A&A 443, 435-449 (2005)
DOI: 10.1051/0004-6361:20053585
C. Wolf1 - M. E. Gray2 - K. Meisenheimer3
1 - Department of Physics, Denys Wilkinson Bldg.,
University of Oxford, Keble Road, Oxford, OX1 3RH, UK
2 - School of Physics and Astronomy,
University of Nottingham, Nottingham, NG7 2RD, UK
3 - Max-Planck-Institut für Astronomie, Königstuhl 17,
69117 Heidelberg, Germany
Received 7 June 2005 / Accepted 3 August 2005
Abstract
We report the discovery of a rich component of dusty star-forming galaxies
contaminating the red-sequence in the supercluster system comprising Abell 901a, 901b and A902 at redshift 0.17. These galaxies do not fit into
the colour-density relation, because their preferred habitat is different
from that of regular red-sequence galaxies, which are typically dust-poor,
old and passively evolving. The dusty red galaxies prefer the medium-density
outskirts of clusters while being rare in both the low-density field and the
high-density cluster cores.
This new result is based on the information content in the medium-band
photometry of the COMBO-17 survey. The photo-z accuracy of the
800 brightest cluster galaxies is <0.01 and of the order of the velocity
dispersion of the cluster. This enables us to select a rich and clean
cluster sample, in which we can trace age-sensitive and dust-sensitive
spectral features independently with the detailed medium-band SED data.
We find the red colour of the dusty galaxies to be a result of dust
extinction combined with relatively old stellar ages.
We speculate that the dusty red galaxies could either be a product of
minor mergers between established old red cluster galaxies with infalling
blue field galaxies, or mark a period in the internal transformation of
blue field galaxies into red cluster galaxies, which is triggered by the
environmental influences experienced during cluster infall.
Key words: surveys - techniques: photometric - methods: data analysis - galaxies: clusters: general - galaxies: evolution
It has been known for decades that massive galaxies in clusters tend to have spheroid-dominated morphologies (Dressler 1980; Dressler et al. 1997) and a paucity of recent star formation (Butcher & Oemler 1984; Lewis et al. 2002). Dressler's morphology-density relation can be seen as a result of an environmental influence on galaxy properties, whereby a higher density of galaxies supresses star formation and leads to pressure-supported morphologies via some still controversial physical mechanism. Alternatively, the galaxies in denser regions could just be exhibiting a more mature state because they were the earliest to collapse and evolved most quickly. Both concepts could explain the observation that the percentage of red old galaxies increases with the local galaxy density, while that of blue young galaxies decreases correspondingly.
So far, it appears that clusters conform with rather universal trends of evolution in colour-magnitude and colour-density relations (McIntosh et al. 2005), if only a simple bimodal distinction of red vs. blue galaxies is considered, which is most simply translated into non-star-forming vs. star-forming galaxies. However, from ISOCAM observations Coia et al. (2005) have concluded that some of the red-sequence galaxies in the cluster Cl 0024+1654 are undergoing bursts of star formation while their red colour is due to their very dusty nature.
In this paper, we present striking evidence that the red-sequence of the clusters Abell 901/902 is highly contaminated by galaxies which are not just old and passively evolving. Instead we find a large population of dusty star-forming galaxies in the red sequence, which do not fit into the simple colour-density relation. These dusty red cluster galaxies can neither be considered a subset of typical red galaxies nor a subset of typical blue galaxies, because their spatial distribution is inconsistent with either one of them.
It is possible that the dusty red star-forming galaxies may be in the process of transformation from typical blue field galaxies to typical red cluster galaxies. But it is equally conceivable that the newly identified population is undergoing just a particular phase in a more complicated evolutionary picture. Miller & Owen (2002) found star-forming galaxies with large amounts of extinction to be concentrated more centrally in clusters than star-forming galaxies in general, suggesting an environmental effect. Coia et al. (2005) have studied dusty starbursts in several clusters and found great variety in abundance even when clusters of similar mass and distance were investigated. They found dusty starbursts most often in clusters with complex dynamics that were suggestive of sub-cluster mergers. In fact, our target Abell 901/902 is a dynamically very complex supercluster environment.
A variety of physical processes are expected to play an important role in the environmentally triggered transformation from star-forming galaxies to red old spheroidal systems: e.g., major mergers between galaxies of comparable size (Barnes 1992); impulsive encounters where galaxies move at too high a relative speed to merge yet are dynamically heated and sometimes eventually disrupted (minor interactions and galaxy harassment; Moore et al. 1999); ram-pressure stripping, where a galaxy's gas content is stripped off as it passes through the hot, extended gas characteristic of large groups and galaxy clusters (Gunn & Gott 1972); and suffocation, where a galaxy loses its warm gas envelope as a reservoir for future cooling and gas accretion (Larson et al. 1980). Bekki (1999) suggested that tidal forces exerted by a cluster onto an infalling group could trigger nuclear starbursts with no merging being involved.
Recent results from a variety of authors indicate that the transformation of massive star-forming galaxies into spheroidal non-star forming galaxies takes place in galaxy groups, rather than in cluster environments (Kodama et al. 2001; Balogh et al. 2002). Therefore, it is necessary not just to study the cores of massive clusters but to explore the properties of galaxies out to large radii, towards densities characteristic of the field. However, detailed studies of galaxy populations in clusters have been very challenging, because without exhaustive spectroscopy it is difficult to identify which galaxies are truly embedded in the cluster environment.
An alternative route presented here for the first time involves the use of high-precision photometric redshifts such as those provided by the medium-band survey COMBO-17 (Wolf et al. 2001). These permit a sufficiently clean selection of cluster galaxies with very little field contamination. The detailed medium-band SEDs contain a wealth of spectral information which allows simultaneous decoupling of redshift, age and dust reddening using purely optical photometric data.
One of the COMBO-17 fields contains a complex supercluster environment which
is very suitable to study the evolutionary phenomena near galaxy clusters and
test the role of the physical mechanisms listed above. This is the A901 field of COMBO-17 which contains the clusters Abell 901a, 901b and 902, as
well as some associated groups, all at redshift 0.165 and within a
projected area of
Mpc/h. This field has already been subject
of weak lensing studies in two and three dimensions using the COMBO-17 data
(Gray et al. 2002; Taylor et al. 2004). A first study of the cluster population itself
investigated the relation of galaxy colours to the dark-matter density as
inferred from weak lensing maps (Gray et al. 2004). The field has since been
observed with XMM-Newton, Spitzer, HST/ACS and GALEX.
