The Nature of the Faint Low-Frequency Radio Source Population

Abstract: We present a multi-wavelength study into the nature of faint radio sources in a deep radio image with the Giant Meterwave Radio Telescope at 612\,MHz covering 1.2 deg$^2$ of the ELAIS N1 region. We detect 2800 sources above 50~$\mu$Jy\,beam$^{-1}$. By matching to multi-wavelength data we obtain a redshift estimate for 63\%, with 29\% based on spectroscopy. For 1526 of the sources with redshifts we use radio and X-ray luminosity, optical spectroscopy, mid-infrared colors, and 24$\mu$m and IR to radio flux ratios to search for the presence of an AGN. The analysis reveals a rapid change in the population as flux density decreases from $\sim$500\,$\mu$Jy to $\sim$100\,$\mu$Jy. We find that 80.3\% of the objects show no evidence of AGN and have multi-wavelength properties consistent with radio emission from star forming galaxies (SFG). We classify 11.4\% as Radio Quiet (RQ) AGN and the remaining 8.3\% as Radio Loud (RL) AGN. The redshift of all populations extends to $z > 3$ with a median of $\sim$1. The median radio and far-IR luminosity increases systematically from SFG, to RQ AGN and RL AGN. The median $q_{\rm 24 \mu m}$ for SFG, $0.89\pm0.01$ is slightly below that for RQ AGN, $1.05\pm0.03$, and both differ substantially from the value for RL AGN of $-0.06\pm0.07$. However, SFG and RQ AGN show no significant difference in far-IR/radio ratios and have statistically indistinguishable star formation rates inferred from radio and far-IR luminosities. We conclude that radio emission from host galaxies of RQ AGN in this flux density regime result primarily from star formation activity.