Galaxy morphologies and environment in the Abell 901/902 supercluster
  from COMBO-17
Mon. Not. R. Astron. Soc. 000, 1–7 () Printed 23 July 2021 (MN LATEX style file v2.2)
Galaxy morphologies and environment in the Abell 901/902
supercluster from COMBO-17
K. P. Lane1?, M. E. Gray1, A. Aragón-Salamanca1, C. Wolf2, K. Meisenheimer 3
1. School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham, NG7 2RD
2. Department of Physics, Denys Wilkinson Bldg., University of Oxford, Keble Road, Oxford OX1 3RH
3. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany
ABSTRACT
We present a morphological study of galaxies in the A901/902 supercluster from the
COMBO-17 survey. A total of 570 galaxies with photometric redshifts in the range 0.155 <
zphot < 0.185 are visually classified by three independent classifiers to MV = −18. These
morphological classifications are compared to local galaxy density, distance from the nearest
cluster centre, local surface mass density from weak lensing, and photometric classification.
At high local galaxy densities log Σ10/Mpc2 > 1.5 a classical morphology-density relation is
found. A correlation is also found between morphology and local projected surface mass den-
sity, but no trend is observed with distance to the nearest cluster. This supports the finding that
local environment is more important to galaxy morphology than global cluster properties. The
breakdown of the morphological catalogue by colour shows a dominance of blue galaxies in
the galaxies displaying late-type morphologies and a corresponding dominance of red galax-
ies in the early-type population. Using the 17-band photometry from COMBO-17, we further
split the supercluster red sequence into old passive galaxies and galaxies with young stars and
dust according to the prescription of Wolf et al. (2005). We find that the dusty star-forming
population describes an intermediate morphological group between late-type and early-type
galaxies, supporting the hypothesis that field and group spiral galaxies are transformed into
S0s and, perhaps, ellipticals during cluster infall.
Key words: galaxies: clusters: general — galaxies: evolution — galaxies: interactions
1 INTRODUCTION
The precise role that environment plays in transforming the mor-
phological and star-formation properties of galaxies as they are
accreted onto groups and clusters remains unclear. Well-known
correlations of cluster properties with environment hinting at evo-
lutionary effects include the cluster morphology-density relation
(Dressler 1980; Dressler et al. 1997) and the increasing fraction
of blue galaxies in clusters at higher redshift (Butcher & Oemler
1978, 1984). A galaxy’s encounters with other galaxies, with a hot
intracluster medium (ICM) or with a tidal cluster potential could all
be pathways to morphological alteration. However, it is also possi-
ble that galaxies in the densest regions formed earlier, evolved more
quickly, and thus display more mature evolutionary states.
Dressler et al. (1997) find that at z ∼ 0.5 the fraction of S0s
is significantly lower than at present, with a proportional increase
in the spiral fraction. As the fraction of ellipticals is already sim-
ilar to that found locally, it is surmised that the low-z S0 popula-
tion formed chiefly from spirals. One way to investigate the effect
of environment is to search for transitional objects in the process
of transformation. For example, ‘passive spirals’ displaying spiral
? email: ppxkl@nottingham.ac.uk
arms but no signatures of star formation could be identified as an
intermediary stage between spirals and S0s (Goto et al. 2003; Pog-
gianti 2004). It is therefore important to examine the variation of
morphological populations with environment (local mass, gas, and
galaxy densities) to determine the physical processes at work.
Historically both relaxed and irregular clusters have been the
focus of morphological analysis. In fact Dressler (1980) showed
that the morphology-density relation is present in both. This im-
plies that morphological segregation is already in progress before
cluster virialisation, if heirarchical models of structure formation
are to be believed. An advanced stage of evolution will lead to clus-
ter properties being strongly correlated, and corresponding signa-
tures of different environmental influences may then be difficult to
untangle. In order to decouple the effects of mass density, galaxy
density, cluster-centric distance and gas content on galaxy mor-
phologies it is necessary to examine systems still in the process
of formation. In a system that has not yet reached equilibrium, the
different components of the structure may still be segregated.
