence of a tight Red
Sequence (RS, Bower et al. (1992), de Propris et al. (1998))
(Gladders & Yee (2000), Baldry et al. (2004)) formed of mas-
sive red elliptical galaxies undergoing passive evolution, and
the analysis of its main parameters (zero point, scatter and
slope) provides a means to quantify the evolution of the galaxies
properties with redshift. It remains, nevertheless, unclear to
what degree the CMR is determined by age and metallicity
effects.
Visvanathan & Sandage (1977),
isa fundamentalscaling
The study of high-z samples of galaxies is also impor-
tant to provide information for the modelling of physical pro-
cesses in semi-analytical techniques. Semi-analytical modelling
(SAM) employing AGN feedback to prevent the overproduc-
tion of blue galaxies have recently succeeded in predicting a
large amount of massive old galaxies (De Lucia et al. (2006),
Bower et al. (2006), Menci et al. (2006), Croton et al. (2006),
Somerville et al. (2008)), however, several issues remain yet to
be solved, such as the incapability to reproduce quantitatively
the colour-bimodality in the colour-magnitude diagram and the
scatter of the red-sequence, which is overestimated by a factor
2-3 (e.g. Menci et al. (2008)).
The exceptional high-resolution provided by the Advanced
CameraforSurveys(ACS)attheHubbleSpaceTelescope(HST)
has greatly contributed to the current knowledge on the evo-
lution of galaxies in dense environments. Results on the eight
z∼1 clusters of the ACS Intermediate Redshift Cluster Survey
(Blakeslee et al. (2003); Mei et al. (2007), Holden et al. (2005),
Mei et al. (2009) and references therein), and studies of in-
dividual distant clusters (RDCS 1252.9-2927 at z=1.235:
Lidman et al. (2004), Demarco et al. (2007); XMMU J2235.3-
2557 at z=1.393: Rosati et al. (2009), Lidman et al. (2008);
XMMXCS J2215.9-1738 at z=1.45, Stanford et al. (2006))
point toward the prevalence of a tight RS up to z=1.4, where the
CMR slope andscatter areobservedtohavea negligibleincrease
with redshift.
In this paper we provide a detailed analysis of the galaxy
properties in XMMU J1229+0151 (hereafter, XMM1229), an
X-ray selected, optically rich and distant cluster (z=0.975 cor-
responding to a lookback time of 7.6 Gyr). We derive ac-
curate colour measurements from the high-resolution ACS
data, and characterize the galaxy morphology via visual in-
spection and by fitting Sersic profiles. Stellar masses, ages
and star formation histories of the cluster’s early-types are
derived by fitting the coadded spectrophotometric data with
Bruzual & Charlot (2003) templates.
The paper is organized as follows: in Sect. 2 we present the
imagingandspectroscopicdata,as well as reductionprocedures.
The ACS morphological analysis is introduced in Sect. 3. In
Sect. 4 we derive the i775− z850CMR, and the results from the
SED fitting are presented in Section 5. In Sect. 6 we investigate
the properties of the brightest cluster galaxies. We conclude in
Sect. 7.
The cosmological parameters used throughout the paper are
H0=70 km/s/Mpc, ΩΛ=0.7 and Ωm=0.3. Filter magnitudes are
presented in the AB system unless stated otherwise.
2. Observations and data reduction
2.1. XMM-Newton data
The cluster XMM1229 was initially detected in a serendipi-
tous cluster survey of the XMM-Newton archive, the XMM-
Newton Distant Cluster Project (XDCP, B¨ ohringer et al. (2005),
Fassbender (2008)). Our target was observed in 25 XMM-
Newton pointings of the bright radio loud quasar 3C 273 at
an off-axis angle of approximately 13 arcmin. We selected
only observations whose exposure time, after cleaning for high
background periods, was larger than 10 ks. Unfortunately,
XMM1229 was not observed by the EPIC-pn camera, since the
pn was always operated in Small Window Mode (except for
Obs Id=0126700201, having a clean exposure time of only ∼
6ks).Therefore,weusedonlythedatafromthetwoXMM/MOS
CCDs. The 11 observations selected for our analysis are listed
in Table 1. The information given is the following: observation
date (column 1), XMM-Newton observation identification num-
ber (column 2) and revolution (column 3), filter (M=medium,
T=thin) and mode (F=full window, S=small window) used (col-
umn 4), good exposure time of XMM/MOS1+MOS2, after
cleaning for high particle backgroundperiods (column 5).
Data were processed using the XMM-Newton Science
Analysis Software (SAS v7.0.0). Light curves for pattern=0
events in the 10 − 15 keV band were produced to search for
periods of background flaring, which were selected and re-
moved by applying a 3σ clipping algorithm. Light curves in the
0.3 − 10 keV band were visually inspected to remove residual
soft-proton induced flares. We selected events with patterns 0 to
12 (single, double and quadruple) and further removed events
with low spectral quality (i.e. FLAG=0). We obtained total ex-
posure times of ∼ 370 and ∼ 400 ks for the XMM/MOS1 and
XMM/MOS2, respectively.
The spectra of the cluster were extracted from a circu-
lar region of radius 30 arcsec centered at RA=12:29:29.2,
Dec=+01:51:26.4. The background was estimated from a cir-
cular region on the same chip of radius ∼ 2 arcmin centered at
RA=12:29:21.2, Dec=+01:51:55.4, after removing cluster and
point sources.
We corrected vignetting effects using the SAS task
EVIGWEIGHT (Arnaud et al. (2001)) to calculate the emission
weighted effective area, by assigning a weight to each photon
equal to the ratio of the effective area at the position of the pho-
ton with respect to the on-axis effective area. Redistribution ma-
trices were generated using the SAS task RMFGEN for each point-
ing, filter, and detector.
Time averaged spectra for the source and the background
were obtained by adding the counts for each channel. Since dif-
ferent filters were used for the observations, we weighted each
instrumenteffectivearea(ARF) andredistributionmatrix(RMF)
with the exposure time of the observation. In order to use the χ2
minimization in the spectral fitting we binned the spectra with a
minimum number of 20 counts per bin.
2.1.1. Spectral analysis
The
XSPEC
a
Liedahl et al. (1995)). We modeled Galactic absorption with
tbabs (Wilms et al. (2000)). We always refer to values of solar
abundances as in Anders & Grevesse (1989).
two XMM/MOS
v11.3.1
single-temperature
spectra were
and
model
analyzed
were
(Kaastra (1992);
with
with(Arnaud (1996))
mekal
fitted

