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We investigate the properties of optically passive spirals and dusty red galaxies in the A901/2 cluster complex at redshift ∼0.17 using rest-frame near-ultraviolet-optical spectral energy distributions, 24-μm infrared data and Hubble Space Telescope morphologies from the STAGES data set. The cluster sample is based on COMBO-17 redshifts with an rms precision of σcz≈ 2000 km s−1. We find that ‘dusty red galaxies' and ‘optically passive spirals' in A901/2 are largely the same phenomenon, and that they form stars at a substantial rate, which is only four times lower than that in blue spirals at fixed mass. This star formation is more obscured than in blue galaxies and its optical signatures are weak. They appear predominantly in the stellar mass range of log M*/M⊙=[10, 11] where they constitute over half of the star-forming galaxies in the cluster; they are thus a vital ingredient for understanding the overall picture of star formation quenching in clusters. We find that the mean specific star formation rate (SFR) of star-forming galaxies in the cluster is clearly lower than in the field, in contrast to the specific SFR properties of blue galaxies alone, which appear similar in cluster and field. Such a rich red spiral population is best explained if quenching is a slow process and morphological transformation is delayed even more. At log M*/M⊙ < 10, such galaxies are rare, suggesting that their quenching is fast and accompanied by morphological change. We note that edge-on spirals play a minor role; despite being dust reddened they form only a small fraction of spirals independent of environment

We present an analysis of V-band radial surface brightness profiles for spiral galaxies from the field and cluster environments using Hubble Space Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). We use a large sample of ∼330 face-on to intermediately inclined spiral galaxies and assess the effect of the galaxy environment on the azimuthally averaged radial surface brightness μ profiles for each galaxy in the outer stellar disc (24 < μ < 26.5 mag arcsec−2). For galaxies with a purely exponential outer disc (∼50 per cent), we determine the significance of an environmental dependence on the outer disc scalelength hout. For galaxies with a broken exponential in their outer disc, either down-bending (truncation, ∼10 per cent) or up-bending (antitruncation, ∼40 per cent), we measure the strength T (outer-to-inner scalelength ratio, log10 hout/hin) of the μ breaks and determine the significance of an environmental dependence on break strength T. Surprisingly, we find no evidence to suggest any such environmental dependence on either outer disc scalelength hout or break strength T, implying that the galaxy environment is not affecting the stellar distribution in the outer stellar disc. We also find that for galaxies with small effective radii (re < 3 kpc) there is a lack of outer disc truncations in both the field and cluster environments. Our results suggest that the stellar distribution in the outer disc of spiral galaxies is not significantly affected by the galaxy environment

We present a high-resolution dark matter reconstruction of the z= 0.165 Abell 901/902 supercluster from a weak lensing analysis of the Hubble Space Telescope STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901α. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter ‘observations'

We study the links between star formation history and structure for a large mass-selected galaxy sample at 0.05 ≤ zphot ≤ 0.30. The galaxies inhabit a very broad range of environments, from cluster cores to the field. Using Hubble Space Telescope (HST) images, we quantify their structure following Hoyos et al., and divide them into disturbed and undisturbed. We also visually identify mergers. Additionally, we provide a quantitative measure of the degree of disturbance for each galaxy (‘roughness’). The majority of elliptical and lenticular galaxies have relaxed structure, showing no signs of ongoing star formation. Structurally disturbed galaxies, which tend to avoid the lowest density regions, have higher star formation activity and younger stellar populations than undisturbed systems. Cluster spirals with reduced/quenched star formation have somewhat less disturbed morphologies than spirals with ‘normal’ star formation activity, suggesting that these ‘passive’ spirals have started their morphological transformation into S0s. Visually identified mergers and galaxies not identified as mergers but with similar roughness have similar specific star formation rates and stellar ages. The degree of enhanced star formation is thus linked to the degree of structural disturbance, regardless of whether it is caused by major mergers or not. This suggests that merging galaxies are not special in terms of their higher-than-normal star formation activity. Any physical process that produces ‘roughness’, or regions of enhanced luminosity density, will increase the star formation activity in a galaxy with similar efficiency. An alternative explanation is that star formation episodes increase the galaxies’ roughness similarly, regardless of whether they are merger induced or not.

We present images of extended H alpha clouds associated with 14 member galaxies in the Coma cluster obtained from deep narrowband imaging observations with the Suprime-Cam at the Subaru Telescope. The parent galaxies of the extended H alpha clouds are distributed farther than 0.2 Mpc from the peak of the X-ray emission of the cluster. Most of the galaxies are bluer than g - r approximate to 0.5 and they account for 57% of the blue ( g - r < 0.5) bright ( r < 17.8 mag) galaxies in the central region of the Coma cluster. They reside near the red- and blueshifted edges of the radial velocity distribution of Coma cluster member galaxies. Our findings suggest that most of the parent galaxies were recently captured by the Coma cluster potential and are now infalling toward the cluster center with their disk gas being stripped off and producing the observed H alpha clouds.