In this paper, we examine the galaxy population in this cluster complex using the optical data from COMBO-17 and taking advantage of accurate photometric redshifts which have errors below 0.01 for most objects studied. We exploit the 17-filter SED data to differentiate between dust-free and obscured galaxies, and thus identify dusty galaxies in the red-sequence from our multi-band photometry alone. As a result, we are able to identify the true old population unambiguously, and investigate the properties of the dusty red-sequence galaxies. The full COMBO-17 data set provides us with a field comparison.
The paper is structured as follows: in Sect. 2, we present the COMBO-17 data and their characteristics. Section 3 reports on the galaxy sample selected from the COMBO-17 survey catalogue. In Sect. 4 we compile our analysis of the cluster population characterizing the several types of galaxies and deriving a type-density relation. Section 5 presents a discussion of our results, and finally we summarize in Sect. 6.
Throughout the paper we use
100 km/(s Mpc) in combination
with
and use h=1 when citing
luminosities in the text. Both apparent and absolute magnitudes are reported
in the Vega system and are total object magnitudes. All rest-frame quantities
are corrected for Galactic foreground reddening, while apparent magnitudes and
observed-frame colours are used as observed. Rest-frame colour indices are
always labelled as such, while observed-frame indices have no special labels.
The COMBO-17 survey has measured accurate photometric redshifts for more
than 30 000 galaxies in three different fields, including the Chandra Deep
Field South (Wolf et al. 2004) and the field of Abell 901/902 which is analysed
in this paper. Photometry in 5 broad (UBVRI) and 12 medium-bands is used to
classify objects into normal stars, white dwarfs, galaxies and QSOs (more
generally type-1 AGN) and to estimate redshifts for extragalactic objects.
Quasars are clearly identified when the contribution of AGN light to the
SED is sufficient to leave recognizable traces in the 17-band SED data.
Comparisons with spectroscopic redshifts and simulations have suggested the
mean error of COMBO-17 redshifts to be
for galaxies
with R<20, while fainter galaxies have higher redshift errors. We found
the 90% completeness limit for galaxies at z<1 to be
,
fainter than the samples analyzed later in this paper.
The COMBO-17 filters have central wavelengths ranging from observed-frame
365 to 915 nm and allow us to derive luminosities for cluster galaxies in A901 between 310 nm and 740 nm rest-frame wavelength directly from observed
photometry. We obtain luminosities by placing the best-fitting template SED
into the observed 17-filter spectrum and integrating the template over the
efficiency curve of the desired redshifted rest-frame band. However, the
analysis here relies mostly on observed-band photometry to eliminate impacts
of errors in the derivation of rest-frame quantities. All luminosities refer
to total object photometry, while the colours and SED shapes are determined
from seeing-adaptive aperture photometry probing the same physical section of
any object in all 17 bands. The faintest objects in the sample contain 60% of their total light in the apertures, but the few most luminous and
largest galaxies emit only
20% of the total in the aperture. As a
result, the SEDs of larger galaxies are more dominated by nuclear light.
All survey details regarding observations, data reduction, classification, redshift estimation, rest-frame properties, completeness, accuracy and reliability of the object catalogue data are discussed in great detail in Wolf et al. (2004). Here, we only repeat a discussion of the galaxy templates as this is relevant for the analysis of the galaxy population in Abell 901/902.
The galaxy template library used in COMBO-17 contains a grid of synthetic
spectra based on the PEGASE code (Fioc & Rocca-Volmerange 1997) for population synthesis models.
The templates span a two-dimensional grid with a range of ages calculated
by the PEGASE code and a range of extinction levels which we applied as
external screens to the SEDs delivered by PEGASE. The setup for PEGASE uses
standard parameters suggested by the codes SSPs and scenarios
with a Kroupa (1993) IMF and no extinction. The star formation history
follows an exponential decay law with a time constant of Gyr. The
SEDs are calculated by PEGASE for various time steps since the beginning of
the first star formation. As templates we use 60 spectra which span ages
(more precisely, time since onset of star formation) from 50 Myr to 15 Gyr.
All templates are then extinguished with six different equidistant degrees
of reddening in the interval of
EB-V=[0.0,0.1,...0.5]. The reddening
law of our choice is the SMC law from the 3-component model by Pei (1992).
We decided to use this law because it provided a reasonably good match
between the rest-frame UV templates and the observed SEDs of galaxies with
known redshift. Note that the detailed choice of extinction law has no
effect on the observed SEDs of galaxies with z<0.6, and hence certainly
not on the cluster A901.
We tune initial model metallicities to give almost solar metallicity for old populations typical of local L* spheroidal galaxies. It is worth noting, however, that the well-known age/metallicity degeneracy is actually helpful in this case: Mismatches between real galaxy and template metallicities can be compensated for by modest changes in template age, while yielding still accurate estimates of redshift and SED shape.
The COMBO-17 observations cover an area of
at full depth, centered on
), which is
.
The interstellar foreground reddening is estimated at
EB-V=0.06
(Schlegel et al. 1998), which is taken into account for fitting templates to the
observed photometry and deriving the correct photometric redshift.
At the cluster redshift z=0.165, the angular scale is 2.0/h kpc per
arcsec. Assuming h=0.7 the full field covers an area of
Mpc. This area contains three pronounced cluster cores called Abell 901a, 901b and 902, as well as various associated groups. A deep WFI
image of the field has been published in Gray et al. (2002).
The field contains 11009 objects brighter than R=23, of which 98% are
classified by COMBO-17. Due to its relatively low
galactic latitude of
,
the field contains a large proportion of stars. The COMBO-17
classifier found 2091 stars and 13 white dwarfs at R<23. A number of
bright stars reduce the effective field slightly and constrain some
multi-wavelength follow-up observations. The brightest star in the field
is an F8 star with V=9.6 named PPM
-09 2962. At
the brightest object is an M8 Mira variable
(Kirkpatrick et al. 1997, IRAS 09540-0946 = GSC 05479-01188) with
and K=5.75 (measured by 2MASS).
After eliminating stars, QSOs, objects too strange to be classified and objects with bad flags, we have 7992 galaxies left with estimated redshifts and R<23 to search for members of the cluster A901/902.
Spectra of supercluster galaxies were obtained using the 2dF instrument on
the AAT in March 2002 and March 2003. Details of these observations are to
be published in Gray et al. (in preparation). A total of 89 galaxies
were targetted using the 1200B grating in a single fibre configuration
during the 2002 run, of which 64 were within
0.15<z<0.18. Three fibre
configurations using the lower resolution 600V grating were obtained during
the 2003 run targetting 368 objects, with 47 repeated from the previous run.
