We explore the amount of obscured star-formation as a function of environment in the A901/902 supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with HST as part of the STAGES and GEMS surveys. We combine the COMBO-17 near-UV/optical SED with Spitzer 24µm photometry to estimate both the unobscured and obscured star formation in galaxies with M * > 10 10 M ⊙ . We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates per unit stellar mass similar to or lower than blue starforming galaxies. More than half of the red star-forming galaxies have low IR-to-UV luminosity ratios, relatively high Sersic indices and they are equally abundant at all densities. They might be gradually quenching their starformation, possibly but not necessarily under the influence of gas-removing environmental processes. The other 40% of the red star-forming galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific star formation rates and are more abundant at intermediate densities.Our results indicate that while there is an overall suppression in the star-forming galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.

Abstract. The FORS Deep Field project is a multi-colour, multi-object spectroscopic investigation of a ∼7 × 7 region near the south galactic pole based mostly on observations carried out with the FORS instruments attached to the VLT telescopes. It includes the QSO Q 0103-260 (z = 3.36). The goal of this study is to improve our understanding of the formation and evolution of galaxies in the young Universe. In this paper the field selection, the photometric observations, and the data reduction are described. The source detection and photometry of objects in the FORS Deep Field is discussed in detail. A combined B and I selected UBgRI JK s photometric catalog of 8753 objects in the FDF is presented and its properties are briefly discussed. The formal 50% completeness limits for point sources, derived from the co-added images, are 25.64, 27.69, 26.86, 26.68, 26.37, 23.60 and 21.57 in U, B, g, R, I, J and Ks (Vega-system), respectively. A comparison of the number counts in the FORS Deep Field to those derived in other deep field surveys shows very good agreement.

We explore the properties of 24 field early‐type galaxies in the redshift range 0.20 < z < 0.75 down to MB≤−19.30 in a sample extracted from the FORS Deep Field and the William Herschel Deep Field. Target galaxies were selected on the basis of a combination of luminosity, spectrophotometric type, morphology and photometric redshift or broad‐band colours. High signal‐to‐noise ratio intermediate‐resolution spectroscopy has been acquired at the Very Large Telescope, complemented by deep high‐resolution imaging with the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) and additional ground‐based multiband photometry. All galaxy spectra were observed under subarcsecond conditions and allow us to derive accurate kinematics and stellar population properties of the galaxies. To clarify the low level of star formation detected in some galaxies, we identify the amount of active galactic nuclei (AGN) activity in our sample using archive data of Chandra and XMM–Newton X‐ray surveys. None of the galaxies in our sample was identified as secure AGN source based on their X‐ray emission. The rest‐frame B‐ and K‐band scaling relations of the Faber–Jackson relation and the Fundamental Plane display a moderate evolution for the field early‐type galaxies. Lenticular (S0) galaxies feature on average a stronger luminosity evolution and bluer rest‐frame colours which can be explained that they comprise more diverse stellar populations compared to elliptical galaxies. The evolution of the FP can be interpreted as an average change in the dynamical (effective) mass‐to‐light ratio of our galaxies as 〈Δlog(M/LB)/z〉=−0.74 ± 0.08. The M/L evolution of these field galaxies suggests a continuous mass assembly of field early‐type galaxies during the last 5 Gyr, which gets supported by recent studies of field galaxies up to z∼ 1. Independent evidence for recent star formation activity is provided by spectroscopic ([O ii] emission, Hδ) and photometric (rest‐frame broad‐band colours) diagnostics. Based on the Hδ absorption feature we detect a weak residual star formation for galaxies that accounts for 5–10 per cent in the total stellar mass of these galaxies. The co‐evolution in the luminosity and mass of our galaxies favours a downsizing formation process. We find some evidence that our galaxies experienced a period of star formation quenching, possible triggered by AGN activity that is in good agreement with recent results on both observational and theoretical side.

