We measure the stellar mass function (SMF) and stellar mass density of galaxies in the COSMOS field up to z ∼ 6. We select them in the near-IR bands of the COSMOS2015 catalogue, which includes ultra-deep photometry from UltraVISTA-DR2, SPLASH, and Subaru/Hyper SuprimeCam. At z > 2.5 we use new precise photometric redshifts with error σ z = 0.03(1 + z) and an outlier fraction of 12%, estimated by means of the unique spectroscopic sample of COSMOS (∼100 000 spectroscopic measurements in total, more than one thousand having robust z spec > 2.5). The increased exposure time in the DR2, along with our panchromatic detection strategy, allow us to improve the completeness at high z with respect to previous UltraVISTA catalogues (e.g. our sample is >75% complete at 10 10 M and z = 5). We also identify passive galaxies through a robust colour-colour selection, extending their SMF estimate up to z = 4. Our work provides a comprehensive view of galaxy-stellar-mass assembly between z = 0.1 and 6, for the first time using consistent estimates across the entire redshift range. We fit these measurements with a Schechter function, correcting for Eddington bias. We compare the SMF fit with the halo mass function predicted from ΛCDM simulations, finding that at z > 3 both functions decline with a similar slope in the high-mass end. This feature could be explained assuming that mechanisms quenching star formation in massive haloes become less effective at high redshifts; however further work needs to be done to confirm this scenario. Concerning the SMF low-mass end, it shows a progressive steepening as it moves towards higher redshifts, with α decreasing from −1.47 +0.02 −0.02 at z 0.1 to −2.11 +0.30 −0.13 at z 5. This slope depends on the characterisation of the observational uncertainties, which is crucial to properly remove the Eddington bias. We show that there is currently no consensus on the method to quantify such errors: different error models result in different best-fit Schechter parameters.

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.

We study the effects of local environment and stellar mass on galaxy properties using a mass complete sample of quiescent and star-forming systems in the COSMOS field at z 3. We show that at z 1, the median star-formation rate (SFR) and specific SFR (sSFR) of all galaxies depend on environment, but they become independent of environment at z 1. However, we find that only for star-forming galaxies, the median SFR and sSFR are similar in different environments, regardless of redshift and stellar mass. We find that the quiescent fraction depends on environment at z 1, and on stellar mass out to z ∼ 3. We show that at z 1, galaxies become quiescent faster in denser environments and that the overall environmental quenching efficiency increases with cosmic time. Environmental and mass quenching processes depend on each other. At z 1, denser environments more efficiently quench galaxies with higher masses (log(M/M ⊙ ) 10.7), possibly due to a higher merger rate of massive galaxies in denser environments, and that mass quenching is more efficient in denser regions. We show that the overall mass quenching efficiency (ǫ mass ) for more massive galaxies (log(M/M ⊙ ) 10.2) rises with cosmic time until z ∼ 1 and flattens out since then. However, for less massive galaxies, the rise in ǫ mass continues to the present time. Our results suggest that environmental quenching is only relevant at z 1, likely a fast process, whereas mass quenching is the dominant mechanism at z 1, with a possible stellar feedback physics.