The primary selection function targetted galaxies selected by photometric
redshift to be within the supercluster redshift and having R<20, with
additional fibres being allocated to secondary targets (QSOs or non-cluster
galaxies) when available. However, the limitations on close packing of
fibres meant that the highest-density regions of the clusters were less
well sampled, thereby missing a number of the brightest galaxies. The
fibers measure
in diameter and measure light from very similar
fractions of any object to the photometric apertures.
Redshifts were obtained by two independent means, by line profile fitting
of the Ca H and K feature in absorption and by cross-correlation with
template spectra using the XCSAO task within IRAF. The two measures agreed
well, with
.
In total,
spectra were obtained for 407 unique objects including various stars and
QSOs that were targets of different interest. After eliminating several
galaxies due to data quality issues, we have 249 galaxy spectra
within the redshift range of the supercluster for statistical analysis.
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Figure 1:
Bright cluster galaxies:
top row, left: photometric redshift vs. apparent magnitude for the
galaxy sample in the A901 field with the cluster at
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Figure 2: Rest-frame colour-magnitude diagrams: cluster ( left) versus field ( right) with cluster red-sequence fit (grey line) and Butcher-Oemler style red-sequence cut (black line). |
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Figure 1 shows a redshift-magnitude diagram of the galaxy sample
focussing on a redshift interval encompassing the target clusters. The bulk
of the bright cluster population can be clearly seen at
,
reaching up to apparent magnitudes of
.
Fainter than R=20 the
photometric redshift errors slowly start to widen the cluster distribution,
which makes it harder to disentangle cluster members from field contaminants.
In fact, at the bright end (R<20) we expect a photometric redshift error of
,
while at R=23 we expect
.
We first decide on a redshift cut for the cluster sample by looking at the
redshift distribution in a bright sample with small photo-z errors, before
defining a deeper sample on the basis of this redshift cut. For this, we use
an iterative approach to determine the mean and rms redshift of the cluster
population. We settled on a redshift cut of
at
R<20. This sample contains 482 galaxies with a redshift distribution of
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(1) |
We now define a deeper cluster sample for the following analysis and keep the
redshift range at
.
The fainter we go, the lower
the completeness of the cluster sample will be as more and more galaxies are
scattered outside the redshift interval due to increasing photo-z errors.
However, widening the interval would clearly include a large amount of field
contaminants, which we would like to avoid. For this paper, we settle to
applying a luminosity cut at MV<-17, which corresponds to
.
The low redshift errors in this bright regime help to keep field contamination
low and cluster completeness high. At R=21.5 we expect redshift errors of
,
which would make the sample still 68% complete at
its luminosity limit.
Hereby, we select 795 galaxies in a comoving volume of 1575 (Mpc/h)3.
Although the clusters occupy only a tiny fraction of this volume, the sample
consists mostly of cluster galaxies due to their large overdensity. We split
the sample into red-sequence and blue cloud following the procedure explored
by Bell et al. (2004) on COMBO-17 galaxies. We apply a cut parallel to the
colour-magnitude relation of the red-sequence, which evolves with redshift.
From the COMBO-17 sample Bell et al. (2004) have suggested two alternative
cuts, one determined internally from COMBO-17 and another one which includes
measurements from the local Universe. Here, we use the COMBO-17 internal cut
defined by
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(2) |
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(3) |
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Figure 3: Selection of field galaxy sample: luminosity-redshift diagrams of red-sequence galaxies show rich groups and clusters in all three COMBO-17 fields. We exclude them to select a field sample (shaded area) from the low-redshift domain of all fields. |
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We select a low-redshift field comparison sample for two purposes: we would like to estimate the field contamination in the redshift slice of the cluster sample from the space density of galaxies outside the cluster using the same luminosity cut; and we would like to compare the properties of the cluster population to that of the field.
We would like the field sample to have a similar mean redshift as the cluster to suppress possible influence from any evolutionary trends and consider all three COMBO-17 fields. We investigate the redshift range of z=[0.05,0.25], where the photo-z catalogue is more than 90% complete at MV=-17 for all galaxy SED types. We use the same luminosity cut and determine the number of galaxies and the volume they are drawn from, hence obtaining a space density.
We want to exclude strong local overdensities, such as the Abell clusters at
in the S11 field and at
in the A901 field as well as
some rich groups at
.
Figure 3 shows overdensities in
redshift space for all three COMBO-17 fields as indicated only by the strongly
clustered red-sequence galaxies. Avoiding obvious overdensities we adopt the
redshift ranges shaded in grey. The field sample has a mean redshift of
,
which is very close to the redshift
of A901. It contains 385 galaxies from a volume of
9750 (Mpc/h)3.
The volume of the cluster sample at
z=[0.155,0.185] is more than six times
smaller, and contains an estimated field contamination of 62 galaxies.
This implies that statistically one in 13 out of the 795 galaxies in the cluster
sample would really be in the physically unrelated field, somewhere along the
line-of sight within
50/h Mpc from the cluster center. Since red-sequence
galaxies are intrinsically more clustered than blue galaxies, we split the
contamination assessment by galaxy colour by applying the evolving red-sequence
cut to the field sample. Our best estimate is that 13 out of 468 red-sequence
galaxies (3%), and 49 out of 327 blue-cloud galaxies (15%) are field
contaminants. No attempt is made in this paper to subtract the field
contamination from any numbers reported for the cluster.
Table 1: Mean properties of the three galaxy SED class samples.
To first order, galaxy SEDs are often considered to form a 1-parameter family,
running from old populations in red spheroidal galaxies to young starbursts in
blue galaxies (Connolly et al. 1995). Also, plenty of photometric redshift codes use a
1-parameter family of templates. In COMBO-17, such an approach was taken until 2002, by when it became clear that this was inappropriate at least for the
galaxies in A901. Using the Kinney et al. (1996) template spectra, 1/4
of the bright galaxies in the cluster had redshift errors up to
0.1 and
would have been left out from the study presented here.