To this end we morphologically classify galaxies in the Abell
901(a,b)/902 supercluster, a structure consisting of three clusters
and associated groups all within 5 × 5 Mpc at z = 0.17. We
use imaging data from the COMBO-17 survey of this region (Wolf
et al. 2001, 2004), which includes observations from the ESO/MPG
c© RAS
2 K. P. Lane et al
Wide-Field Imager in broad-band UBV RI and a further 12
medium-band optical filters. The 17-band photometry provides pre-
cision photometric redshifts (mean error σz/(1 + z) < 0.01 for
R < 20 galaxies) and spectral energy distributions (SEDs). Addi-
tionally, a deep R-band image (R < 25.5) with 0.7′′seeing pro-
vides excellent image quality for visual classifications and weak
gravitational lensing. Further multiwavelength coverage from X-
ray to MIR of the 0.5◦ × 0.5◦ field includes observations with
XMM-Newton, GALEX, Spitzer, a NIR extension to COMBO-17
using Omega2000 on Calar Alto, plus spectra of the 300 brightest
cluster members from 2dF. This extensive data set makes the su-
percluster a uniquely well-positioned subject for detailed studies of
galaxy evolution. The utility of this field will be greatly extended
by a forthcoming HST mosaic as part of the STAGES survey.
Additional interest comes from the fact that the supercluster
is dynamically complex, with no clear scaling relations between
mass from weak lensing maps (Gray et al. 2002; Taylor et al. 2004),
galaxy number density, velocity dispersion, and X-ray luminosity
(Gray et al, in prep). This provides an excellent opportunity to in-
vestigate which environmental properties have the greatest influ-
ence on galaxy evolution, bearing in mind the important caveat
that all such quantities are seen in projection. Here we investi-
gate correlations of visually classified galaxy morphologies with
environment. Throughout we use a concordance cosmology with
Ωm = 0.27, Ωλ = 0.73, and H0 = 71 km s
−1 Mpc−1 so that 1
arcmin = 168 kpc at the redshift of the supercluster.
2 VISUAL GALAXY CLASSIFICATION
The cluster sample was chosen by a 0.155 < z < 0.185 cut
in photometric redshift at an initial absolute magnitude cut of
MV = −19 (all magnitudes are Vega). According to Wolf et al.
(2005), σz < 0.01 for this sample gives 99% completeness, if a
Gaussian distribution is assumed. Using the same redshift range
but increasing the magnitude depth to MV = −18 increases the
error in the photo-z to σ ≈ 0.015 and therefore the completeness is
∼ 68% at this luminosity limit. The above sample of cluster galax-
ies is estimated to have ∼ 60 non-cluster contaminants (Wolf et al.
2005). This represents 8.6% of our sample and so only introduces
small uncertainties into our results.
Classifications were performed independently by three of the
authors (KPL, MEG, AAS) and combined in order to reduce clas-
sifier bias. Galaxies were classified according to de Vaucouleurs’
T-type scheme, in which -5 is elliptical, -2 = S0, 1 = Sa, 2 = Sab, 3
= Sb, 4 = Sbc, 5 = Sc, 6 = Scd, 7 = Sd, 8 = Sdm, 9 = Sm/Irr (de Vau-
couleurs 1959). However, in terms of real ability to discriminate by
the classifiers this version of the T scale is distorted: for example,
a gap of three between E and S0s is far too wide. We therefore
adopted an alternative T-type scale where the difference between
adjacent T-types is set to one and the gap between a given Hub-
ble type and its intermediary (e.g. Sa/Sab) is taken as a difference
of 0.5. Additionally, the scheme allows combined classifications in
the case where a classifier cannot positively separate two adjacent
T-types (the most common example being E/S0 or S0/E). The rel-
ative weighting assigned to the primary and secondary choices is
discussed below. Comments were recorded if the morphology was
abnormal in any way and galaxies were flagged if they showed vi-
sual signs of asymmetry, merger, or interaction.
Combining the three classification sets gives some measure
of the reliability of the classifications. Fig. 1 shows the fraction
of galaxies with a disagreement of more than two alternative T-
Figure 1. Effects of magnitude on classification precision. The fraction of
galaxies with a spread in classification of > 2 alternative T-types from 3
independent classifiers is shown as a function of absolute magnitude. By
MV = −18 the fraction of galaxies with a spread in classifications > 2 is
almost 20%.
types over the three independent classifications. The level of dis-
agreement increases with greater magnitude as the galaxies be-
come fainter and their structure harder to discern. Above an abso-
lute magnitude of -18 the disagreement of > 2 alternative T-types
reaches ∼ 20% of galaxies. A magnitude limit of MV = −18
was therefore adopted for further morphological analysis resulting
in 570 galaxies.
