Abstract. Studies of distant galaxies have shown that ellipticals and large spirals (Schade et al. 1999, ApJ, 525, 31; Lilly et al. 1998, ApJ, 500, 75) were already in place 8 Gyr ago, leading to a very modest recent star formation (Brinchmann & Ellis 2000, ApJ, 536, L77) in intermediate mass galaxies (3−30 × 10 10 M ). This is challenged by a recent analysis (Heavens et al. 2004, Nature, 428, 625) of the fossil record of the stellar populations of ∼10 5 nearby galaxies, which shows that intermediate mass galaxies formed or assembled the bulk of their stars 4 to 8 Gyr ago. Here we present direct observational evidence supporting this findings from a long term, multi-wavelength study of 195 z > 0.4 intermediate mass galaxies, mostly selected from the Canada France Redshift Survey (CFRS). We show that recent and efficient star formation is revealed at IR wavelengths since ∼15% of intermediate mass galaxies at z > 0.4 are indeed luminous IR galaxies (LIRGs), a phenomenon far more common than in the local Universe. The star formation in LIRGs is sufficient in itself to produce 38% of the total stellar mass of intermediate mass galaxies and then to account for most of the reported stellar mass formation since z = 1. Observations of distant galaxies have also the potential to resolve their star formation and mass assembly histories. The high occurrence of LIRGs is easily understood only if they correspond to episodic peaks of star formation, during which galaxies are reddened through short IREs (infrared episodes). We estimate that each galaxy should experience 4 to 5 × (τ IRE /0.1 Gyr) −1IREs from z = 1 to z = 0.4, τ IRE being the characteristic timescale. An efficient and episodic star formation is further supported by the luminosity-metallicity relation of z ∼ 0.7 emission line galaxies, which we find to be on average metal deficient by a factor of ∼2 when compared to those of local spirals. We then examine how galaxy IREs can be related to the emergence at high redshift of the abundant population of galaxies with small size (but not with small mass), blue core and many irregularities. We show that recent merging and gas infall naturally explain both morphological changes and episodic star formation history in a hierarchical galaxy formation frame. We propose a simple scenario in which 75 ± 25% of intermediate mass spirals have recently experienced their last major merger event, leading to a drastic reshaping of their bulges and disks during the last 8 Gyr. It links in a simple manner distant and local galaxies, and gives account of the simultaneous decreases during that period, of the cosmic star formation density, of the merger rate, and of the number densities of LIRGs, compact and irregular galaxies, while the densities of ellipticals and large spirals are essentially unaffected. It predicts that 42, 22 and 36% of the IR (episodic) star formation density is related to major mergers, minor mergers and gas infall, respectively.

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 σ cz ≈ 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

We present the first results from the ongoing LAGER project (Lyman Alpha Galaxies in the Epoch of Reionization), which is the largest narrowband survey for z ∼ 7 galaxies to date. Using a specially built narrowband filter NB964 for the superb large-area Dark-Energy Camera (DECam) on the NOAO/CTIO 4m Blanco telescope, LAGER has collected 34 hours NB964 narrowband imaging data in the 3 deg 2 COSMOS field. We have identified 23 Lyman Alpha Emitter (LAE) candidates at z = 6.9 in the central 2-deg 2 region, where DECam and public COSMOS multi-band images exist. The resulting luminosity function can be described as a Schechter function modified by a significant excess at the bright end (4 galaxies with L Lyα ∼ 10 43.4±0.2 erg s −1 ). The number density at L Lyα ∼ 10 43.4±0.2 erg s −1 is little changed from z = 6.6, while at fainter L Lyα it is substantially reduced. Overall, we see a fourfold reduction in Lyα luminosity density from z = 5.7 to 6.9. Combined with a more modest evolution of the continuum UV luminosity density, this suggests a factor of ∼ 3 suppression of Lyα by radiative transfer through the z ∼ 7 intergalactic medium (IGM). It indicates an IGM neutral fraction x HI ∼ 0.4-0.6 (assuming Lyα velocity offsets of 100-200 km s −1 ). The changing shape of the Lyα luminosity function between z 6.6 and z = 6.9 supports the hypothesis of ionized bubbles in a patchy reionization at z ∼ 7.

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 multi-cluster system at z~0.165 has been the subject of an 80-orbit F606W HST/ACS mosaic covering the full 0.5x0.5 (~5x5 Mpc^2) span of the supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX, Spitzer, 2dF, GMRT, 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 dataset 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 environment we will be able to evaluate the relative importance of the dark matter halos, 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 Sersic 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.Comment: 29 pages, 22 figures; accepted to MNRAS. Full data release available at http://www.nottingham.ac.uk/astronomy/stage