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Figure 4: Observed-frame colour-colour digrams: top row, left: two observed-frame colour-indices, each straddling the 4000 Å-break, show galaxies as a 1-parameter family. The grey line roughly indicates the location of the gap in the colour bimodality. Top row, center: when one colour index does not encompass the 4000 Å -break, the galaxies fan out into a wider range. Top row, right: a medium-band colour index probing the 4000 Å-break. Bottom row: template colours (black lines) are plotted over the galaxy sample (grey points). The left-most line is an age sequence from 50 Myr ( bottom left) to 15 Gyr ( top right) without dust reddening. The four other lines show colours for the same age sequence with reddening of EB-V=[0.2,0.3,0.4,0.5] omitting the line at 0.1 for clarity. |
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However, using the 2-D template set outlined above the redshifts of virtually all cluster galaxies were correctly estimated. The 2-parameter aspect of the galaxy family is of central importance, to accommodate spectra with different curvature while keeping the same overall colour. This issue becomes apparent when looking at colour-colour diagrams of the cluster in Fig. 4. Here, the panels on the left-hand side show a narrow distribution of galaxy colours in observed-frame B-R over U-I. The approximate position of the gap between red-sequence and blue cloud galaxies is given by a line cutting across the distribution, although the gap is washed out by the CMR slope in combination with a wide luminosity interval collapsed in this plot. The center panels show galaxies more spread out in the colour plane B-R over R-I, and suggest the existence of a distinctive elongated clump at the red end, almost detached from the main, larger and less concentrated, distribution.
The observed-frame B-R encloses the 4000 Å-break at
as does
the colour index U-I, in contrast to the colour index R-I which probes
the red end of the observed stellar spectrum. The spread in R-I colour at
fixed B-R colour means that the 4000 Å-break is not a safe predictor for
the colour at longer (or even shorter) wavelengths. Since the 4000 Å-break
is mostly driven by the mean stellar age of a galaxy, the spread in colours
requires another important factor.
A comparison with the age dust template grid shows dust reddening
to be a possible second independent factor in galaxy colours. The pronounced
red clump coincides with templates of old age and no dust reddening, and is
hereafter referred to as dust-free old clump. It may represent typical
spheroidal galaxies in the cluster, while galaxies with redder R-I colours
at fixed B-R value correspond to dust-reddened objects with younger mean
stellar age. This interpretation of galaxies as a 2-D age
dust
family can further be tested with the COMBO-17 medium-band colours. In the
right-hand panels of Fig. 4 we see again the clear 2-D parameter
family. Colour indices from neighboring medium-band filters probe small-scale
variations in the SED. Near the 4000 Å-break these differ between galaxies
that are red because of age and those that are red because of dust. We note,
that medium-band indices where both filters are redwards of the 4000 Å-break
do not differentiate age and dust, and galaxies appear again as 1-D families.
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Figure 5: Three galaxy types: left: stellar age vs. EB-V from estimated template parameters. Center/Right: broad-band/medium-band colour. Top row: dust-free old clump in the red-sequence. Middle row: dusty population in the red-sequence. Bottom row: the blue cloud. |
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Figure 6: Colours and spectra: top: the mean template of the dusty red-sequence galaxies is redder at the red end and bluer at the blue end than that of the dust-free old clump galaxies. Middle: the dust-free old clump reflects older age in a stronger 4000 Å -break and Ca H and K-lines than the dusty red galaxies. The spectral differences in these templates match up with the mean colour differences of the two SED classes. Bottom: similar trends can be found in the mean observed spectra (we note that the flux calibration is incorrect towards the red). |
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The colour-colour diagrams already suggested that we have two contributions to the red-sequence of Abell 901/902: a pronounced dust-free old clump; and a more widely scattered collection of galaxies with higher dust reddening and lower mean stellar age. We thus try go beyond a simple classification of galaxies into a red-sequence and a blue cloud from their colour bimodality. We want to split the red-sequence itself into two different types in order to investigate their possibly different properties in terms of distribution in space, luminosity, their environment and their spectral properties. We try to separate the pronounced dust-free old clump from dusty red-sequence galaxies by cutting in template parameter space rather than colour space. We find ad-hoc that a simple reddening cut at EB-V=0.1 delivers a reasonably clean separation between the two features (see Fig. 5).
For the purpose of this paper, we will now provisionally classify the galaxy population into three categories:
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Figure 7: Top: mean 2dF spectra for three galaxy types (restframe). Vertical offsets are used for clarity. The hashed area is affected by night-sky emission lines. Bottom: the mean dusty red spectrum divided by the mean old red spectrum. |
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We would now like to investigate the average spectral properties among the three galaxy SED classes just defined. We will ignore the age-metallicity degeneracy here, since a detailed discussion of ages and star formation histories is beyond the scope of this paper, and the degeneracy will not matter much for the overall shape of continuum spectra and colour indices.
From the template parameter values we find mean dust extinction levels and
galaxy ages given our choice of star formation histories and metallicities.
The dust-free old clump is distinguished from dusty red-sequence contaminants
by a high mean age of 6.2 Gyr and a low extinction of
as opposed to 3.5 Gyr and
.
The dusty red-sequence contaminants appear with similar dust extinction levels
as the blue cloud galaxies do, but with a higher galaxy age that could reflect
a larger proportion of old stars. However, there could be potentially stronger
extinction affecting specifically the youngest stellar population.
In Fig. 6 we show the two mean templates for the dust-free and the dusty red-sequence galaxies along with the full quantum efficiency curves of several COMBO-17 filters. In the top panel, we can see how both templates have the same U-I colour, but differ in between those filters. The dusty red template is bluer at the blue end of the spectrum due to an increase of young stars. At the long-wavelength end it is redder than the dust-free old clump, presumably due to dust reddening. Comparing only the slope of the continuum between neighboring filters reveals only little difference between them. However, in the colour-colour diagrams of Fig. 5 the two types of red galaxies are more clearly differentiated because their principal colour difference is orthogonal to the extent of the dust-free old clump.
The middle panel shows template details around the 4000 Å-break and a few
medium-band filters (here filter names are made from their central wavelength).
Here, the templates are normalised to the flux in the 464-filter, which contains
the Ca H and K lines. The different template flux levels within the filters match
up in fine detail to the different mean colours in the sample: when compared to
the dusty red galaxies, the dust-free old clump is redder in U-420, bluer in 420-464, again redder in 464-518 and similar in 518-571. The detailed
correspondence between template details and sample colours vindicates the use
of the template grid with its age dust structure.
The similarity in broad-band colours between old red and dusty red galaxies
in such a low-redshift cluster echoes a similar ambiguity in interpreting
the colours of Extremely Red Objects (EROs) found at higher redshift. These
are believed to comprise dust-free, old galaxies as well as dust-reddened
star-forming galaxies, mostly at
,
but also include edge-on
disk galaxies at lower-redshift (Yan & Thompson 2003).