These three classification sets were combined into one over-
all set according to a set of rules based on the prescription used by
the EDisCS group (Desai et al. 2006). For each galaxy, the final
classification was computed by an equally weighted combination
of the three sets. In the case of a combined classification where
a classifier was unable to discriminate between adjacent T-types,
the primary and secondary classifications were assigned 3/4 and
1/4 of that classifier’s weighting, respectively. The T-type with the
highest combined weighting was taken as the final classification for
that galaxy. If two T-types had equal combined weightings the fi-
nal classification was randomly selected between them. Finally, in
the case where there were more than two equally weighted types
the differences between the types were computed. If one difference
was 6 3 on the alternative T-type scale the final classification was
again chosen randomly between the two types spanning this gap.
Otherwise if there was more than one gap of this size or none 6 3
then the median of the equally highly-weighted T-types was taken
as the final classification. The final Hubble type classification cata-
log was comprised of 275 ellipticals, 126 S0, 59 Sa, 15 Sab, 49 Sb,
8 Sbc, 9 Sc, 2 Scd, 19 Sm/Irr and 6 unknown galaxies. For use in
the following morphology-environment studies spirals and Irr were
binned together (167 in all), but given the small numbers it would
make no difference if we considered only pure spirals.
The visual distinction between an S0 galaxy and an elliptical
is difficult, especially if the S0 is face on. Following Dressler et al.
(1997) we gauge the reliabilty of our S0/E separation by compar-
ing the ellipticities of our classified galaxies to the ellipticities of
c© RAS, MNRAS 000, 1–7
Galaxy morphologies and environment in the Abell 901/902 supercluster from COMBO-17 3
Figure 2. Comparison of the ellipticities of the samples classified as Ellip-
ticals and S0s in the A901/2 field with the ellipticities of the corresponding
morphological samples as found in the Coma cluster (Andreon et al. 1996).
The good agreement between the distributions of ellipticities in our sam-
ples and those in the Coma cluster, combined with the different ellipticity
distributions of our Elliptical and S0 samples, provides some confidence in
our ability to separate Elliptical and S0 classes.
galaxies in the Coma cluster (Andreon et al. 1996, see Fig 2). El-
lipticities were measured from the R-band image using SExtractor
(Bertin & Arnouts 1996). Kolmogorov-Smirnov tests give confi-
dence intervals of 98.9% for ellipticals and 53.1% for S0s being
drawn from the same populations as their Andreon et al. (1996)
counterparts. This compares with an ∼ 10−6 confidence that our
ellipticals and S0s are drawn from the same population. These re-
sults provide some confidence that we have reliably separated these
two classes.
3 LINKING MORPHOLOGY TO CLUSTER
ENVIRONMENT
The morphology-density, or Σ − T , relation has been observed
in a large range of galaxy environments, from rich cluster cores,
groups, to field densities (e.g. Postman & Geller 1984; Treu et al.
2003). Several physical mechanisms may be responsible for trans-
forming galaxy morphologies. These will be effective in different
regimes: e.g. in groups where galaxy density increases over the
field, but where relative velocities are still low, galaxy-galaxy in-
teractions including major mergers may play a large role (Barnes
1992). The high velocities reached by galaxies in the cores of rich
clusters make mergers less likely, but increase the efficacy of high-
speed interactions such as harassment (Moore et al. 1999). Like-
wise, cluster-specific mechanisms such as tidal stripping (Merritt
1983) or the removal of a galaxy’s hot (Larson et al. 1980) or cold
(Gunn & Gott 1972) gas by ram-pressure processes require a steep
potential or high ICM densities and so are more likely to take effect
in the inner regions of clusters.
In light of the many physical mechanisms that may be at work
in different regimes, we examine galaxy morphology as a function
of several different proxies for ‘environment’: local galaxy density,
projected mass density from lensing, and distance to the nearest
cluster centre.