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J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 13
Fig.1. HST/ACS colour image of XMMU J1229+0151 with X-ray contours. The image is centered on the cluster X-ray emission
and has a size of 1.5 arcmin2.
Table 1. Log of the archival XMM-Newton observations of
XMM1229.
Date
(1)
2000-06-13
2000-06-14
2000-06-15
2000-06-16
2000-06-18
2001-06-13
2003-07-07
2004-06-30
2005-07-10
2007-01-12
2007-06-25
Obs. Id.
(2)
0126700201
0126700301
0126700601
0126700701
0126700801
0136550101
0159960101
0136550801
0136551001
0414190101
0414190301
Rev.
(3)
0094
0094
0095
0095
0096
0277
0655
0835
1023
1299
1381
Filt./Mode
(4)
M/F
M/F
M/S
M/S
M/S
T/S
T/S
T1-M2/S
M/S
M/S
M/S
Texp[ks]
(5)
11.7+11.6
56.4+56.1
24.0+23.7
17.5+17.8
40.8+41.1
40.1+40.1
51.3+54.6
14.3+47.7
26.9+26.7
57.3+55.5
26.8+26.2
The fits were performed over the 0.5 − 6 keV band. We
excluded energies below 0.5 keV, due to calibration uncertain-
ties, and above 6 keV, where the background starts to domi-
nate. Furthermore, due to the relatively low S/N of the observa-
tions, we noticethat instrumentalKα emission lines1fromAl (at
∼ 1.5 keV) and Si (at ∼ 1.7 keV) may affect the spectral analysis
significantly. Therefore, we also excluded photons in the energy
range 1.4 − 1.8 keV from our spectral analysis. In the selected
energy bands we have a total of ∼ 1300 and ∼ 1200 net counts
for the XMM/MOS1 and XMM/MOS2, respectively.
The free parameters in our spectral fits are temperature,
metallicity, redshift and normalization, although we also per-
1http://xmm.vilspa.esa.es/external/xmm user support/documentation
/uhb/node35.html
125
10−5
10−4
10−3
Counts/sec/keV
Energy (keV)
Fig.2. X-ray spectra of XMM1229 from the XMM/MOS1
(black) and XMM/MOS2 (red) detectors. The solid lines show
the best-fit models. The Fe K-line is prominent at 3.2 keV.
formed the fit freezing the redshift to 0.975, the median spec-
troscopic redshift of the confirmed galaxies. Local absorption is
fixed to the Galactic neutral hydrogen column density, as ob-
tained from radio data (Dickey & Lockman (1990)).
The results of the spectral analysis are listed in Table 2,
where the quoted errors are at the 1-σ confidence level. The
information provided is the following: detector used, (column
1), temperature (column 2), iron abundance (column 3), redshift
(column 4), χ2and number of degrees of freedom (column 5).
The last line of the table refers to the spectral fit with the red-
shift set to 0.975.

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4 J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 1
Table 2. Results of the X-ray spectral analysis.
Detector
(1)
MOS1
MOS2
MOS1+2
MOS1+2
kT [keV]
(2)
6.2+1.0
−0.8
6.2+1.1
−0.6
6.25+0.69
−0.55
6.4+0.7
−0.6
ZFe[Z⊙]
(3)
0.37+0.20
0.42+0.22
0.38 ± 0.14
0.34+0.14
−0.13
z
(4)
0.96+0.02
0.95+0.03
0.96 ± 0.02
0.9751
χ2/d.o.f.
(5)
45.1/51
50.1/44
95.4/98
96.8/99
−0.18
−0.03
−0.21
−0.02
1fixed redshift
The rest-frame luminosity corrected for Galactic absorp-
tion in the 0.5 − 2.0 keV range is (1.3 ± 0.2) × 1044erg s−1,
for an aperture of 30 arcsec radius, which corresponds to a
physical size of 240 kpc. To obtain the total cluster lumi-
nosity we resort to extrapolating the measured aperture lumi-
nosity to a radius of 2 Mpc, assuming an isothermal β-model
(Cavaliere & Fusco-Femiano (1976)), a well-known analytical
formula dependent on a slope β and a core radius rc, that de-
scribes to a good degree the surface brightness profile of regu-
lar clusters. Unfortunately, the signal-to-noise ratio is not good
enough to fit a β-model, therefore we assume that XMM1229
is a cluster with a standard X-ray morphology i.e., without
signs of merging or a strong cool core, and we use the typ-
ical values β=0.7 and rc=250 kpc, obtaining LX (r<2 Mpc)
∼ 3 × 1044erg s−1.
2.2. HST/ACS i775and z850band imaging
In the framework of the Supernova Cosmology Project
(Dawson et al., (2009)) we obtained the HST/ACS Wide Field
Camera (WFC) data. Images in the F775W (i775) and F850LP
(z850) passbands centered on the cluster X-ray centroid were ac-
quired in December 2006, with total exposures of 4110 sec and
10940 sec respectively. The i775and z850are the most efficient
filters in supernova searches and, although they are not opti-
mal for a cluster at this redshift, the i775 encloses the D4000
break, which is redshifted to 7920 Å at the cluster redshift.
ACS has a field of view (FoV) of 3.3 x 3.3 arcmin and a pixel
scale of 0.05”/pix. The images were processed using the ACS
GTO Apsis pipeline (Blakeslee et al. (2003)), with a Lanczos3
interpolation kernel. The photometric zero points are equal to
34.65 and 34.93 in the i775 and z850 bands respectively, fol-
lowing the prescription of Sirianni et al. (2005). To account for
the galactic extinction we applied to our photometric catalog
the correction factor E(B − V)=0.017 retrieved from the NASA
ExtragalacticDatabase2and usingthe dust extinctionmaps from
Schlegel et al. (1998). The corresponding correction in the opti-
cal bands is E(i775-z850)=0.010 mag.
2.3. VLT/FORS2 spectroscopy
Spectroscopic observations were carried out with Focal
Reducer andLow Dispersion
Appenzeller et al. (1998)) on Antu (Unit 1 of the ESO
Very Large Telescope (VLT)) as part of a program to search
for very high redshift Type Ia supernova in the hosts of early
type galaxies of rich galaxy clusters (Dawson et al., (2009)).
In this respect, the field XMMU J1229+0151 was very rich in
candidates, with three candidates occurring during the three
Spectrograph (FORS2:
2http://nedwww.ipac.caltech.edu/
Table 3. FORS2 Observing Log
MaskType SlitsGrism & FilterTexp
(s)
8 x 750
8 x 700
4 x 700
4 x 700
9 x 900
AirmassDate
(UT)
2006 Jan 01
2006 Jan 30
2006 Jan 31
2006 Feb 01
2006 Jun 20-21
1
2
3
4
5
MOS
MOS
MOS
MOS
MXU
12
18
18
18
34
300I+OG590
300I+OG590
300I+OG590
300I+OG590
300I+OG590
1.3
1.2
1.1
1.2
1.3
months monitoring. One candidate was identified as a Type Ia
supernova at the cluster redshift (Dawson et al., (2009)).
FORS2 was used with the 300I grism and the OG590 or-
der sorting filter. This configuration has a dispersion of 2.3
Angstroms per pixel and provides a wavelength range starting
at 5900 Å and extending to approximately 10000 Å. Since the
observations had to be carried out at short notice (the SN had to
be observed while it was near maximum light), most of the ob-
servationswere donewith the multi-objectspectroscopic(MOS)
mode of FORS2. The MOS mode consists of 19 moveable slits
(with lengthsthat varybetween 20”and 22”)that can be inserted
into the focal plane. The slit width was set to 1”. On one occa-
sion, when the MOS mode was unavailable because of technical
reasons, the field was observed with the MXU mode of FORS2.
The MXU mode consists of precut mask that is inserted into
the FORS2 focal plane once the field has been acquired. Since
the length of the slit can be made much shorter in the MXU
mode than in the MOS mode, the number of targets that could
be observed in the MXU mode was a factor of two larger than
the number of targets that could be observed in the MOS mode.
However, the time to prepare, cut and insert a mask is usually a
couple of days, whereas the MOS observationscan be done with
a few hours notice.
The field of XMMU J1229+0151 was observed with four
different configurations, 4 MOS and one MXU. The details of
the observations are given in Table 3. The MOS configurations
were used when the supernovae (there were three supernova
visible in the field of XMMU J1229+0151 at the same time)
were near maximum light. The MXU mask was used several
months later when the supernovae were significantly fainter.
In all masks, slits were placed on the supernova, thus spectra
of the supernovae together with their hosts and spectra of the
hosts without the supernovaewere obtained.The otherslits were
placed on candidate cluster galaxies or field galaxies. For each
MOS setup, between 4 and 9 exposures of 700 to 900 seconds
were taken. Between each exposure, the telescope was moved
a few arc seconds along the slit direction. These offsets, which
shift the spectra along detector columns, allow one to remove
detector fringes, as described in Hilton et al. (2007), which also
details how the FORS2 data was processed.
A total of 100 slits over four masks were used to observe 74
individual targets. The targets were selected by colour and mag-
nitude, using the R- and z-band pre-imaging. Priority 1 targets
had (R-z)>1.8 and z<23. Priority 2 targets had 1.8<(R-z)<1.6
and z<23. Some cluster members were observed in more than
one mask. From these 74 targets, 64 redshifts were obtained,
and 27 of these are cluster members - the redshift distribution of
the targets in shown in Fig. 3. A total of 21 confirmed galaxies
are within the FoV of ACS.
Cluster membership was attributed by a reasonable selection
ofgalaxieswithin± 2000km/srelativetothepeakoftheredshift