However, in order to deduce the true nature of the dusty red-sequence galaxies we should investigate their observed spectra rather than the templates. We have averaged the spectra for the three types using equal weights for every galaxy. Most spectra fall into the dust-free old category because these galaxies are most abundant at high luminosity and dominate any bright flux-limited sample. With 69 objects the dusty red-sequence category contains enough data to allow a detailed comparison with galaxies from the dust-free old clump.
We show the mean spectra in Fig. 7 with relative flux offsets for
clarity. From old red over dusty red to blue young galaxies, there is a clear
trend of increasing Oxygen and Hydrogen emission line fluxes. Going from old
red to dusty red galaxies, emission fill-in can be seen within the H and H
absorption lines. Going again from old via dusty to blue, the
Ca absorption lines and the G-band decrease in depth. These trends may appear
quite weak when going from old red to dusty red galaxies but can be seen more
clearly in the ratio spectrum of Fig. 7, where dusty red galaxies are
divided by old red galaxies, and even weak O III emission lines appear.
We compare the two types of red galaxies more carefully in Fig. 6.
Redwards of the Ca lines, we find again the same trends first identified from
the medium-band colour index 464-518 and from the templates. Bluewards of Ca H and K the spectra are relatively noisy and we see less pronounced differences.
The mean spectrum of dust-free old galaxies reflects an old stellar population
which is passively evolving and shows no signs of ongoing star formation.
We see some H-absorption which hints at recent star formation. The
equivalent width of H
is 2.3 Å (rest-frame), while normal cluster
ellipticals are not supposed to show any clear H
-lines. Measuring lines
in the mean spectrum should be robust in terms of measuring the average
equivalent width, but it tells us nothing about the population mix producing this
average. The mean spectrum may contain a fraction of k+a galaxies (post-starburst
galaxies also known as E+A galaxies) with pronounced H
-lines.
In the dusty red-sequence galaxies we see weak O II emission as well as
H-absorption, and conclude that they must be forming stars at some
rate currently. Given the template solution fitting the observed colour spectrum,
we find that they are moved to the red sequence via a combination of dust reddening
typical of blue cloud galaxies, together with older ages typical of red-sequence
galaxies.
We find equivalent widths in emission of
(O II)
Å and in absorption of
(H
)
Å.
In contrast, the average blue cloud galaxy has not only stronger Oxygen emission
(
(O II)
Å) but also stronger H
absorption (
(H
)
Å).
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Figure 8:
Cluster ( top) versus field ( bottom):
left: the age ![]() |
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The equivalent width of H
holds clues about the recent variations in
the star formation rate. Values of
(H
Å imply that a
recent starburst was followed by quenching of the star formation, such as in
k+a galaxies. If the star formation had continued, related emission would have
filled in the line reducing the EW (see Mercurio et al. 2004, and references
therein). Dressler et al. (1999) introduced the notion of e(a) galaxies,
which display not only strong H
absorption as in k+a galaxies, but
also O II emission suggesting ongoing star formation. These objects are
explained by Poggianti & Wu (2000) as dust-enshrouded star bursts where
age-dependent dust absorption explains the apparent contradiction from the
lines: strong H
troughs arise from an A star population that has left
their dusty birth place, while the very young O stars are still hidden such that
they can not fill in the absorption with emission from their ionized environment.
At this stage, we will not offer a detailed explanation of the spectra of our class of dusty, star-forming red-sequence galaxies, nor speculate about deeply dust-enshrouded star formation. A more detailed analysis of individual spectra is the subject of another paper (Gray et al., in preparation), and Spitzer observations of this cluster field have just been obtained.
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Figure 9:
Distribution of galaxy types in (x,y,z):
top row: sky maps. The dust-free old galaxies are concentrated towards
the cluster cores, while the other two types are more scattered over the field.
Middle row: the cluster cores form two main velocity components, a North
group and a South group separated by
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We now compare the galaxy properties between the cluster sample and the field sample in order to assess to what extent field contamination could bias the cluster results. Especially, we would like to see whether the relatively high fraction of intermediate-age dust-reddened galaxies within the red-sequence of the cluster is a result of field contamination to the cluster sample.
In Fig. 8 we compare age dust-parameters and
colour- magnitude diagrams of the cluster and field samples. We find that
the cluster contains an enhancement of both dust-free old populations and
intermediate-age dust-reddened populations (at
3 Gyr on our mean
stellar age scale). On the other hand, the cluster shows a shortage of blue
cloud galaxies, which is particularly apparent among young populations with
ages of <1 Gyr. In the cluster, more than a third of all galaxies belong to
the dust-free old clump, while in the field only 9% do. More than 20% of the
cluster sample are in the red-sequence despite not belonging to the dust-free
old clump, while only 11% of field galaxies do. The dusty red-sequence
galaxies reach higher luminosities in the cluster than in the field.
In Sect. 3.2 we reported numbers on field contamination to the cluster sample as expected from the galaxy densities measured at non-cluster redshifts. The result was a considerable contamination to the blue cloud, but in the cluster red-sequence only 3% were expected to be field galaxies. This contamination rate is much below the observed contribution from dust-reddened galaxies to the cluster red-sequence. These intermediate-age dust-reddened galaxies are therefore a genuine cluster-related phenomenon, which can not be explained by effects relating to selection or contamination.
While the increased red-sequence fraction and the shortage of young vigorously star-forming galaxies in clusters has been established long ago, the increase in dust-reddened intermediate-age populations is a new trend identified here from optical data alone.
The spatial distribution of the three galaxy types in the cluster is shown in Fig. 9. The top row shows sky maps with positions straight from the camera images. It is immediately clear that galaxies from the dust free old clump are most highly clustered, and reach the highest projected densities at the cores of the various subclusters. In contrast, dusty red galaxies and blue cloud galaxies show no high concentrations within this supercluster environment. However, we can decuce from the contamination estimate, that the total density of blue galaxies is still six times higher than average field galaxy density. The maps may suggest to the eye that both dusty red and blue galaxies trace roughly the shape of overdensities outlined by the dust-free old population, just without their strong clustering.
Going beyond 2-D projections we try to investigate the 3-D distribution in
(x,y,z)-coordinates, although it is clear, that the z-coordinate reflects
a combination of spatial position with peculiar velocity. The middle row in
Fig. 9 shows projected maps in the (y,z)-plane (North is up). The
strongly clustered old galaxies reveal two concentrations, a Northern group
(ellipse) and a Southern group (circle), with a rest-frame velocity difference
of
km s-1 centered on
cz = 49 500 km s-1.
