We explore the simple inter-relationships between mass, star formation rate, and environment in the SDSS, zCOSMOS, and other deep surveys. We take a purely empirical approach in identifying those features of galaxy evolution that are demanded by the data and then explore the analytic consequences of these. We show that the differential effects of mass and environment are completely separable to z ~ 1, leading to the idea of two distinct processes of "mass quenching" and "environment quenching." The effect of environment quenching, at fixed over-density, evidently does not change with epoch to z ~ 1 in zCOSMOS, suggesting that the environment quenching occurs as large-scale structure develops in the universe, probably through the cessation of star formation in 30%-70% of satellite galaxies. In contrast, mass quenching appears to be a more dynamic process, governed by a quenching rate. We show that the observed constancy of the Schechter M* and α s for star-forming galaxies demands that the quenching of galaxies around and above M* must follow a rate that is statistically proportional to their star formation rates (or closely mimic such a dependence). We then postulate that this simple mass-quenching law in fact holds over a much broader range of stellar mass (2 dex) and cosmic time. We show that the combination of these two quenching processes, plus some additional quenching due to merging naturally produces (1) a quasi-static single Schechter mass function for star-forming galaxies with an exponential cutoff at a value M* that is set uniquely by the constant of proportionality between the star formation and mass quenching rates and (2) a double Schechter function for passive galaxies with two components. The dominant component (at high masses) is produced by mass quenching and has exactly the same M* as the star-forming galaxies but a faint end slope that differs by Δα s ~ 1. The other component is produced by environment effects and has the same M* and α s as the star-forming galaxies but an amplitude that is strongly dependent on environment. Subsequent merging of quenched galaxies will modify these predictions somewhat in the denser environments, mildly increasing M* and making α s slightly more negative. All of these detailed quantitative inter-relationships between the Schechter parameters of the star-forming and passive galaxies, across a broad range of environments, are indeed seen to high accuracy in the SDSS, lending strong support to our simple empirically based model. We find that the amount of post-quenching "dry merging" that could have occurred is quite constrained. Our model gives a prediction for the mass function of the population of transitory objects that are in the process of being quenched. Our simple empirical laws for the cessation of star formation in galaxies also naturally produce the "anti-hierarchical" run of mean age with mass for passive galaxies, as well as the qualitative variation of formation timescale indicated by the relative α-element abundances.

We present the spectrophotometric properties of a sample of 141 emission-line galaxies at redshifts in the range 0.2 < z < 1.0 with a peak around z ∈ [0.2, 0.4]. The analysis is based on medium resolution (R s = 500−600), optical spectra obtained at VLT and Keck. The targets are mostly "Canada-France Redshift Survey" emission-line galaxies, with the addition of field galaxies randomly selected behind lensing clusters. We complement this sample with galaxy spectra from the "Gemini Deep Deep Survey" public data release. We have computed absolute magnitudes of the galaxies and measured the line fluxes and equivalent widths of the main emission/absorption lines. The last two have been measured after careful subtraction of the fitted stellar continuum using the platefit software originally developed for the SDSS and adapted to our data. We present a careful comparison of this software with the results of manual measurements. The pipeline has also been tested on lower resolution spectra, typical of the "VIMOS/VLT Deep Survey" (R s = 250), by resampling our medium resolution spectra. We show that we can successfully deblend the most important strong emission lines. These data are primarily used to perform a spectral classification of the galaxies in order to distinguish star-forming galaxies from AGNs. Among the initial sample of 141 emission-line galaxies, we find 7 Seyfert 2 (narrow-line AGN), 115 star-forming galaxies and 16 "candidate" star-forming galaxies. Scientific analysis of these data, in terms of chemical abundances, stellar populations, etc., will be presented in subsequent papers of this serie.

We present the gas-phase oxygen abundance (O/H) for a sample of 131 star-forming galaxies at intermediate redshifts (0.2 < z < 1.0). The sample selection, the spectroscopic observations (mainly with VLT/FORS) and associated data reduction, the photometric properties, the emission-line measurements, and the spectral classification are fully described in a companion paper (Paper I). We use two methods to estimate the O/H abundance ratio: the "standard" R 23 method which is based on empirical calibrations, and the CL01 method which is based on grids of photo-ionization models and on the fitting of emission lines. For most galaxies, we have been able to solve the problem of the metallicity degeneracy between the high-and low-metallicity branches of the O/H vs. R 23 relationship using various secondary indicators. The luminosity -metallicity (L − Z) relation has been derived in the B-and R-bands, with metallicities derived with the two methods (R 23 and CL01). In the analysis, we first consider our sample alone and then a larger one which includes other samples of intermediate-redshift galaxies drawn from the literature. The derived L − Z relations at intermediate redshifts are very similar (same slope) to the L − Z relation obtained for the local universe. Our sample alone only shows a small, not significant, evolution of the L− Z relation with redshift up to z ∼ 1.0. We only find statistical variations consistent with the uncertainty in the derived parameters. Including other samples of intermediate-redshift galaxies, we find however that galaxies at z ∼ 1 appear to be metal-deficient by a factor of ∼3 compared with galaxies in the local universe. For a given luminosity, they contain on average about one third of the metals locked in local galaxies.

In 2011 Laporte et al. reported a very high redshift galaxy candidate: a lensed J-band dropout (A2667-J1). J1 has a photometric redshift of z = 9.6-12, the probability density function for which permits no low-or intermediate-z solution. We here report new spectroscopic observations of this galaxy with Very Large Telescope/X-Shooter, which show clear [O III] λ5007Å, Lyα, Hα and Hβ emission and place the galaxy firmly at z = 2.082. The oxygen lines contribute only ∼25 per cent to the H-band flux and do not significantly affect the dropout selection of J1. After correcting the broad-band fluxes for line emission, we identify two roughly equally plausible natures for A2667-J1: it is either a young heavily reddened starburst or a maximally old system with a very pronounced 4000-Å break, upon which a minor secondary burst of star formation is superimposed. Fits show that to make a 3σ detection of this object in the B band (V band), imaging of depth AB = 30.2 (29.5) would be required -despite the relatively bright near-infrared (NIR) magnitude, we would need optical data of equivalent depth to the Hubble Ultra Deep Field to rule out the mid-z solution on purely photometric grounds. Assuming that this stellar population can be scaled to the NIR magnitudes of recent Hubble Space Telescope/Wide Field Camera 3 (WFC3) IR-selected galaxies, we conclude that unfeasibly deep optical data (reaching AB ∼ 32) would be required for the same level of security. There is a population of galaxies at z ≈ 2 with continuum colours alone that mimic those of our z = 7-12 candidates.