A Multiwavelength Analysis of the Faint Radio Sky (COSMOS-XS): the Nature of the Ultra-faint Radio Population

<jats:title>Abstract</jats:title> <jats:p>Ultra-deep radio surveys are an invaluable probe of dust-obscured star formation, but require a clear understanding of the relative contribution from radio active galactic nuclei (AGNs) to be used to their fullest potential. We study the composition of the <jats:italic>μ</jats:italic>Jy radio population detected in the Karl G. Jansky Very Large Array COSMOS-XS survey based on a sample of 1540 sources detected at 3 GHz  over an area of ∼350 arcmin<jats:sup>2</jats:sup>. This ultra-deep survey consists of a single pointing in the well-studied COSMOS field at both 3 and 10 GHz and reaches rms sensitivities of 0.53 and 0.41 <jats:italic>μ</jats:italic>Jy beam<jats:sup>−1</jats:sup>, respectively. We find multiwavelength counterparts for 97% of radio sources, based on a combination of near-UV/optical to sub-millimeter data, and through a stacking analysis at optical/near-IR wavelengths we further show that the sources lacking such counterparts are likely to be high-redshift in nature (typical <jats:italic>z</jats:italic> ∼ 4−5). Utilizing the multiwavelength data over COSMOS, we identify AGNs through a variety of diagnostics and find these to make up 23.2 ± 1.3% of our sample, with the remainder constituting uncontaminated star-forming galaxies. However, more than half of the AGNs exhibit radio emission consistent with originating from star formation, with only 8.8 ± 0.8% of radio sources showing a clear excess in radio luminosity. At flux densities of ∼30 <jats:italic>μ</jats:italic>Jy at 3 GHz, the fraction of star formation-powered sources reaches ∼90%, and this fraction is consistent with unity at even lower flux densities. Overall, our findings imply that ultra-deep radio surveys such as COSMOS-XS constitute a highly effective means of obtaining clean samples of star formation-powered radio sources.</jats:p>