There is also indication for
filamentary structure. The (y,z)-maps can not easily show clustering as such,
because galaxies in deep potential wells show wide velocity dispersions. We note,
that projections in the (x,z)-plane happen to not reveal conspicuous structures.
The distribution of dusty red galaxies follows a similar two group-structure,
possibly with a slight offset to lower redshifts in the North group. The blue
galaxies mostly avoid the concentrated areas which are heavily populated by
red galaxies of both kinds, most notably the Northern group, and almost seem
to live in the gaps left by the (y,z)-structure of red galaxies.
This is an intriguing piece of evidence when combined with the (x,y)-maps,
where old red galaxies are clearly clustered, while the dusty red and blue
galaxies have low clustering in common. But in the (y,z)-maps dusty red
galaxies occupy a similar region as old red galaxies, while blue galaxies
tend to avoid them. Most of the redshift signal is likely to originate from
peculiar velocities, since a line-of-sight distance equal to the lateral
size of our field causes only 350 km s-1 of redshift. Hence, a tentative
consistent explanation of the maps could place the blue galaxies in a large
low-density volume where large distances from massive cluster cores leave
them with low random velocities and a natural spread in positional redshift.
The old red galaxies form concentrated cluster cores that move at relative
bulk velocities of 700 km s-1 (N+/S-) through the blue envelope.
The dusty red galaxies would be associated in velocity and position with the
cores populating their outskirts with less concentration.
Finally, we determine velocity histograms for each galaxy type independently
(bottom row in Fig. 9). The full samples suggest high velocity
dispersion, where especially the dusty red galaxies appear to have
km s-1. There are indications of a bimodal distribution
among both types of red galaxies, arising from the two groups in velocity space.
When analyzing only the encircled groups, we find
km s-1 (rest-frame) for all four red samples. Such a split does not make
sense for blue galaxies that show no structure in velocity space anyway.
However, there are differences between the North and South group: all three galaxy types in the South group have mean redshifts and velocity dispersions that are consistent with identical given our number statistics. In the North group however, we find almost no bright blue galaxies (hardly any member in the spectroscopic sample) and dusty red galaxies could be displaced to the North and to lower redshift compared to the old red galaxies. We note, that the empty regions in these z-diagrams are truely devoid of galaxies and not artificially empty due to selection effects. The only selection at work was a luminosity cut for the spectroscopic sample.
A complete analysis of the structure of this supercluster complex is beyond the scope of this paper. But we have learned, that the spatial and velocity distribution of the three galaxy types is consistent with dusty red galaxies forming an intermediate population whose preferred habitat are the regions where the high-density cluster cores interface with the lower-density field. The preferred habitats of old red and young blue galaxies are to either side of the interfacing dusty red galaxies.
We now investigate how the fraction of each type among the galaxy population
depends on environment, assessed by the projected galaxy density
.
We define
for any galaxy as the number of galaxies per
or (Mpc/h)2, within a circle whose radius is the
average of the distance to the 9th and 10th nearest neighbor. In
Fig. 10 these fractions are plotted in five bins of galaxy
density, which have bin limits of
.
The geometric average of the whole cluster sample is
or
.
In addition, we plotted the type fractions for a field sample and place
the data points at
,
the mean density
for the estimated level of field contamination. We remind the reader, that we
have not applied any statistical subtraction of the field contamination.
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Figure 10:
Type-density relation: the cluster environment covers projected
galaxy densities of
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Figure 11: Top row: luminosity distributions: the dust-free old populations follow almost a Gaussian, while the blue cloud could more easily be described by a Schechter function. The cluster shows an excess of dust-free old populations, especially fainter than L*, and of dusty red-sequence contaminants, mostly brighter than L*. The field and cluster samples are each normalised by the number of their galaxies. |
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We determine luminosity distributions of cluster and field galaxies by type,
where both have been normalised to their respective total number of galaxies
at MV<-17. The cluster is not corrected for incompleteness induced by
increasing redshift errors towards the faint end, which may have shifted
points by -0.15 dex at
if we assume
68% completeness
(see Sect. 3.2). The field sample is >90% complete at all
luminosities shown here, and redshift errors do not affect its completeness
given a smooth underlying field. No attempt is made to correct these observed
luminosities to any sort of bolometric luminosities, e.g. by taking into
account the dust extinction. Since completely obscured stellar populations
would neither affect the measured luminosities nor the reddening observed
in the visible stellar light, we leave any improvement involving corrections
for dust until we have MIR data available.
Looking at differences between the types, we find a deficit of faint blue cloud galaxies in the cluster. The faint-end slope is not significantly different after adjusting the cluster for a roughly estimated incompleteness. This deficit is compensated by an enhancement of red-sequence galaxies at all luminosities. We find an excess of dust-free old populations at all luminosities but especially among sub-L* galaxies fainter than MV=-20. We also find an excess of dusty red-sequence contaminants which is especially strong among galaxies brighter than MV=-19. At the faint end of our sample dusty red-sequence galaxies appear to have similar fractions in the cluster and the field sample.
The difference between the cluster and the field can be summarized with two phenomenological aspects: first, in the outskirts of the cluster a fraction of the more luminous blue cloud galaxies appear as dusty intermediate-age contaminants to the red-sequence. Here, the stellar population could be of genuinely intermediate age or it could be composed of a large old population with a smaller younger population. Secondly, in the more concentrated cluster regions a fraction of less luminous blue galaxies appear instead as red galaxies and part of the dust-free old clump, indicating that their transformation has already happened longer ago.
Recently, galaxies have been split into a red, old passively evolving and a blue, young, star-forming population. The long-established colour-density relation means red galaxies are strongly concentrated in dense cluster cores while blue galaxies overwhelmingly dominate the low-density field. In this paper we have added complexity to the simple colour-density relation from a third type of galaxies identified here to be related to clusters. They do not fit into the simple bimodal description of red-vs.-blue galaxies, and instead show signs of an ongoing transition induced by a cluster-specific mechanism. This process involves the appearance as a red-sequence galaxy which derives its colour from both dust reddening and old age of the stellar population, while actively forming stars. These properties are indicated by galaxy colours, 4000 Å-break and O II emission. Galaxies of this kind may not be restricted to the red-sequence, but in A901/902 they constitute more than a third of the red-sequence population while only <2/3 are dust-free and old.
