ATLAS 1.4 GHz data release 2 – II. Properties of the faint polarized sky

Abstract

This is the second of two papers describing the second data release (DR2) of the Australia Telescope Large Area Survey (ATLAS) at 1.4 GHz. In Paper I we detailed our data reduction and analysis procedures, and presented catalogues of components (discrete regions of radio emission) and sources (groups of physically associated radio components). In this paper we present our key observational results. We find that the 1.4 GHz Euclidean normalised differential number counts for ATLAS components exhibit monotonic declines in both total intensity and linear polarization from millijansky levels down to the survey limit of ~100 uJy. We discuss the parameter space in which component counts may suitably proxy source counts. We do not detect any components or sources with fractional polarization levels greater than 24%. The ATLAS data are consistent with a lognormal distribution of fractional polarization with median level 4% that is independent of flux density down to total intensity ~10 mJy and perhaps even 1 mJy. Each of these findings are in contrast to previous studies; we attribute these new results to improved data analysis procedures. We find that polarized emission from 1.4 GHz millijansky sources originates from the jets or lobes of extended sources that are powered by an active galactic nucleus, consistent with previous findings in the literature. We provide estimates for the sky density of linearly polarized components and sources in 1.4 GHz surveys with ~10" resolution.