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We study the composition of the faint radio population selected from the Karl G. Jansky Very Large Array Cosmic Evolution Survey (VLA-COSMOS) 3 GHz Large Project, which is a radio continuum survey performed at 10 cm wavelength. The survey covers a 2.6 square degree area with a mean rms of ~2:3 μJy/beam, cataloging 10 830 sources above 5σ, and enclosing the full 2 square degree COSMOS field. By combining these radio data with optical, near-infrared (UltraVISTA), and mid-infrared (Spitzer/IRAC) data, as well as X-ray data (Chandra), we find counterparts to radio sources for ~93% of the total radio sample reaching out to z ≲ 6; these sources are found in the unmasked areas of the COSMOS field, i.e., those not affected by saturated or bright sources in the optical to near-infrared (NIR) bands. We further classify the sources as star-forming galaxies or AGN based on various criteria, such as X-ray luminosity; observed mid-infrared color; UV-far-infrared spectral energy distribution; rest-frame, near-UV optical color that is corrected for dust extinction; and radio excess relative to that expected from the star formation rate of the hosts. We separate the AGN into subsamples dominated by low-to-moderate and moderate-to-high radiative luminosity AGN, i.e., candidates for highredshift analogs to local low- and high-excitation emission line AGN, respectively. We study the fractional contributions of these subpopulations down to radio flux levels of ~11 μJy at 3 GHz (or ~20 μJy at 1.4 GHz assuming a spectral index of -0.7). We find that the dominant fraction at 1.4 GHz flux densities above ~200 μJy is constituted of low-to-moderate radiative luminosity AGN. Below densities of ~100 μJy the fraction of star-forming galaxies increases to ~60%, followed by the moderate-to-high radiative luminosity AGN (~20%) and low-to-moderate radiative luminosity AGN (~20%). Based on this observational evidence, we extrapolate the fractions down to sensitivities of the Square Kilometer Array (SKA). Our estimates suggest that at the faint flux limits to be reached by the (Wide, Deep, and UltraDeep) SKA1 surveys, a selection based only on radio flux limits can provide a simple tool to efficiently identify samples highly (>75%) dominated by star-forming galaxies. [ABSTRACT FROM AUTHOR]

Context. Active galactic nuclei (AGN) are thought to play an important role in galaxy evolution. It has been suggested that AGN feedback could be partly responsible for quenching star-formation in the hosts, leading to transition from the blue cloud to the red sequence. The transition seems to occur faster for the most massive galaxies, where traces of AGN activity have been found as early as at z < 0.1. The correlation between AGN activity, aging of the stellar populations, and stellar mass still needs to be fully understood, especially at high redshifts. Aims. Our aim is to investigate the link between AGN activity, star-formation, and stellar mass of the host galaxy at 0 < z < 1, looking for spectroscopic traces of AGN and aging of the host. This work provides an extension of the existing studies at z < 0.1 and contributes to shed light on galaxy evolution at intermediate redshifts. Methods. We used the zCOSMOS 20k data to create a sample of galaxies at z < 1. We divided the sample into several mass-redshift bins to obtain stacked galaxy spectra with an improved signal-to-noise ratio (S/N).We exploited emission-line diagnostic diagrams to separate AGN from star-forming galaxies. Results. We found an indication of a role for the total galaxy stellar mass in leading galaxy classification. Stacked spectra show AGN signatures above the log M*/M⊙ > 10.2 threshold. Moreover, the stellar populations of AGN hosts are found to be older than star-forming and composite galaxies. This could be due to the the tendency of AGN to reside in massive hosts. Conclusions. The dependence of the AGN classification on the stellar mass agrees with what has been found in previous research. Together with the evidence of older stellar populations inhabiting the AGN-like galaxies, it is consistent with the downsizing scenario. In particular, our evidence points to an evolutionary scenario where the AGN-feedback is capable of quenching the star formation in the most massive galaxies. Therefore, the AGN-feedback is the best candidate for initiating the passive evolutionary phase of galaxies. [ABSTRACT FROM AUTHOR]