These dusty star-forming contaminants to the red-sequence have very little in common with the dominant red-sequence population. The dusty galaxies share a spatial distribution and an avoidance of dense cluster regions with the blue cluster galaxies. But in contrast to blue galaxies, they are not common in the low-density galaxy field outside of the cluster. We may think of them as an intermediate population which could form a spatial interface between blue field galaxies and red cluster galaxies in cores.
There is further evidence that the dusty red galaxies form an intermediate
population between blue and red galaxies. They contain a significant old
stellar population as indicated by their Ca H and K lines and 4000 Å-break and
they are mostly of medium luminosity, at least in Abell 901/902. We repeat
here, that their mean level of dust extinction is measured to be modest
(
)
and very similar to that in typical blue cloud galaxies.
It has been known for a long time, that a small fraction of red-sequence galaxies are edge-on spiral galaxies, which are red because we only have a clear view of their old red bulge, while their younger disk is reddened by dust lanes. So, edge-on spirals form a fraction of the dusty red galaxies in both the field and the cluster sample. However, any orientation effect can not lead to environmental differences in the fraction of edge-on spirals versus classical blue spirals. Hence, the dominant component of dusty red galaxies must be the result of an evolutionary phenomenon. Where did they come from and where do they go? Generically, we consider two possible origins:
Given the abundance of galaxies of the third kind, the two possible origins translate furthermore into two routes of cluster assembly. Route (1) means, that clusters (defined by a concentration of massive old red galaxies) grow via transformation of infalling galaxies intro proper cluster galaxies, e.g. major mergers leading to dusty starbursts among infalling blue cloud galaxies. Route (2) means that clusters grow via the growth of its individual galaxies, where infalling galaxies are incorporated into existing large and red cluster galaxies via minor mergers.
We will try to collect further clues from the literature in the following.
Kodama et al. (2001) have found that the dominance of blue galaxies changes
into one of red galaxies at densities typical for galaxy groups rather than
in cluster cores. They quote a transition density of
Mpc-2 for
MV<MV*+4, H0=50 km/(s Mpc) and q0=0.1
(taking into account their Erratum on the calculation of
). In our
cosmology this equates into
Mpc-2for
.
We can study lower overdensities in our field thanks
to the much reduced field contamination given much more accurate photo-z's.
We find our dusty red galaxies at densities below the transition quoted by
Kodama et al. (2001), while indeed the fraction of old red galaxies increases
closer to their transition density.
While the densities where the change happens may be typical for groups, we do not see particular group signatures among the dusty red galaxies in A901. If the dusty red galaxies just formed an interface between the blue and red habitats, this would support origin (2) which requires combining progenitors from both habitats. In contrast, should a more careful analysis find our dusty red galaxies to live in groups escaping our attention here, this would support origin (1) as an internal process among the infalling blue galaxies which is only triggered by the cluster presence. Again, it remains unclear whether the dusty red galaxies in A901 mark one or the process turning blue galaxies into red galaxies. They may not tell us anything about other processes happening in groups.
We try to consider alternative scenarios where the colours of our third party of allegedly dusty red-sequence galaxies is not a result of dust. We consider mixed stellar populations and additional nuclear light sources:
Post-starburst galaxies of k+a type: such galaxies are roughly characterized by a linear combination of an E-galaxy SED with an A-star SED. A stars are bluer than old red galaxies throughout the visual wavelength range. As a result, any sum of these components is bluer than a pure old red galaxy. Also, Balogh et al. (2005) have excluded the possibility that the colours of k+a's are significantly affected by dust. Hence, they can not show the very red colour observed at the long-wavelength end in Fig. 6 which is also seen in the R-I colours in Fig. 5. We expect k+a galaxies to reside on the bluer end of the dust-free old clump and to some extent in the blue cloud.
Additional nuclear light sources: when considering these or indeed any internal colour gradients, we need to be aware of possible aperture bias. Our SEDs and their colours are determined from aperture photometry which probes a region of fixed physical size for any galaxy at the cluster distance. As a result, all nuclear components in the spectrum will appear enhanced as compared to SEDs from total object photometry. HST images will soon permit to quantify such effects. The origin of the nuclear component could be in dusty star-forming regions or potentially in active nuclei, and is constrained by our observations of continuum colours and emission lines. Active nuclei are almost certainly ruled out, because (i) our spectra show only very weak Oxygen lines, (ii) the objects are not detected in a 90 ks XMM image, and (iii) the abundance of these objects. Furthermore, the continuum shape does not permit AGN light as a sole modification on top of an old red SED: the deviation from an old red SED is increasingly blue on the blue side and increasingly red on the red side.
Coia et al. (2005) have discussed the properties and abundance of mid-IR sources in clusters from ISOCAM observations. Generally, MIR-luminosity is a good tracer of total star formation, given that the largest part of star formation is usually dust-enshrouded in larger galaxies. They found that half of their sources (heavily star-forming cluster galaxies) reside in the red-sequence. While they are obviously not dust-poor old populations following passive evolution, their red colour is instead explained by dust reddening. In contrast to the main red cluster population, the dusty red starbursts were not concentrated in high-density regions.
If such galaxies existed in Abell 901/902, we would identify at least the red-sequence part of the population. Most likely they are similar objects as our dusty star-forming red-sequence contaminants: a property common to both is their tendency to avoid the high-density cluster cores. In the cluster Cl 0024+1654 the MIR sources have a broader velocity dispersion than the main cluster population, just like the dusty red galaxies do in the Abell 901/902 complex. We note, that Coia et al. (2005) found red starburst galaxies associated with close neighbors in all four cases where HST imaging was available.
Poggianti & Wu (2000) have discussed in detail a type of galaxy identified
initially by Dressler et al. (1999) in distant cluster studies, which may
resemble our dusty red galaxies to some extent. They have found e(a)
galaxies showing both O II emission and H-absorption. Using
line ratios and FIR fluxes, they explained these galaxies consistently as
dusty starbursts with an age-dependent extinction level. Furthermore, they
found 75% of their e(a) galaxies to be mergers or interacting pairs with
close companions, supporting a merger origin to the starbursts.
Our dusty red galaxies show similar features, but their mean H absorption of only
Å does not let them pass the e(a) criteria. Rather than speculating about the precise nature of our dusty
star-forming red-sequence galaxies we defer this discussion until
observations with HST and Spitzer have been analyzed, which will deliver
a wealth of detailed information about total star formation rates and
morphologies.