3.1 Linking morphology to local projected galaxy density
The projected local galaxy density, Σ10, was found by calculating
the area encompassed by the galaxy in question and its 9 nearest
neighbours. Only galaxies to MV = −18 were used when calcu-
lating Σ10 as fainter galaxies will decrease completeness.
Galaxies lying nearer to the edge of the image field than their
9th nearest neighbour were removed from the catalogue, 37 in all.
This was to avoid anomalously low densities resulting from missing
neighbouring galaxies outside the field-of-view.
Due to uncertainties in the photometric redshifts (σ ≈ 0.015
at the magnitude limit used in this study,MV < −18), we estimate
that ∼ 95 potential field galaxies cannot be ruled out as cluster
members. This implies the minimum density we can measure is ∼
3.6Mpc−2. However, this is much smaller than the lowest densities
considered in our study (∼ 25Mpc−2).
The Σ−T relation (Fig. 3) shows a strong increase in the frac-
tion of ellipticals at high densities, a corresponding fall in the frac-
tion of spirals and the S0 fractions show no correlation. This is the
classical morphology-density relation as seen in numerous other
studies (e.g. Oemler 1974; Dressler 1980; Dressler et al. 1997; Treu
et al. 2003). Error bars are multinomial and were determined using
Monte Carlo simulations.
By analysing data in the range 0 < z < 1, including Dressler
(1980) at z ∼ 0 and Dressler et al. (1997) at z ∼ 0.5, Smith et al.
(2005) and Postman et al. (2005) find that the gradient of the early
type Σ−T relation increases with lowering z due to reducing num-
bers of spirals and increasing numbers of S0s, especially in high
density regions. It is difficult to compare the results presented in
Fig. 3 with these studies because they use depths comparable to that
used by Dressler (1980). To enable a comparison we cut our sample
to the same depth as used by Dressler (1980) (MV = −19.6), after
correcting for differences in the cosmology used. Doing so reduces
the number of sources in our classified sample by more than a fac-
tor of 2 which increases the error bars to point where no trends can
be seen. However it is noted that in our samples there are smaller
fractions of S0s and larger fractions of Es when compared with
c© RAS, MNRAS 000, 1–7
4 K. P. Lane et al
Dressler (1980). This could be due to cluster to cluster variation or
systematic differences in classification.
If we compare our full sample with that of Dressler (1980)
relative to the cluster core Σ10, the gradient in the Σ − T relation
for our elliptical population is shallower than found in the highest
density regions by Dressler (1980). There are also smaller fractions
of S0s and larger fractions of spirals as compared with Dressler
(1980). This is in agreement with the findings of Smith et al. (2005)
and Postman et al. (2005). At lower densities we find larger frac-
tions of ellipticals than Dressler (1980). That our fractions of S0s
are so much smaller than Dressler (1980), and that our elliptical
fractions are so much larger, is unexpected. However, S0 fraction
does vary by large amounts from cluster to cluster and the crite-
ria used to separate E and S0 classes varies from study to study.
We also note that the fraction of early types (elliptical + S0) in our
highest density bins is in keeping with the positive trend found in
the fraction of early type galaxies with decreasing redshift in high
density regions (Smith et al. 2005). Again it is noted that this com-
parison is made using our full sample to MV = −18 to enable
sample errors to be reduced to the point where trends can be seen.
With the adopted cosmology, 1 arcsecond corresponds to
2.80kpc at the cluster redshift. Similarly the seeing limited resolu-
tion of our image is 1.96kpc at the cluster redshift. This compares
with ∼ 700pc for Postman et al. (2005), using HST ACS data, at
z ∼ 1, and ∼ 500pc for Dressler et al. (1997), using HST WFPC2
data, at z ∼ 0.5. Our resolution may result in later type spirals be-
ing classified as earlier types since fine structure will be harder to
discern. This situation will be improved by using HST ACS data
obtained as part of the STAGES project.
