Large-scale structures (LSS) out to z < 3.0 are measured in the Cosmic Evolution Survey (COSMOS) using extremely accurate photometric redshifts (photoz). The Ks-band selected sample (from Ultra-Vista) is comprised of 155,954 galaxies. Two techniques -adaptive smoothing and Voronoi tessellation -are used to estimate the environmental densities within 127 redshift slices. Approximately 250 statistically significant overdense structures are identified out to z = 3.0 with shapes varying from elongated filamentary structures to more circularly symmetric concentrations. We also compare the densities derived for COSMOS with those based on semi-analytic predictions for a ΛCDM simulation and find excellent overall agreement between the mean densities as a function of redshift and the range of densities. The galaxy properties (stellar mass, spectral energy distributions (SEDs) and star formation rates (SFRs)) are strongly correlated with environmental density and redshift, particularly at z < 1.0−1.2. Classifying the spectral type of each galaxy using the rest-frame b-i color (from the photoz SED fitting), we find a strong correlation of early type galaxies (E-Sa) with high density environments, while the degree of environmental segregation varies systematically with redshift out to z ∼ 1.3. In the highest density regions, 80% of the galaxies are early types at z=0.2 compared to only 20% at z = 1.5. The SFRs and the star formation timescales exhibit clear environmental correlations. At z > 0.8, the star formation rate density (SFRD) is uniformly distributed over all environmental density percentiles, while at lower redshifts the dominant contribution is shifted to galaxies in lower density environments.

The relation between the stellar mass (M⋆) and the star formation rate (SFR) characterizes how the instantaneous star formation is determined by the galaxy past star formation history and by the growth of the dark matter structures. We deconstruct the M⋆ − SFR plane by measuring the specific SFR functions in several stellar mass bins from z = 0.2 out to z = 1.4 (specific SFR = SFR/M⋆, noted sSFR). Our analysis is primary based on a 24µm selected catalogue combining the COSMOS and GOODS surveys. We estimate the SFR by combining mid-and far-infrared data for 20500 galaxies. The sSFR functions are derived in four stellar mass bins within the range 9.5 < log(M⋆/M ⊙ ) < 11.5. First, we demonstrate the importance of taking into account selection effects when studying the M⋆ − SFR relation. Secondly, we find a mass-dependent evolution of the median sSFR with redshift varying as sSFR ∝ (1 + z) b , with b increasing from b = 2.88 ±0.12 to b = 3.78 ±0.60 between M⋆ = 10 9.75 M ⊙ and M⋆ = 10 11.1 M ⊙ , respectively. At low masses, this evolution is consistent with the cosmological accretion rate and predictions from semi-analytical models (SAM). This agreement breaks down for more massive galaxies showing the need for a more comprehensive description of the star formation history in massive galaxies. Third, we obtain that the shape of the sSFR function is invariant with time at z < 1.4 but depends on the mass. We observe a broadening of the sSFR function ranging from 0.28 dex at M⋆ = 10 9.75 M ⊙ to 0.46 dex at M⋆ = 10 11.1 M ⊙ . Such increase in the intrinsic scatter of the M⋆ − SFR relation suggests an increasing diversity of SFHs as the stellar mass increases. Finally, we find a gradual decline of the sSFR with stellar mass as log10(sSFR) ∝ −0.17M⋆. We discuss the numerous physical processes, as gas exhaustion in hot gas halos or secular evolution, which can gradually reduce the sSFR and increase the SFH diversity.

We explore the evolved galaxy population in the proto‐clusters around four high‐z radio galaxies at 2 ≲z≲ 3 based on wide‐field near‐infrared (NIR) imaging. Three of the four fields are known proto‐clusters as demonstrated by overdensities of line‐emitting galaxies at the same redshifts as the radio galaxies found by narrow‐band surveys and spectroscopic follow‐up observations. We imaged the fields of three targets (PKS 1138−262, USS 0943−242 and MRC 0316−257) to a depth of Ks∼ 22 (Vega magnitude, 5σ) over a 4 × 7 arcmin2 area centred on the radio galaxies with a new wide‐field NIR camera, Multi‐Object Infra‐Red Camera and Spectrograph (MOIRCS), on the Subaru Telescope. Another target (USS 1558−003) was observed with Son of ISAAC on the New technology Telescope (NTT) to a depth of Ks= 20.5 (5σ) over a 5 × 5 arcmin2 area. We apply colour cuts in J−Ks and/or JHKs in order to exclusively search for galaxies located at high redshifts: z > 2. To the 5σ limiting magnitudes, we see a significant excess of NIR‐selected galaxies by a factor of 2 to 3 compared to those found in the field of GOODS‐South. The spatial distribution of these NIR‐selected galaxies is not uniform and traces structures similar to those of emission‐line galaxies, although the samples of NIR‐selected galaxies and emitters show little overlap, from which we conclude that the former tend to be an evolved population with much higher stellar mass than the latter, young and active emitters. We focus on the NIR colour–magnitude sequence of the evolved population and find that the bright‐end (Mstars > 1011 M⊙) of the red sequence is well populated by z∼ 2 but much less so in the z∼ 3 proto‐clusters. This may imply that the bright‐end of the colour–magnitude sequence first appeared between z= 3 and 2, an era coinciding with the appearance of sub‐mm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high‐density environments by vigorous star formation and assembly.

We present the first results from a near-IR spectroscopic survey of the COSMOS field, using the Fiber Multi-Object Spectrograph on the Subaru telescope, designed to characterize the star-forming galaxy population at 1.4 < z < 1.7. The high-resolution mode is implemented to detect Hα in emission between 1.6−1.8 µm with f Hα 4 × 10 −17 erg cm −2 s −1 . Here, we specifically focus on 271 sBzK-selected galaxies that yield a Hα detection thus providing a redshift and emission line luminosity to establish the relation between star formation rate and stellar mass. With further J-band spectroscopy for 89 of these, the level of dust extinction is assessed by measuring the Balmer decrement using co-added spectra. We find that the extinction (0.6 A Hα 2.5) rises with stellar mass and is elevated at high masses compared to low-redshift galaxies. Using this subset of the spectroscopic sample, we further find that the differential extinction between stellar and nebular emission E star (B − V )/E neb (B − V) is 0.7-0.8, dissimilar to that typically seen at low redshift. After correcting for extinction, we derive an Hα-based main sequence with a slope (0.81 ± 0.04) and normalization similar to previous studies at these redshifts.

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