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J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 15
Fig.3.Redshiftdistributionof thegalaxiesinthe clusterXMMU
J1229+0151. Vertical red-dashed lines refer to redshift cuts at
z=0.965, 0.990 used to select the cluster members. This region
is shown in more detail in the top-right inslet.
distribution, or ∼ 3-σ. We assign the mean value of the redshift
distribution of the 27 cluster members to the cluster redshift,
z=0.975 and assume a conservative redshift error ∆z=10−3. The
cluster velocity dispersion was determined with the 27 galaxy
redshifts, using the software ROSTAT of Beers et al. (1990). We
obtained σ=683±62 km/s, where the error refers to the formal
bootrap error obtained with 1000 samples. This value is in per-
fect agreement with the result obtained using the methodology
proposed by Danese et al. (1980).
Even though we have a limited number of cluster members
which could introduce a bias in our computation of σ due to
the presence of substructures we, nevertheless, investigate the
connection between the state of the hot intra-cluster medium
(ICM) and the cluster galaxy population by means of the well-
known Temperature-σ relation (e.g. Wu et al. (1999)). The ob-
served T-σ relation for high-z clusters (e.g. Rosati et al. (2002))
implies that we would expect a higher velocity dispersion of
about 900±300 km/s for the cluster temperature. We note how-
ever that there is a significant scatter in the T-σ relation, and our
value is within the 30% scatter.
In Table 4 we list the cluster members. The information pro-
vided is the following: galaxy ID (column 1); RA (column 2)
and DEC (column 3); redshift (column 4); spectral classifica-
tion (column 5) and morphological type (column 6). The spec-
tral classification is done according to the scheme proposed in
Dressler et al. (1999), based on the strength of the [OII] and Hδ
lines. The k class refers to passive (no [OII] emission) galaxies.
This class is subdivided in two types, depending on the strength
of the Hδlines: k+a have moderate (3 < EW Hδ< 8) Hδabsorp-
tion, and a+k show strong (EW Hδ> 8) Hδabsorption. The e
spectral class refers to galaxies with [OII] emission and is sub-
divided into three types: e(a) present strong Balmer absorption,
e(c) have weak or moderate Balmer absorption, and e(b) show
very strong [OII] lines.
2.4. NTT/SOFI J- and Ks-band imaging
NIR imaging in the J- and Ks-bands were acquired using SOFI
(Moorwood et al. (1998)) at the New Technology Telescope
(NTT) at the ESO/La Silla observatory. The observations were
taken in March 2007, as part of the NIR follow-up of the XDCP
survey programme. The instrument was operated in the Large
Table 4. Spectroscopic confirmed members.
ID
(1)
5417
3428
3430
3025
4055
3301
4155
5411
20008
3497
20010
4126
3507
20013
20014
3949
30004
3495
3524
5499
3205
4661a
5001a
4956a
4794a
4910a
4800a
RA (J2000)
(2)
187.3857875
187.3793000
187.3720375
187.3771333
187.3573750
187.3466250
187.3885500
187.3718958
187.3734583
187.3724875
187.3726625
187.3900292
187.3716250
187.3684167
187.3654583
187.3696083
187.3697875
187.3715708
187.3807000
187.3844542
187.3747292
187.3631292
187.3500083
187.3367208
187.3342500
187.3213042
187.3186708
DEC (J2000)
(3)
1.8712528
1.8563222
1.8560639
1.8363889
1.8601056
1.8502667
1.8644889
1.8717778
1.8726667
1.8579083
1.8579944
1.8628750
1.8571444
1.8559167
1.8485556
1.8602611
1.8601389
1.8582111
1.8676667
1.8683722
1.8461806
1.8977194
1.8870944
1.8890611
1.8927333
1.8925528
1.8948944
z
(4)
0.977
0.984
0.974
0.979
0.968
0.969
0.969
0.974
0.973
0.982
0.977
0.973
0.976
0.979
0.969
0.976
0.970
0.980
0.969
0.973
0.984
0.975
0.973
0.978
0.974
0.976
0.976
Class
(5)
a+k
a+k
k
e(c)
k
e(c)
k+a
k
e(a)
a+k
k
k
k
k
k
k
k
k
a+k
k
a+k
k+a
k
k
k
e(b)
k
Type
(6)
S0
S0
Ell
Ell
Sb
Ell
Ell
Ell
Irr
Ell
Ell
Ell
Ell
Ell
S0
Ell
S0/Ell
Ell
Ell
Ell
Ell
–
–
–
–
–
–
agalaxy outside the FoV of ACS
field mode, corresponding to a 5x5 arcmin FoV, with a pixel
scale of 0.288 arsec/pix. Since the NIR background is gener-
ally highly variable, a large dithering pattern has to be applied,
thus we set the automatic jitter box to a width of 30 arcsec. Total
exposure times amount to 1hr in Ks and 45 min in J. The J-band
data have a seeing of 0.98” whereas the Ks-band have an image
quality of 0.69”.
Photometric calibration standards (Persson et al. (1998))
were acquiredseveraltimes duringthe observationrun.The zero
points (ZP) were computed using the reduced standards (back-
groundsubtracted,countrate image)with the followingrelation:
ZP = mag + 2.5log(countrate)+ atmcorr∗ airmass
(1)
where mag refers to the standard star magnitude, and atmcorr
refers to the wavelength dependent atmospheric correction. The
stellar flux was measured within circular apertures with 6” ra-
dius; such a large radius ensures that we account for the bulk of
the flux. The backgroundwas estimated with a 3σ clippingalgo-
rithm. The scatter of the zero points is 0.015 mag and 0.04 mag
for the J and Ks filter, respectively. We converted VEGA mag-
nitudes to the AB photometric system with the ESO web-tool
available at http://archive.eso.org/apps/mag2flux.
The data was reduced with the package ESO/MVM
(Vandame (2004)) using the HST/ACS catalog to match the as-
trometry. We used SExtractor (Bertin & Arnouts (1996)) in dual
image mode to perform the source detection in the Ks-band, and
the photometry of both images.

