The distribution of polarized radio sources $>$15$\mu$Jy in GOODS-N

Abstract

We present deep VLA observations of the polarization of radio sources in the GOODS-N field at 1.4 GHz at resolutions of 1.6" and 10". At 1.6", we find that the peak flux cumulative number count distribution is N($>$p) $\sim$ 45 * (p/30$\mu$Jy)$^{-0.6}$ per square degree above a detection threshold of 14.5 $\mu$Jy. This represents a break from the steeper slopes at higher flux densities, resulting in fewer sources predicted for future surveys with the SKA and its precursors. It provides a significant challenge for using background RMs to study clusters of galaxies or individual galaxies. Most of the polarized sources are well above our detection limit, and are radio galaxies which are well-resolved even at 10", with redshifts from $\sim$0.2 - 1.9. We determined a total polarized flux for each source by integrating the 10" polarized intensity maps, as will be done by upcoming surveys such as POSSUM. These total polarized fluxes are a factor of 2 higher, on average, than the peak polarized flux at 1.6"; this would increase the number counts by $\sim$50% at a fixed flux level. The detected sources have rotation measures (RMs) with a characteristic rms scatter of $\sim$11$\frac{rad}{m^2}$ around the local Galactic value, after eliminating likely outliers. The median fractional polarization from all total intensity sources does not continue the trend of increasing at lower flux densities, as seen for stronger sources. The changes in the polarization characteristics seen at these low fluxes likely represent the increasing dominance of star-forming galaxies.