Fasano et al. (2000) use ground based data with resolution 2-4
kpc at the redshifts of their clusters (0.1 . z . 0.25). Again no
comparisons can be drawn when our sample is cut to the same abso-
lute magnitude (MV = −20). However, when comparing our full
sample we find good agreement between our MD relation and the
MD relation found by Fasano et al. (2000) in their high elliptical-
concentration clusters. Our data fits the trend of rising S0 fraction,
falling Sp fraction and no evolution in the fraction of E with red-
shift. We also find our data to be consistent with the rising trends
in the NS0/NE and NS0/Nsp fractions with lowering redshift, as
presented in Fasano et al. (2000).
Note that the dynamic range in our Σ10 measurements is
somewhat smaller than in the above studies, particularly at low
densities. However, this does not have a significant effect on the
comparisons discussed above.
3.2 Linking morphology to projected cluster mass
As the mass of a cluster is made up predominantly of dark mat-
ter, one might expect various environmental properties, such as lo-
cal galaxy density and the intracluster medium (ICM), to trace the
potential wells described by the dark matter mass of the cluster.
This may well be true in a virialised cluster where there is a po-
sitional degeneracy between such environmental factors, however
maps of projected mass and galaxy distribution (Gray et al. 2002)
and extended X-ray emission (Gray et al, in prep) show that in the
A901/902 supercluster such scaling relations are not self-consistent
from cluster to cluster. For example, Abell 901b displays a promi-
nent mass peak and LX = 2.35 × 1044 erg s−1, yet is relatively
deficient in galaxy numbers. This then provides an opportunity to
ascertain if cluster mass has a direct effect on galaxy morphology,
or whether it is merely a tracer of other morphology affecting envi-
ronmental properties.
Figure 3. Morphological type, as a fraction of the total, with increasing
local density. At high density there is a clear upward trend in ellipticals and
a downward trend in spirals. There appears to be no trend in S0s. We ig-
nored bins with fewer than 5 galaxies due to the large sampling errors. This
corresponds to log Σ10 < 1.3. The upper panel shows the total number of
galaxies in each density bin.
The projected surface mass density for this region was recon-
structed by Gray et al. (2002) from an analysis of weak lensing of
faint galaxies in the same COMBO-17 image. The surface mass is
measured as a dimensionless quantity κ, where κ = Σ/Σcrit is the
ratio of the projected surface mass density to the critical surface
mass density for lensing for a fixed source and lens redshift. For
the supercluster lens at z = 0.17 and a population of faint lensed
galaxies with 〈z〉 ∼ 1 we have Σcrit = 5.0 × 1015hM� Mpc
The Gray et al. (2002) map includes smoothing with a Gaussian
of σ = 60 arcsec, and the rms noise in the map was estimated as
σκ = 0.027 through simulations.
Fig. 4 shows a clear relation between projected mass and mor-
phology (a κ− T relation) similar to the observed Σ− T relation,
but only at high mass densities (corresponding to the 3σ κ error
regime). An upward trend with κ is found in ellipticals and a down-
ward trend in S0s, however spirals do not show any correlation with
projected mass. The different trends observed between Σ10 and κ
suggest that both are tracers of morphology. Whether they are in-
depenent or, as is more likely, they are tracers of different aspects
of the same environmental driver for morphology, remains to be
determined. Fewer than 30% of galaxies in the highest κ bins are
classified as spirals. This is analogous to the star-formation–κ re-
lation found for the supercluster by Gray et al. (2004), where the
highest density regions were found to be populated almost exclu-
sively by galaxies with quiescent SEDs.
3.3 Linking morphology to cluster radius
Any correlation between clustercentric radius and the morphology
of a galaxy (R − T relation) will most likely be a reflection of
global rather than local properties of the cluster since radius is not
a localised quantity.
c© RAS, MNRAS 000, 1–7
Galaxy morphologies and environment in the Abell 901/902 supercluster from COMBO-17 5
Figure 4. Morphological type, as a fraction of the total, with increasing
projected surface mass density, κ, from lensing. At high surface density
an upward trend in ellipticals and a downward trend in S0s is observed. No
clear trend is found in the spiral population. The upper panel shows the total
number of galaxies in each κ bin. The bin width corresponds to the noise of
the κ maps, σκ = 0.027.