Page 6

6 J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 1
Fig.4. i775-z850image galleryof the 21 spectroscopicallyconfirmedmembers in the ACS FoV, orderedin increasing i775-z850colour.
Individual stamps are centered on the cluster members and have a size of 5” x 5”. Top labels correspondto the spectroscopic galaxy
ID and bottom labels refer to the visual morphological classification.
3. Structural analysis
3.1. Surface brightness profile fitting
The radial surface brightness profiles of galaxies can be de-
scribed by the Sersic law (Sersic (1968)),
Σ(r) ∝ exp(r/re)1/n− 1(2)
where Σ(r) is the surface brightness at radius r, the Sersic in-
dex, n, characterizes the degree of concentration of the profile;
and the effective radius, Re, corresponds to the projected radius
enclosing half of the galaxy light.
Using the ACS i775 and z850 data we made a 2D
bulge/disk galaxy decomposition with the software GIM2D
(Simard et al. (2002)). The galaxy model is the sum of a bulge
component (Sersic profile) and an exponential disk, depending
on a total of eleven parameters. Of these parameters, three de-
scribe the shape of the Sersic profile, including the index n,
which we constrained to 0< n <4. The upper bound is in-
troduced because n=4 corresponds to the de Vaucouleurs pro-
file, a purely empirical fit to the profiles of elliptical galaxies
and bulges (de Vaucouleurs (1961)). Allowing larger values of
n usually does not improve the fit, however the covariance be-
tween n and Recan lead to an overestimation of Refor large n
(Blakeslee et al. (2006)). The median Sersic index n of the spec-
troscopically confirmed galaxies is 3.9 and the median effective
radius is 5.5 pixel (0.28”).
The distribution of Reis consistent in both bands within the
1-σ errors, with an average error of 0.77 and 0.53 pix in the i775
andz850bandrespectively.Thecomparisonbetweentheeffective
radii obtained in the two bands Re(i775) - Re(z850) is shown in
Fig. 5. This difference is useful to assess an imperfect matching
of the PSFs or the presence of colour gradients. However, we
find a very good agreement between the two radii therefore we
do not expect those effects. In this figure we also present the
results of fitting a ”red-sequence” sample of early-type galaxies
which is introduced in Sect. 4.2. The reduced χ2of the best-fit
models is ∼1 for the majority of the galaxies, emphasizing the
good quality of the fit.
3.2. Visual morphological classification
In addition to the profile fitting, we made a visual classification
of the spectroscopic members using morphological templates
from Postman et al. (2005). In Fig. 4 we show postage stamps of
the cluster members in the i775passband labelled with the mor-
phological type. We note two red galaxy pairs (ID=20010/3497,
30004/3949). In Fig. 6 we show the distribution of the fit pa-
rameters n and Reof both the spectroscopic and ”red-sequence”
samples (see Sect. 4.2 for details on the latter), complemented
with the visual classification.
The morphologyof the spectroscopic galaxies in XMM1229
is clearly dominated by elliptical galaxies (15/21) with only
one galaxy classified as spiral (ID=4055) and one irregular
(ID=20008),unlikeotherdistantclusters(see foreg.theEDisCS
high-redshift sample, z ≤0.8, De Lucia et al. (2004)). The re-
maining four cluster members are classified as S0s. We stress
that we targeted red galaxies for spectroscopy, hence this was a