We have analyzed the galaxy population of the supercluster system Abell 901a,
901b and 902 located at redshift 0.17. From the COMBO-17 catalogue, we
have selected the bright galaxy population with MV<-17 by using a photo-zselection of
z=[0.155,0.185]. Given the photo-z accuracy of <0.01, this
sample is virtually complete at MV<-19 and could be
30% incomplete
at MV=-17. For the
250 most luminous cluster galaxies we obtained
additional spectroscopic data in the blue part of the spectrum using 2dF. The
supercluster is a system of great morphological complexity with several large
concentrations which are presumably unrelaxed and will merge in the future.
Our results are as follows:
This work has only used ground-based optical data. We used the discriminative power and concentrated information content of the medium-band survey COMBO-17 to study the cluster population with high completeness and low contamination. Just from the filter SEDs we could select cluster members and identify dusty contaminants to the red-sequence. We have thus established a third class of cluster members beyond the simple red-blue division. From previous work of the colour-density relation it would have been counter-intuitive to predict a rich population of dusty red galaxies in higher-density environments. However, we have shown them to be a phenomenon which is genuinely related to galaxy clusters and preferentially populate their medium-density envelopes interfacing between the young blue field and the old red cores.
In the near future, many medium-band surveys which are wider or deeper than COMBO-17 will target further galaxy clusters, such as a wide-area medium-band survey with the VST. These will enlarge the sample of clusters studied with the technique presented here, and will reveal possible variations in the dusty red galaxy content, which would provide further clues to the physical origin of this phenomenon.
Acknowledgements
C.W. was supported by a PPARC Advanced Fellowship. M.E.G. was supported by a PPARC Postdoctoral Fellowship and an Anne McLaren Research Fellowship. We appreciate discussions with Alfonso Aragon-Salamanca, Michael L. Balogh, Steven Bamford, Eric F. Bell, Mustapha Mouhcine and Stella Seitz, and thank an anonymous referee for useful comments. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Spectroscopy of bright galaxies was available through the 2dFGRS survey on
the S11 field with 39 galaxies including the cluster A1364 at
and 14 galaxies on the CDFS field, most at
(Colless et al. 2001). More
observations on the A901 field provided a larger and deeper sample of 351
galaxies at
including the cluster A901/902 at
.
The
total number of independent redshifts is 404. Figure A.1 displays
the redshift quality of these bright galaxies. We find that 77% of the
galaxies have redshift errors
below 0.01 while three
objects (i.e. less than 1%) deviate by more than 5-
from the true
redshift.
Looking specifically at the A901/2 cluster system, we notice a consistent
small offset of
between photometric and
spectroscopic redshifts. The reason for such offsets lies in systematic
differences between the predicted and measured photometry of the galaxies.
Measurements can be off due to small photometric calibration errors, and
colour predictions can be off due to slightly unrealistic template SEDs or
even slightly wrong filter transmission curves. At
nm, the
offset corresponds to a 2.5 nm error in estimating the location of spectral
features from the multi-band photometry. However, another galaxy cluster,
Abell 1384 lies partially within the COMBO-17 S11 field at
and is estimated with a larger offset of
in the opposite direction, suggesting that field-dependent errors
in the calibration are more relevant than filter curve errors.
Spectroscopy of fainter galaxies is available via the VIMOS VLT Deep
Survey (VVDS, Le Fèvre et al. 2004) on the CDFS field. Using galaxies with VVDS redshift reliability of 95% or greater, we find five outliers among 334 galaxies (1.5%) at R<23 with true redshift errors on the order of 0.1,
while the remaining 98% have a distribution of photo-z errors
with a mean offset of
and an rms scatter of
.
The redshift grid for the galaxy colour library covers the range from
z=0 to z=1.40 in 177 steps. These are equidistant on a
scale with steps of 0.005 and of course limit the redshift resolution
when reconstructing galaxy density features in redshift space. According
to the sampling theorem, features in redshift space can be recovered if
their wavelength is at least twice as large as a grid step. Thus, we have
to expect that features with wavelength
will be
smoothed by our redshift estimation even under perfect conditions where
systematic problems in photometric calibration or SED match are absent.
This will not significantly restrict the power of our dataset,
because our redshift estimation is probably not consistently much more
accurate than 0.01 across all redshifts and SEDs we are interested in.
Galaxies with passively evolving old stellar populations have traditionally been selected from colour-magnitude diagrams. In the absence of any prior redshift information, observed-frame colours allow to search for clusters of galaxies using a red-sequence selection (e.g., Gladders & Yee 2000, 2005). This method capitalizes on the fact, that usually galaxies in any cluster red-sequence show redder observed-frame colours than foreground galaxies. If redshifts and rest-frame data are available, the cluster population can be separated from the field with vastly improved clarity and can include blue cluster members as well.
In rest-frame colour-magnitude diagrams (at least anywhere at z<1) the galaxy population appears bimodal with a narrow red-sequence and a wide blue cloud (Bell et al. 2004; Baldry et al. 2004; Strateva et al. 2001), provided a suitable colour index is used. The selection of an old stellar population via a red-sequence cut works best when a colour index is used that depends most sensitively on mean stellar age, i.e. a colour index made from two passbands enclosing the 4000 Å-break, such as rest-frame U-V. Colour indices made from two passbands which reside both on one side of this break, such as rest-frame R-I or NUV colour indices depend to a lesser degree on age and to an increased degree on dust reddening.
We illustrate this conclusion with colour-magnitude diagrams of A901,
where the location of the dust-free old clump is highlighted. Here, we would
like to suppress possible errors introduced by translating measured data into
rest-frame quantities and plot measured magnitudes and colours instead. At
,
the observed-frame B-R corresponds approximately to rest-frame
U-V and R to MV (except for a zeropoint shift). Enclosing the 4000 Å
-break we observe the strongest possible sensitivity to age and find the red
sequence at the red end of the galaxy distribution (see Fig. B.1).
However, a Butcher-Oemler-style red-sequence cut defined by a line parallel to
the colour-magnitude relation, but
bluer, still selects a number of
younger, dust-reddened systems in our cluster sample.
If we use the colour index V-I instead, which corresponds approximately to
the relative impact of dust reddening on the colour is
increased compared to that of stellar age. As a result, the red sequence moves
bluewards, further into the bulk of the galaxy population. A Butcher-Oemler
style selection will now be contaminated even more by dust-reddened galaxies.
However, using more detailed photometric information as in this paper helps to
achieve a much cleaner selection of dust-free old galaxies.