The clustercentric radius is the distance to the nearest cluster,
where the cluster centre was defined as the peak of the κ map, al-
though BCGs could have been used without much change. Fig. 5
shows that there is no clear trend between galaxy morphology and
cluster radius, with only a small rise in elliptical fractions and a
small decrease in late-type and S0 fractions at small radii. These
small radii correspond to regions which would be the least affected
by the large scale cluster merger. This result may then reflect the
virialized cluster cores where radius is degenerate with projected
mass.
The presence of a relation between morphology and lo-
cal galaxy density, combined with this apparent lack of a rela-
tion between morphology and clustercentric radius adds further
weight to the hypothesis that local conditions have more effect on
galaxy morphology than global cluster properties. Previous stud-
ies (Dressler et al. 1997) have shown that any radial dependence of
morphology is most likely a reflection of the Σ − T relation due
to the one-to-one correspondence between density and radius in re-
laxed clusters. For the A901/902 system in particular the cluster
properties and radius are more decoupled than in relaxed systems
due to the dynamical complexity. However, one caveat relevant to
both the Σ − T and R − T correlations is the possibility of pro-
jection effects, particularly in the region between the A901a and
A901b clusters. This possibility has been checked by masking out
this region and is found to have no appreciable effect on the above
results and hence is ruled out as a source of uncertainty.
4 LINKING MORPHOLOGY TO PHOTOMETRIC
CLASSIFICATION
In Wolf et al. (2005) the Abell 901/902 cluster galaxy sample ex-
amined here was divided into three subpopulations: red, passively
Figure 5. Morphological type, as a fraction of the total, with increasing
distance to nearest cluster centre. No clear trend is observed with increasing
radial distance. Data points with zero sample size have undetermined errors.
evolving galaxies; blue star-forming galaxies; and a previously un-
known third population of red galaxies revealed by the 17-band
photometry. This third population consists of cluster galaxies also
located along the cluster red sequence, but containing significant
amounts of young stars and dust (hereafter referred to as ‘dusty red
galaxies’).
Examining the photometric class of each morphological type,
Fig. 6 shows a clear trend. The majority of morphologically early-
type galaxies (E,S0) are photometrically passive and red. Late-type
galaxies (Sb,Sc) are predominantly blue. The third population of
dusty red galaxies, on the other hand, shows a distinct distribution
of intermediate morphologies. In this case intermediate types have
been binned with the next highest integer alternative T-type for clar-
ity, e.g. Sab is binned with Sb, Sbc with Sc and so on.
Two possible origins for this intermediate population of dusty
red galaxies are posed by Wolf et al. (2005). Firstly, that they orig-
inate in the blue cloud and are in a state of being transformed into
red cluster galaxies or, secondly, that they are the result of mi-
nor mergers of infalling galaxies with established cluster galaxies.
To try and distinguish between these two formation scenarios the
merger or interaction state of each galaxy was noted.
Of the classified galaxies which are photometrically found to
be dusty and red, only 14.2±9.4% were found by at least one clas-
sifier to be in a state of interaction with a neighbouring galaxy or
undergoing a merger. This compares to 10.6±6.5% of the clas-
sified passive red galaxies and 27.5±8.5% of the classified blue
galaxies. Within the uncertainties the three groups have consistent
merger/interaction fractions, however, the low fraction of interac-
tions/mergers for dusty red galaxies is inconsistent with a minor
merger scenario for their formation. However, it should be noted
that a large fraction of mergers may go undetected by a visual mor-
phological analysis since this technique is only sensitive to asym-
metries or tidal features in morphology which may not be present
in a minor merger at scales larger than our resolution (∼ 2 kpc).
c© RAS, MNRAS 000, 1–7
6 K. P. Lane et al
Therefore the minor merger scenario for formation of dusty red
galaxies cannot be ruled out, but does look unlikely.
The dusty red galaxies represent a significant proportion of the
overall galaxy population (22.4%) and do not appear to be a sub-
set of the blue or passive red galaxy populations. The differences
in morphology are paralleled by differences in average spectra and
spatial distributions shown in Wolf et al. (2005). In particular, they
occupy regions of medium densities, avoiding high densities nearer
the cluster core as well as low density regions in the cluster periph-
ery (Wolf et al. 2005).
These pieces of evidence would then suggest that one ma-
jor route in which infalling galaxies can be incorporated into the
cluster is via transformative processes that do not necessarily in-
volve mergers. Galaxies entering the cluster may have their star-
formation ultimately quenched, but after an initial phase of en-
hanced star-formation (Milvang-Jensen et al. 2003; Bamford et al.