Page 7

J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 17
Fig.5. Comparison of the Reobtained with GIM2D in the i775
and z850bands. Spectroscopic members are represented in solid
circles and ”red-sequence”galaxies (see Sect. 4.2)) are shown in
open circles. The dashed line indicates the one-to-one relation.
The bottomplot shows the differencein the Revalues forthe two
bands normalized by the average Re. The dashed line represents
the constant zero value.
Fig.6. Sersic index n as a function of the effective radius Re
obtained with GIM2D, using the ACS/i775band. The spectro-
scopic sample is evidenced by open squares. The morphology
of the cluster members is dominated by elliptical galaxies (red
circles) characterized by a high n. Four galaxies are classified as
S0 (green triangles), one member is an Sb galaxy (blue square),
with n < 1 and one galaxy has an irregular shape (magenta
5-pointed star). The 31 ”red-sequence” early-type galaxies (see
Sect. 4.2) are also displayed with the same symbols without the
open squares.
colour,notamorphologicalselection,thereforewe donotexpect
to have a bias on ellipticals with respect to S0s.
Fig.7. Differential PSF blurring effect in i and z-bands: at r=5
pix (0.25“) the PSF correction is 0.034 mag (vertical line).
4. The i775− z850colour-magnitude relation
4.1. Galaxy photometry
We use SExtractor in dual image mode to perform the source
detection in the z850 band, and the photometry in both bands.
The image quality of the i775band is sightly better than the z850
band, with a Point Spread Function (PSF) FWHM of 0.085“, as
opposedto 0.095”in the z850. The effectof the z850PSF broaden-
ing has been investigated in other works (e.g. Mei et al. (2006))
and is attributed to the long-wavelength halo of the ACS/WFC
(Sirianni et al. (2005)). This effect, although small, bears impli-
cations on the galaxy colour measurement and has to be ac-
counted for. Thus, for each passband we constructed empirical
PSFs bycomputingthe medianprofileofa handfulofstars in the
science images for which we measure growth curves normalized
to the central intensity. We obtain a differential (z850-i775) me-
dian radial profile that shows a steep behavior for radii smaller
than 3 pix - see Fig. 7.
Thei775−z850colourisdeterminedinsmallaperturestoavoid
intrinsicgalaxycolourgradients(see e.g.Scodeggio (2001)fora
discussion on the effect of internal colour gradients). We choose
a fixed aperture of 5 pix (0.25”) since at this radius the steep and
uncertain PSF broadening is no longer dominant (see Fig. 7),
and we apply a correction of 0.034 mag to the i775band in or-
der to match the poorer seeing of the z850band. Total z850band
magnitudes are obtained with SExtractor parameter MAGAUTO.
4.2. Colour-magnitude relation
The colour-magnituderelation is presented in Fig 8. We flag the
35 confirmed interlopers in the ACS field (cyan crosses), since
nearly a fourth of them (9/35) are located on the red-sequence.
We perform a robust linear fit using bi-square weights
(Tuckey’s Biweight) to the CMR of the confirmedpassive mem-
bers. The bi-square weights are calculated using the limit of 6
outlier-resistant standard deviations. The process is performed
iteratively until the standard deviation changes by less than the
uncertainty of the standard deviation of a normal distribution.
The linear fit has a slope of -0.039±0.013 and a zero point
CMRZP=0.86±0.04, which was determined with a bi-weighted
mean. The quoted uncertainty on the slope corresponds to the
estimated standard deviation of the fit coefficient. The scatter of
the CMR including only the passive galaxies is 0.039 mag.
Since the spectroscopic sample does not populate well the
faintendofthered-sequence,weselecteda ”red-sequence”sam-
ple, based on a combination of morphological and colour crite-
ria. We applied a generous colour cut of 0.5 < i775− z850 <

Page 8

8 J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 1
Fig.8. i775-z850 colour-magnitude relation of XMM1229. The
black solid line refers to the linear fit to the passive cluster mem-
bers, which are shown in red circles. The dashed lines corre-
spondtothe3-σregion.Theconfirmedgalaxieswith[OII]emis-
sion are displayed in blue triangles; members with strong Hδ
absorption (see Sect. 5.2) are displayed in green squares. The
cyan crosses refer to spectroscopically confirmed interlopers.
The ”red-sequence” sample is presented with open black sym-
bols: circles refer to ellipticals, squares refer to S0s and inverted
triangles correspond to Sb galaxies. A merging pair of ellipti-
cal galaxies is marked with two filled 5-pointed-stars. The black
dots correspond objects within 1 arcmin from the X-ray cluster
center.
1.3 for 20 < z850 < 24, based on the properties of the bluest
star forming cluster galaxies and the magnitude limit set by
Postman et al. (2005) to ensure a reliable morphological clas-
sification. In addition, we constrained the search radius to 1’
from the cluster X-ray center, corresponding to 478 kpc at the
cluster redshift. This is a reasonable area to search for cluster
members, and avoids contamination of non-members. We can
also express this radius as a fraction of the fiducial radius R200
which was estimated using the R200− T X-ray scaling relations
of Arnaud et al. (2005). Thus, we determine the search radius of
478 kpc to be equal to 0.4×R200.
We found58galaxiesinthis regionwhichwerevisuallyclas-
sified using the templates from Postman et al. (2005). The se-
lection of the red-sequence galaxies was based on the 3-σ clip-
ping of the linear fit to the confirmed passive members. Thirty-
one galaxies are the region delimited by the 3-σ clipping, for a
z850magnitude cut at 24 mag. Again the fraction of ellipticals,
22/31, is much larger than the fraction of S0s, 9/31. The scatter
of the red-sequence combining the two samples (spectroscopic
and ”red-sequence”) is equal to 0.048 mag.
4.3. Model colour-magnitude relation
Traditionally, galaxy colours are measured either using aperture
magnitudes with corrections which take into account PSF differ-
ences, or by using aperture magnitudes after deconvolving the
PSF as, for e.g., in Blakeslee et al. (2003). Instead, in this work
we explored a method to derive galaxy colours based on model
magnitudes, as commonly used in the Sloan Digital Sky Survey
(e.g. Blanton et al. (2005)). In this method, the PSFs of the 2
filters (which are estimated independently in the two bands as
described in the previous section) are convolved with the galaxy
profile models. Hence, this is a direct method were convolution
andnot deconvolutionisperformed.We thereforeusethesurface
brightnessbest-fit modelswith additionalgaussiannoise to mea-
sure aperture and total magnitudes. Similarly to the ”data” CMR
(Sect.4.2),thecolourmeasurementsareperformedinfixedaper-
tures with r=5 pix. An alternative approachwould be to measure
aperture magnitudes over the individual galaxy effective radius,
however this strategy proved unreliable since for many galaxies
Reis smaller than3pix(0.15”),whichis reallytoosmall tomake
a propercolourmeasurement.Total magnitudeswerederivedus-
ing apertures with radius of 10×Reinstead of using SExtractor
MAGAUTO, which we found to be inaccurate in comparison with
large aperture magnitudes.These discrepancies are visible in the
total z850magnitude of the brightest galaxies in the two CMR’s
represented in Figs. 8 and 9.
The procedure to fit the CMR and obtain its zero point
is identical to the method described earlier in Sect. 4.2, only
that now we use both the early-type cluster members and the
”red-sequence” sample. If we consider only the confirmed pas-
sive members to perform the linear fit we obtain a zero point
CMRZP=0.83±0.04,a slope of -0.031±0.016and a scatter equal
to 0.042±0.011. The error of the scatter is estimated with 100
Monte Carlo simulations of the galaxy models, varying the
Sersic index and the effective radius within the 1-σ confidence
errors. The uncertainty associated with the scatter is estimated
by fitting a gaussian to the distribution of the scatter measured
in the models and assigning the standard deviation of the distri-
bution to the error. In order to perform a composite linear fit to
both the spectroscopic and ”red-sequence” samples we applied
a magnitude cut at z850=24 mag, a limit that ensures a reliable
visual classification of the ”red-sequence” sample (see for e.g.
Postman et al. (2005)). We obtain a CMR zero point equal to
0.81±0.04, the total intrinsic scatter slightly increases to 0.050
mag and the slope, -0.031±0.008,remains nearly unchanged.
We would like to remark that the scatter of the colour-
magnitude relations derived from SAM is a factor 2-3 larger
than the observational scatter (see for e.g. Menci et al. (2008)).
In semi-analytical modelling the scatter is obtained by comput-
ing the total galaxy magnitudes, which is precisely known in
simulations. A possible reason for the discrepancy between the
observational scatter and the one obtained with simulations is
the existence of colour gradients which are taken into account in
the total galaxy magnitudes used in SAM to measure the scatter,
whereas in the observations we limit the colour measurement
to a small central aperture, thus minimizing the effect of such
gradients. To investigate this effect we measured the i775− z850
colour of the passive members using the galaxy models, increas-
ing the fixed colour aperture to r=10,15 pix, respectively 0.5”,
0.75” - going beyond these radii would produce noisy measure-
ments since we would run into the background.The correspond-
ing scatter is then0.068,0.088,respectively.This result suggests
the presence of colour gradients in the galaxy sample.
5. Analysis of the spectral energy distributions
The observed spectral energy distribution of a galaxy is a record
of its stellar population history. The SED fitting method relies
on the comparison of the observed SED with synthetic SED’s.
The latter are then convolvedwith the transmission curves of the
filters used in the observations and the output magnitudes are
compared with the observed magnitudes. Galaxy SED’s were

