The VLA-COSMOS 3 GHz Large Project: Multiwavelength counterparts and the
  composition of the faint radio population
  (abridged) We study the composition of the faint radio population selected
from the VLA-COSMOS 3GHz Large Project. The survey covers a 2.6sq.deg. area
with a mean rms of ~2.3uJy/b, cataloging 10830 sources (>5sigma). Combining
these radio data with optical, near-infrared (UltraVISTA), mid-infrared
(Spitzer/IRAC) data, and X-ray data (Chandra), we find counterparts to radio
sources for ~93% of the radio sample (in the areas of the COSMOS field not
affected by saturated or bright sources in the optical to NIR bands), reaching
out to z<6. We further classify the sources as star forming galaxies or AGN
based on various criteria, such as X-ray luminosity, observed MIR color, UV-FIR
spectral-energy distribution, rest-frame NUV-optical color corrected for dust
extinction, and radio-excess relative to that expected from the hosts'
star-formation rate. We separate the AGN into sub-samples dominated by
low-to-moderate and moderate-to-high radiative luminosity AGN, candidates for
high-redshift analogues to local low- and high-excitation emission line AGN,
respectively. We study the fractional contributions of these sub-populations
down to radio flux levels of ~11uJy at 3GHz (or ~20uJy at 1.4GHz assuming a
spectral index of -0.7). We find that the dominant fraction at 1.4GHz flux
densities above ~200uJy is constituted of low-to-moderate radiative luminosity
AGN. Below densities of ~100uJy 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 SKA. Our
estimates suggest that at the faint flux limits to be reached by the 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.