Aims. Galaxy scaling relations such as the Tully-Fisher relation (between the maximum rotation velocity V max and luminosity) and the velocity-size relation (between V max and the disk scale length) are powerful tools to quantify the evolution of disk galaxies with cosmic time. Methods. We took spatially resolved slit spectra of 261 field disk galaxies at redshifts up to z ≈ 1 using the FORS instruments of the ESO Very Large Telescope. The targets were selected from the FORS Deep Field and William Herschel Deep Field. Our spectroscopy was complemented with HST/ACS imaging in the F814W filter. We analyzed the ionized gas kinematics by extracting rotation curves from the two-dimensional spectra. Taking into account all geometrical, observational, and instrumental effects, these rotation curves were used to derive the intrinsic V max .Results. Neglecting galaxies with disturbed kinematics or insufficient spatial rotation curve extent, V max was reliably determined for 124 galaxies covering redshifts 0.05 < z < 0.97. This is one of the largest kinematic samples of distant disk galaxies to date. We compared this data set to the local B-band Tully-Fisher relation and the local velocity-size relation. The scatter in both scaling relations is a factor of ∼2 larger at z ≈ 0.5 than at z ≈ 0. The deviations of individual distant galaxies from the local Tully-Fisher relation are systematic in the sense that the galaxies are increasingly overluminous toward higher redshifts, corresponding to an overluminosity ∆M B = −(1.2 ± 0.5) mag at z = 1. This luminosity evolution at given V max is probably driven by younger stellar populations of distant galaxies with respect to their local counterparts, potentially combined with modest changes in dark matter mass fractions. The analysis of the velocity-size relation reveals that disk galaxies of a given V max have grown in size by a factor of ∼1.5 over the past ∼8 Gyr, most likely through accretion of cold gas and/or small satellites. From scrutinizing the combined evolution in luminosity and size, we find that the galaxies that show the strongest evolution toward smaller sizes at z ≈ 1 are not those that feature the strongest evolution in luminosity, and vice versa.

We present a study of galaxy mergers and the influence of environment in the Abell 901/902 supercluster at z ∼ 0.165, based on 893 bright (R Vega 24) intermediate-mass (M * 10 9 M) galaxies. We use HST ACS F606W data from the Space Telescope A901/902 Galaxy Evolution Survey, COMBO-17, Spitzer 24 μm, and XMM-Newton X-ray data. Our analysis utilizes both a physically driven visual classification system and quantitative CAS parameters to identify systems which show evidence of a recent or ongoing merger of mass ratio >1/10 (i.e., major and minor mergers). Our results are (1) after visual classification and minimizing the contamination from false projection pairs, we find that the merger fraction f merge is 0.023 ± 0.007. The estimated fractions of likely major mergers, likely minor mergers, and ambiguous cases are 0.01 ± 0.004, 0.006 ± 0.003, and 0.007 ± 0.003, respectively. (2) All the mergers lie outside the cluster core of radius R < 0.25 Mpc: the lack of mergers in the core is likely due to the large galaxy velocity dispersion in the core. The mergers, instead, populate the region (0.25 Mpc < R 2 Mpc) between the core and the cluster outskirt. In this region, the estimated frequency of mergers is similar to those seen at typical group overdensities in N-body simulations of accreting groups in the A901/902 clusters. This suggests the ongoing growth of the clusters via accretion of group and field galaxies. (3) We compare our observed merger fraction with those reported in other clusters and groups out to z ∼ 0.4. Existing data points on the merger fraction for L L * galaxies in clusters allow for a wide spectrum of scenarios, ranging from no evolution to evolution by a factor of ∼5 over z ∼ 0.17-0.4. (4) In A901/902, the fraction of interacting galaxies, which lie on the blue cloud is 80% ± 18% (16/20) versus 34% ± 7% or (294/866) for non-interacting galaxies, implying that interacting galaxies are preferentially blue. (5) The average star formation rate (SFR), based on UV or a combination of UV+IR data, is enhanced by a factor of ∼1.5-2 in mergers compared to non-interacting galaxies. However, mergers in the A901/902 clusters contribute only a small fraction (between 10% and 15%) of the total SFR density, while the rest of the SFR density comes from non-interacting galaxies.