Page 9

J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 19
Fig.9. i775-z850colour-magnitude relation of XMM1229 using
the best-fitgalaxymodels.The blacksolidline refersto the com-
posite linear fit to both the passive cluster members (red circles).
Thedashedlinesdelimitthe3-σregionandthedottedlinemarks
the z850magnitude cut at 24 mag.
determined by measuring the flux within a fixed aperture of 3
arcsec in the four available passbands.
Given the large disparity in the resolution of the ground-
and space-based data, a careful matching of the different PSFs
must be done, when constructing the multi-wavelength catalog
for sampling the galaxies’ SEDs. The method we used to de-
rive aperture corrections is the following: we smoothed the i775,
z850and Ks-band images with gaussian kernels to match the see-
ing of the J-band (∼1”) and made growth curves of stars in the
original and degraded (smoothed) images. We then obtained a
differential median radial profile for each band with which we
derive corrections at a given radius. In the multi-colour catalog
we use galaxy magnitudes corrected to match the fluxes to the
worst seeing image (J-band), measured within 0.5” radius aper-
tures for the ACS bands, and 1.16” for the NIR data. We opted
to work with magnitudes extrapolated to 3” radius, which safely
enclose the bulk of the galaxy flux.
A total of 20 spectroscopicmembers are commonto the FoV
of SOFI and ACS, four of which constitute two red galaxy pairs
that are not properly resolved in the NIR data. For this reason
we had to exclude them from the spectral analysis. Only four
of the remaining 16 studied galaxies show [OII] emission lines
(IDs: 3025, 3205, 3301, 4055) signaling ongoingstar formation,
and the first two also present [OIII] lines. The first three galaxies
have been visually classified as ellipticals, although galaxy3301
has a low Sersic index, n=1.5. Galaxy ID=4055 is an edge-on
spiral which is reflected in the low Sersic index (n=1.2).
Additionally, we also fitted the SEDs of the ETGs in the
”red-sequence” sample lying on the ACS CMR red-sequence.
As mentioned earlier, we find 31 ETG in the locus of the red-
sequence. The poorer quality of the NIR data can only resolve
18 of these galaxies.
5.1. Spectrophotometric properties: masses, ages
Stellarmasses,agesandstarformationhistoriesarederivedfrom
the synthetic galaxy fluxes, assuming solar metallicity and a
Salpeter (Salpeter (1955)) initial mass function (IMF), with a
mass cutoff[0.1-100]M⊙. We performathreeparameter(ageT,
τ, mass)fit totheSEDsusingagridofBruzual & Charlot (2003)
Fig.10. SED fit of one of the three brightest galaxies, ID=3025
(solid line). The flux measurements in the i775, z850, J and Ks-
bands (respectively from left-to-right) are shown in circles, with
1-σ error bars, alongwith the filters transmission curves(dashed
lines).
models characterized by a delayed exponential star formation
rate:
rameter τ spans a range of [0.2- 5.8] Gyr, where 5.8 Gyr is the
age of Universe at the cluster redshift. As an example, in Fig. 10
we present the fit to the SED of 3025, one of the three brightest
galaxies(seeSect.6),togetherwiththefiltertransmissioncurves.
The star formation (SF) weighted age represents the mean
age of the bulk of the stars in a galaxy (depending on the τ pa-
rameter), and is defined as:
t
τ2.exp(−t
τ), performing a minimization of the χ2. The pa-
tSFR=
?T
0dt′(T − t′)Ψ(t′)
?T
0dt′Ψ(t′)
(3)
where Ψ is the star formation rate expressed as
Ψ = τ−2te
−t
τ+ Aδ(t − tburst.) (4)
The parameter A refers to the amplitude of an instantaneous
burst at time tburst > τ, as described in Gobat et al. (2008).
Galaxy SF weighted ages do not change significantly if
other models (Maraston (2005)) and different IMF’s are used
(Chabrier (2003), Kroupa & Weidner (2003)), however the stel-
lar masses are dependent on the chosen IMF.
The spectroscopic cluster members form a fairly old popula-
tion, with a median SF weighted age of 4.3 Gyr, and with stellar
masses in the range 4×1010-2.3 ×1011M⊙, see Table 5 for the
listing of the fitted masses (column 2) and ages (column 3). The
”red-sequence” sample, which allows us to probe fainter galax-
ies, appears to be less massive, with a median stellar mass of 5.5
×1010M⊙. However, since we do not have redshifts for these
galaxies, we cannot draw strong conclusions about their masses
and formation ages.
In Fig. 11 we investigatethe correlationbetween the star for-
mationweightedageandstellarmasscontentinboththespectro-
scopic(filledcircles)and”red-sequence”samples(opencircles).
We find a strong mass-age correlation which is confirmed with a
Spearman rho rank of 0.61 with a significance p of 1.4×10−4
(p is a value lying in the range [0.0 - 1.0], where p=0 indi-
cates a very significant correlation and p=1 means no corre-
lation). This mass-age correspondence evidences a well-known
anti-hierarchicalbehavior(downsizing),wherethe most massive
galaxies are also the oldest.

