Recent observations have demonstrated a significant growth in the integrated stellar mass of the red sequence since z=1, dominated by a steadily increasing number of galaxies with stellar masses M* < 10^11 M_sun. In this paper, we use the COMBO-17 photometric redshift survey in conjunction with deep Spitzer 24 micron data to explore the relationship between star formation and the growth of stellar mass. We calculate `star formation rate functions' in four different redshift slices, splitting also into contributions from the red sequence and blue cloud for the first time. We find that the growth of stellar mass since z=1 is consistent with the integrated star formation rate. Yet, most of the stars formed are in blue cloud galaxies. If the stellar mass already in, and formed in, z<1 blue cloud galaxies were to stay in the blue cloud the total stellar mass in blue galaxies would be dramatically overproduced. We explore the expected evolution of stellar mass functions, finding that in this picture the number of massive M* > 3x10^10 M_sun blue galaxies would also be overproduced; i.e., most of the new stars formed in blue cloud galaxies are in the massive galaxies. We explore a simple truncation scenario in which these `extra' blue galaxies have their star formation suppressed by an unspecified mechanism or mechanisms; simple cessation of star formation in these extra blue galaxies is approximately sufficient to build up the red sequence at M*<10^11 M_sun.Comment: 9 Pages; ApJ in pres

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z ∼ 0.24-0.80 (lookback time T b ∼ 3-7 Gyr) using ∼3600 (M 1 × 10 9 M ) galaxies with GEMS Hubble Space Telescope, COMBO-17, and Spitzer data. Our results are as follows. (1) Among ∼790 high-mass (M 2.5 × 10 10 M ) galaxies, the visually based merger fraction over z ∼ 0.24-0.80, ranges from 9% ± 5% to 8% ± 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of ∼0.5 Gyr, it follows that over T b ∼ 3-7 Gyr, ∼68% of high-mass systems have undergone a merger of mass ratio >1/10, with ∼16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous "major or minor" mergers. The average merger rate is ∼ a few ×10 −4 galaxies Gyr −1 Mpc −3 . Among ∼2840 blue-cloud galaxies of mass M 1.0 × 10 9 M , similar results hold. (2) We compare the empirical merger fraction and merger rate for high-mass galaxies to three Λ cold dark matter-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of 5 dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ∼3698 M 1.0 × 10 9 M galaxies, we find that the mean star formation rate (SFR) of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z ∼ 0.24-0.80. Visibly merging systems only account for a small fraction (<30%) of the cosmic SFR density over T b ∼ 3-7 Gyr. This complements the results of Wolf et al. over a shorter time interval of T b ∼ 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

We combine Spitzer 24 mm observations with data from the COMBO-17 survey for ∼15,000 galaxies 0.2 ! z ≤ 1 to determine how the average star formation rates (SFRs) have evolved for galaxy subpopulations of different stellar masses. In the determination of , we consider both the ultraviolet (UV) and the infrared (IR) luminosities, and ASFRS account for the contributions of galaxies that are individually undetected at 24 mm through image stacking. For all redshifts, we find that higher mass galaxies have a substantially lower specific SFR, , than lower mass ASFRS/AM S * ones. However, we find the striking result that the rate of decline in cosmic SFR with redshift is nearly the same for massive and low mass galaxies, i.e., not a strong function of stellar mass. This analysis confirms one version of what has been referred to as "downsizing," namely, that the epoch of major mass buildup in massive galaxies is substantially earlier than the epoch of mass buildup in low-mass galaxies. Yet it shows that star formation activity is not becoming increasingly limited to low-mass galaxies toward the present epoch. We argue that this suggests that heating by AGNpowered radio jets is not the dominant mechanism responsible for the decline in cosmic SFR since , which is z ∼ 1 borne out by comparison with semianalytic models that include this effect.

Context. This paper prepares a series of papers analysing the Intermediate MAss Galaxy Evolution Sequence (IMAGES) up to a redshift of one. Intermediate mass galaxies (M J ≤ −20.3) are selected from the Chandra Deep Field South (CDFS) for which we identify a serious lack of spectroscopically determined redshifts. Aims. Our primary aim in this study is therefore to obtain a sample of intermediate-mass galaxies with known spectroscopic redshift to be used for further analysis of their 3D-kinematics. We also intend to test whether this important cosmological field may be significantly affected by cosmic variance. Methods. The spectroscopic observations were carried out using VIMOS on the ESO VLT. The data reduction was done using a set of semi-automatic IRAF procedures developed by our team. Results. We have spectroscopically identified 691 objects including 580 galaxies, 7 QSOs, and 104 stars. The overall completeness achieved is about 76% for objects with I AB ≤ 23.5 in the CDFS after excluding instrumental failures. This study provides 531 new redshifts in the CDFS. It confirms the presence of several large-scale structures in the CDFS, which are found to be more prominent than in other redshift surveys. To test the impact of these structures in the GOODS-South field, we constructed a representative redshift catalog of 640 galaxies with I AB ≤ 23.5 and z ≤ 1. We then compared the evolution of rest-frame U, B, V, and K galaxy luminosity densities to the one derived from the Canada France Redshift Survey (CFRS). The GOODS South field shows a significant excess of luminosity densities in the z = 0.5-0.75 range, which increases with the wavelength, reaching up to 0.5 dex at 2.1 µm. Stellar mass and specific star formation evolutions might be significantly affected by the presence of the peculiar large-scale structures at z = 0.668 and at z = 0.735, which contain a significant excess of evolved, massive galaxies when compared to other fields. Conclusions. This leads to a clear warning about the results based on the CDFS/GOODS South fields, especially those related to the evolution of red luminosity densities, i.e. stellar mass density and specific star formation rate. Photometric redshift techniques, when applied to that field, are producing quantities that are apparently less affected by cosmic variance (0.25 dex at 2.1 µm), however, at the cost of any ease in disentangling the evolutionary and cosmic variance effects.