We study the links between star formation history and structure for a large mass-selected galaxy sample at 0.05 ≤ zphot ≤ 0.30. The galaxies inhabit a very broad range of environments, from cluster cores to the field. Using Hubble Space Telescope (HST) images, we quantify their structure following Hoyos et al., and divide them into disturbed and undisturbed. We also visually identify mergers. Additionally, we provide a quantitative measure of the degree of disturbance for each galaxy (‘roughness’). The majority of elliptical and lenticular galaxies have relaxed structure, showing no signs of ongoing star formation. Structurally disturbed galaxies, which tend to avoid the lowest density regions, have higher star formation activity and younger stellar populations than undisturbed systems. Cluster spirals with reduced/quenched star formation have somewhat less disturbed morphologies than spirals with ‘normal’ star formation activity, suggesting that these ‘passive’ spirals have started their morphological transformation into S0s. Visually identified mergers and galaxies not identified as mergers but with similar roughness have similar specific star formation rates and stellar ages. The degree of enhanced star formation is thus linked to the degree of structural disturbance, regardless of whether it is caused by major mergers or not. This suggests that merging galaxies are not special in terms of their higher-than-normal star formation activity. Any physical process that produces ‘roughness’, or regions of enhanced luminosity density, will increase the star formation activity in a galaxy with similar efficiency. An alternative explanation is that star formation episodes increase the galaxies’ roughness similarly, regardless of whether they are merger induced or not.

We study the links between star formation history and structure for a large mass-selected galaxy sample at 0.05 ≤ zphot ≤ 0.30. The galaxies inhabit a very broad range of environments, from cluster cores to the field. Using Hubble Space Telescope (HST) images, we quantify their structure following Hoyos et al., and divide them into disturbed and undisturbed. We also visually identify mergers. Additionally, we provide a quantitative measure of the degree of disturbance for each galaxy (‘roughness’). The majority of elliptical and lenticular galaxies have relaxed structure, showing no signs of ongoing star formation. Structurally disturbed galaxies, which tend to avoid the lowest density regions, have higher star formation activity and younger stellar populations than undisturbed systems. Cluster spirals with reduced/quenched star formation have somewhat less disturbed morphologies than spirals with ‘normal’ star formation activity, suggesting that these ‘passive’ spirals have started their morphological transformation into S0s. Visually identified mergers and galaxies not identified as mergers but with similar roughness have similar specific star formation rates and stellar ages. The degree of enhanced star formation is thus linked to the degree of structural disturbance, regardless of whether it is caused by major mergers or not. This suggests that merging galaxies are not special in terms of their higher-than-normal star formation activity. Any physical process that produces ‘roughness’, or regions of enhanced luminosity density, will increase the star formation activity in a galaxy with similar efficiency. An alternative explanation is that star formation episodes increase the galaxies’ roughness similarly, regardless of whether they are merger induced or not.

We study the links between star formation history and structure for a large mass-selected galaxy sample at 0.05 ≤ zphot ≤ 0.30. The galaxies inhabit a very broad range of environments, from cluster cores to the field. Using Hubble Space Telescope (HST) images, we quantify their structure following Hoyos et al., and divide them into disturbed and undisturbed. We also visually identify mergers. Additionally, we provide a quantitative measure of the degree of disturbance for each galaxy (‘roughness’). The majority of elliptical and lenticular galaxies have relaxed structure, showing no signs of ongoing star formation. Structurally disturbed galaxies, which tend to avoid the lowest density regions, have higher star formation activity and younger stellar populations than undisturbed systems. Cluster spirals with reduced/quenched star formation have somewhat less disturbed morphologies than spirals with ‘normal’ star formation activity, suggesting that these ‘passive’ spirals have started their morphological transformation into S0s. Visually identified mergers and galaxies not identified as mergers but with similar roughness have similar specific star formation rates and stellar ages. The degree of enhanced star formation is thus linked to the degree of structural disturbance, regardless of whether it is caused by major mergers or not. This suggests that merging galaxies are not special in terms of their higher-than-normal star formation activity. Any physical process that produces ‘roughness’, or regions of enhanced luminosity density, will increase the star formation activity in a galaxy with similar efficiency. An alternative explanation is that star formation episodes increase the galaxies’ roughness similarly, regardless of whether they are merger induced or not.

We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at z∼ 0.165 has been the subject of an 80-orbit F606W Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) mosaic covering the full span of the supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX, Spitzer, 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues

We present a new automatic method to identify galaxy mergers using the morphological information contained in the residual images of galaxies after the subtraction of a smooth Sérsic model. The removal of the bulk signal from the host galaxy light is done with the aim of detecting the much fainter and elusive minor mergers. The specific morphological parameters that are used in the merger diagnostic suggested here are the residual flux fraction (RFF) and the asymmetry of the residuals [A(Res)]. The new diagnostic has been calibrated and optimized so that the resulting merger sample is very complete. However, the contamination by non-mergers is also high. If the same optimization method is adopted for combinations of other structural parameters such as the Concentration, Asymmetry, clumpineSs (CAS) system, the merger indicator we introduce yields merger samples of equal or higher statistical quality than the samples obtained through the use of other structural parameters. We investigate the ability of the method presented here to select minor mergers by identifying a sample of visually classified mergers that would not have been picked up by the use of the CAS system, when using its usual limits. However, given the low prevalence of mergers among the general population of galaxies and the optimization used here, we find that the merger diagnostic introduced in this work is best used as a negative merger test, that is, it is very effective at selecting non-merging galaxies. In common with all the currently available automatic methods, the sample of merger candidates selected is heavily contaminated by non-mergers, and further steps are needed to produce a clean merger sample. This merger diagnostic has been developed using the Hubble Space Telescope/ACS F606W images of the A901/902 multiple cluster system (z= 0.165) obtained by the Space Telescope A901/902 Galaxy Evolution Survey team. In particular, we have focused on a mass- and magnitude-limited sample (log M/M⊙ >