Page 10

10 J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 1
Fig.11. Photometric masses of the 16 spectroscopic cluster
galaxies (filled circles) and 18 ”red-sequence” ETGs (open
circles) as a function of their star formation weighted ages.
Spectroscopic members with [OII] emission or morphologically
classified as late-type galaxies are signaled with square sym-
bols. Mean error bars corresponding to three mass bins (bin1:
m <4.5×1010M⊙, bin2: 4.5×1010M⊙< m <1×1011M⊙, bin3:
m >1×1011M⊙) are shown on the top.
Fig.12. SF weighted age versus radial distance to the cluster
center. Spectroscopic members with [OII] emission or morpho-
logically classified as late-type galaxies are signaled with square
symbols.
We also investigate the dependence of the galaxy radial dis-
tance to the cluster center with mass and SF weighted age. We
find that the most massive elliptical galaxies populate the clus-
ter core, and conversely the four late-type galaxies are situ-
ated at the periphery of the cluster, at about 1 arcmin from the
center, indicating star formation taking place in these regions.
This morphological segregation is well established at lower red-
shifts (e.g. Biviano et al. (2002)), nonetheless it is interesting to
note that at redshift ∼ 1, the late type galaxies are already set-
tled at the outskirts of the cluster. This segregation was also
found by Demarco et al. (2005) and Homeier et al. (2005) in the
study of a cluster with z=0.837, as well as in RDCS 1252.9-
2927at z=1.234(Demarco et al. (2007)). The dependenceof the
star formation weighted age with the cluster centric distance
(Fig. 12) shows that the galaxy age scatter increases at larger
radii. This is indicative of younger/morediverse SF histories for
Fig.13. Correlation between galaxy i775− z850colour and pho-
tometric mass. Spectroscopic members with [OII] emission or
morphologically classified as late-type galaxies are signaled
with square symbols.
Table 5. SED analysis of the spectroscopic members.
Galaxy ID
(1)
3301
4055
20014
20013
5411
3507
3495
3430
3205
3524
3428
3025
5417
5499
4155
4126
Mass (1010M⊙)
(2)
5.3+1.8
−3.1
1.8+1.1
−1.3
2.4+2.2
−0.3
4.9+0.8
−2.0
6.4+2.3
−2.7
26+2
−11
7.4+3.6
−2.8
23+3
−11
8.3+0.3
−2.3
18+2
−11
6.5+1.0
−2.7
20+6
−9
8.9+5.1
−1.8
8.3+3.7
−3.7
6.8+0.9
−2.7
6.8+1.3
−0.8
tSFRAge (Gyr)
(3)
4.09+0.34
−2.82
3.94+0.58
−3.14
1.18+1.34
−0.1
4.18+0.18
−2.19
4.55+1.19
−2.15
5.74+0
−2.50
4.46+1.28
−2.06
5.74+0
−2.84
3.86+0.15
−1.56
5.99+0
−3.80
4.27+0.19
−2.17
4.85+0.89
−2.36
3.49+2.25
−0.64
4.27+0.68
−2.28
4.43+0.18
−2.24
2.50+0.91
−0.31
galaxies located in the outer regions of the cluster, which have
presumably accreted later onto the cluster. This result has also
been found in other work, e.g. Mei et al. (2009).
Finally,we analyze the relation between the i775−z850colour
and the mass (Fig. 13) and we observe the expected trend of the
most massive galaxies being redder.
5.2. Star formation histories
The spectra of 12 confirmed passive members were coadded to
obtain the stacked spectrum. However, four (ID=3428, 3524,
4155 and 5417) of these 12 galaxies have strong Hδabsorption,
EW(Hδ) ≥ 3 (these are a+k/k+a spectral types, see Table 4) and
therefore we removed them from the stacking procedure. Three
of these galaxies are at about 1 arcmin from the X-ray cluster
center and only galaxy (ID=3428) is closer to the core, at ∼ 0.5
arcmin from the center. In Fig. 14 we present the coadded spec-
trum of 8 spectroscopic passive members with weak or no Hδ