2005). A triggered starburst, possibly via interaction with the ICM,
would introduce dust via supernovae feedback to produce the tran-
sitional dusty, red phase. Ultimately the gas supply will be ex-
hausted and star formation quenched, leaving the remaining stars
to evolve passively on the red sequence.
Dressler et al. (1999) and Poggianti et al. (1999) find similar
spectroscopic populations of dusty starburst galaxies, or e(a) galax-
ies, at z ∼ 0.4 − 0.5. They attribute these to the progenitors of
post-starburst k+a/a+k galaxies. For the COMBO-17 A901/2 field
spectra have been obtained for 64 cluster galaxies using the 2dF
instrument (see Wolf et al. 2005). The average spectra for dusty
red galaxies is seen to be inconsistent with that of k+a galaxies.
The weak [OII] emission as well as Hδ absorption observed in
the average spectra of these dusty red galaxies are consistent with
e(a) type galaxies. To find such a large fraction (22.4%) of poten-
tial k+a progenitor galaxies at z ∼ 0.17 is surprising given that
Dressler et al. (1999) find that 18% of their cluster sample exhibit
k+a/a+k spectral types at z ∼ 0.5. However, the continuing cluster-
cluster merger seen in the A901/2 system could well produce an
increased incidence of e(a) galaxies due to the large number of in-
falling galaxies.
5 CONCLUSIONS
The complex dynamics present in the Abell 901/902 systems pro-
vide an ideal testing ground for the distinction between the local
and global processes driving galaxy morphology. In this paper we
have examined relations between visual galaxy morphologies and
local measures of environment including galaxy density, projected
surface mass density from lensing, and clustercentric radius. The
presence of a strong Σ − T and κ − T relations and absence of
a corresponding Σ − R relation shows that despite the large scale
complexities of the clusters, local conditions are still well corre-
lated to morphology.
Furthermore, the photometric breakdown of morphologies
provides a tantalising glimpse of galaxy transformation during in-
fall, in action. By determining the interaction and merger state of
each classified galaxy it was shown that the population of red se-
quence galaxies with young stars and dust of Wolf et al. (2005)
is not likely to be explained by minor mergers of infalling galax-
ies with cluster members. It is more plausible then that the dusty
red galaxies are experiencing additional, cluster-specific phenom-
ena during infall, causing their star-formation to become dust ob-
scured and reddened. This would then support a picture of cluster
formation in which accreted galaxies can be transformed through
Figure 6. The photometric colour of each morphological type as a fraction
of the total type population. Dusty red galaxies appear to form an interme-
diate regime between star-forming late-type galaxies and early-type passive
galaxies. Data points with very large sampling error have been omitted.
processes other than major or minor mergers, most likely through
induced star-bursts and associated dust obscuration before the gas
supply is stripped and/or exhausted, and star-formation stops.
This study will be extended in the forthcoming STAGES sur-
vey of the A901/902 supercluster. The survey consists of an 80 or-
bit mosaic using HST/ACS of the 0.5× 0.5 degree region and will
be combined with the 17-band photometric redshifts and other de-
tailed multiwavelength data sets. This mosaic will be used for mor-
phological classifications not only for the bright end of the cluster
luminosity function probed here, but also the dwarf galaxy popula-
tion, which may be more sensitive to environmental processes due
to their lower gravitational potentials. In this way we will build up
an even more detailed picture of galaxy evolution within a complex
environment.
ACKNOWLEDGMENTS
KPL was supported by a PPARC studentship. MEG was supported
by an Anne McLaren Research Fellowship from the University of
Nottingham. C. Wolf was supported by a PPARC Advanced fellow-
ship. Thanks go M. Merrifield and O. Almaini for useful and infor-
mative discussions. We thank the anonymous referee for comments
which greatly improved the reliability of the results presented.
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	Introduction
	Visual galaxy classification
	Linking morphology to cluster environment
	Linking morphology to local projected galaxy density
	Linking morphology to projected cluster mass
	Linking morphology to cluster radius
	Linking morphology to photometric classification
	Conclusions