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J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 111
Fig.14. Stacked spectrum (in blue) of all passive members
which do not show strong Hδabsorption (k type, see Table 4).
The red line refers to the best-fit model to the stacked spectrum
andthegreenlinereferstothe best-fitmodelto theaverageSED.
The region around the atmospheric A-band at 7600 Å (dashed
lines) is difficult to subtract and was therefore ignored in the fit.
absorption. The best-fit SED is shown in green and the spectral
fit is shown in red.
Star formation histories were derived only for the eight
galaxies which do not have significant Hδabsorption. The star
formation weighted age and formation redshift obtained by the
best fitting models (i.e. those within the 3-σ confidence), is 3.7
+0.4
−0.5Gyr and zf= 3.0 ± 0.5 respectively, when using the com-
bined spectrophotometric data. It is not surprising that there is
discrepancy between the average age obtained by fitting the in-
dividual SEDs (4.3 Gyr) and that derived from the composite
spectrophotometric data, as we are using the spectrum and SED
to place complementary constraints on the star formation his-
tories (the former has resolution but poor wavelength coverage,
while the latter has coverage but poor resolution). This discrep-
ancy can stem from the fact that the SED unfortunatelydoes not
probe the rest-frame UV and would be thus somewhat insensi-
tive to recent star formation.
6. Is there a Brightest Cluster Galaxy?
The cores of rich galaxy clusters most often host a massive
and bright giant elliptical galaxy - the brightest cluster galaxy
(BCG). In XMM1229, instead of one prominent BCG, we find
three bright galaxies within ∼ 0.5 mag. The total z850-band mag-
nitudes are derived by integrating the best-fit surface brightness
model to a large radius, r=10×Re. In Table 6 we summarize the
most relevant characteristics of these galaxies: total z850magni-
tudefrombest-fitmodel(column2),distancetotheX-raycluster
center (column 3), photometric mass (column 4), star formation
weighted age (column 5). As expected, the three bright galax-
ies are the most massive galaxies, with masses of the order of 2
×1011M⊙. The galaxy ID=3025located at 1.3“ from the cluster
center shows strong [OII] emission, indicating ongoing star for-
mation which is confirmed by a lower star formation weighted
age of 4.85 Gyr, approximately 1 Gyr younger than the other
two brightest galaxies. In addition,this galaxy is fainter by ∼ 0.2
mag in Ks and ∼ 0.15 mag in J, with respect to the other two
bright galaxies.
Table 6. Properties of the three brightest galaxies.
ID
(1)
3025
3430
3507
z850mag
(2)
21.051 ± 0.002
21.055 ± 0.002
21.468 ± 0.002
dist [”]
(3)
78
5
1
Mass [1011M⊙]
(4)
2.0+0.6
−0.9
2.3+0.3
−1.1
2.6+0.3
−1.1
Age [Gyr]
(5)
4.9+0.89
−2.36
5.7−2.84
5.7−2.50
7. Discussion and conclusions
XMMU J1229+0151 is a rich, X-ray luminous galaxy cluster at
redshift z=0.975, that benefited from a good multi-wavelength
coverage and is therefore an adequate laboratory for studying
galaxy evolution. The high quality ACS imaging data combined
with the FORS2 spectra allowed us to derived accurate galaxy
photometry,and the with the additional NIR J- and Ks-bands we
performed an SED analysis.
– From the X-ray spectral analysis we obtained a global
cluster temperature of 6.4 keV and a luminosity Lx[0.5 −
2.0]keV=1.3 ×1044erg s−1, indicating that XMM1229 is a
massivecluster.Fixingtheredshifttothespectroscopicvalue
we obtain the metal abundance Z/Z⊙= 0.34+0.14
– We measured the cluster velocity dispersion σ using the
27 galaxy redshifts obtained with FORS2, σ=683±62 km/s.
The velocity dispersion is below the one expected from the
mean T-σ relation (Rosati et al. (2002)) for the cluster tem-
perature, however it is still within the large scatter of the re-
lation.
– Using the morphological templates of M. Postman we made
a visual classification of the cluster galaxies. This evalu-
ation indicates a predominance of ellipticals (15/21), with
only four members classified as S0, one irregular galaxy and
one late-type Sb galaxy. In order to investigate whether the
shortageof S0s and also to populatethe faint end of the clus-
ter red-sequence, we constructed a ”red-sequence” sample,
based on the galaxies morphology, colour and total magni-
tude.We findthat the fractionofellipticals in the locusof the
red-sequencepertainingto the latter galaxysample, 22/31,is
a factor three larger than the the number of S0s in the spec-
troscopic sample (9/31). Furthermore, there are two pairs of
red galaxies in the spectroscopic sample.
– In addition to the visual assessment we also fitted Sersic
models to the surface brightness profiles of the two galaxy
samples. The distribution of the best-fit structural parame-
ters n peaks at 3.9 suggesting a majority of bulge dominated
galaxies. The median effective radius is 0.275”, approxi-
mately the radius chosen for measuring the i775-z850colour
(r=0.25”).
– Two methods were explored to measure the scatter of the
CMR: (i) in a first approach, as standard in the literature, we
correct the different PSFs of the i775and z850bands to mea-
sure accurate galaxy aperture magnitudes, and (ii) in an al-
ternative approach, we use the best-fit galaxy model magni-
tudes obtained by fitting the surface brightness profiles. The
CMR at this high redshift is found already to be very tight,
with an intrinsic scatter of 0.04 mag when taking into ac-
count only the passive members, a spread which is similar to
the local clusters, thus confirming that the cluster ETGs as-
sembled early on and in short timescales. The scatter of the
red-sequence is essentially the same from these two inde-
−0.13.

Page 12

12 J.S.Santos et al.: Multi-wavelength observations of a rich galaxy cluster at z ∼ 1
pendent methods, showing that the second method is robust
againstuncertaintiesarisingfromPSF corrections.Theslope
of the red-sequence including only the cluster members is -
0.031±0.016, and slightly decreases to -0.022±0.008 when
accounting also for the ”red-sequence” galaxies.
These results are in agreement with the conclusions drawn
fromtheACSIntermediateRedshiftClusterSurvey(seee.g.,
Mei et al. (2006), Blakeslee et al. (2003), Mei et al. (2009)),
where no significant redshift evolution was found in the
CMR scatter and slope. It is worth noting that in the referred
papers, galaxy colours were measured in apertures of vari-
able size corresponding to the effective radius.
– The spectrophotometric analysis shows a red-sequence pop-
ulated by moderately massive galaxies, with a median stellar
mass of 7.4 ×1010M⊙. The combined SED + spectral fit to
the stacked spectrum of the passive members allowed us to
constrain the ages of the ETGs to 3.7+0.4
to a formation redshift zf = 3.0 ± 0.5, similarly to other z ∼
1 clusters (e.g Gobat et al. (2008))
– The inferred star formation histories imply that the cluster
galaxies have completed most of the chemical enrichment,
which is consistent with the high metal abundance of the
ICM, Z ∼ 1/3 Z⊙, as derived from our X-ray analysis (see
Table 2).
– As widely reported in the literature, we find a clear signature
of significant downsizing, since the correlation between stel-
lar mass and galaxy age favors an anti-hierarchical behavior
where the most massive galaxies are the oldest, which also
tend to be closer to the cluster core (Fig. 11, Fig. 12).
−0.5Gyr,corresponding
Acknowledgements. We acknowledge the excellent support provided by the
staff at the Paranal observatory. In particular, we wish to acknowledge their
assistance in setting up the observations with the MXU mode of FORS2
when technical problems prevented us from using the MOS mode. We thank
M. Postman for providing us with his templates for the galaxy morpho-
logical classification. JSS would like to thank D. Pierini, M. Nonino, S.
Borgani and M. Girardi for useful discussions. JSS acknowledges support
by the Deutsche Forschungsgemeinschaft under contract BO702/16-2. RG ac-
knowledges support by the DFG cluster of excellence Origin and Structure
of the Universe (www.universe-cluster.de). This research has made use of
the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under contract with
the National Aeronautics and Space Administration.
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