The Evolution of the Low-Frequency Radio AGN Population to $z\simeq 1.5$ in the ELAIS N1 Field

Abstract

We study the cosmic evolution of radio sources out to $z \simeq 1.5$ using a GMRT 610 MHz survey covering $\sim$1.86 deg$^2$ of the ELAIS N1 field with a minimum/median rms noise 7.1/19.5\,$μ$Jy / beam and an angular resolution of 6\,arcsec. We classify sources as star forming galaxies (SFGs), radio-quiet (RQ) and radio-loud (RL) Active Galactic Nuclei (AGN) using a combination of multi-wavelength diagnostics and find evidence in support of the radio emission in SFGs and RQ AGN arising from star formation, rather than AGN-related processes. At high luminosities, however, both SFGs and RQ AGN display a radio excess when comparing radio and infrared star formation rates. The vast majority of our sample lie along the $\rm{SFR - M_{\star}}$ "main sequence" at all redshifts when using infrared star formation rates. We derive the 610 MHz radio luminosity function for the total AGN population, constraining its evolution via continuous models of pure density and pure luminosity evolution with $\rm{Φ^{\star}\,\propto\,(\,1+\,z)^{(2.25\pm0.38)-(0.63\pm0.35)z}}$ and $\rm{L_{610\,MHz}\,\propto\,(\,1+\,z)^{(3.45\pm0.53)-(0.55\pm0.29)z}}$ respectively. For our RQ and RL AGN, we find a fairly mild evolution with redshift best fitted by pure luminosity evolution with $\rm{L_{610\,MHz}\,\propto\,(\,1+\,z)^{(2.81\pm0.43)-(0.57\pm0.30)z}}$ for RQ AGN and $\rm{L_{610\,MHz}\,\propto\,(\,1+\,z)^{(3.58\pm0.54)-(0.56\pm0.29)z}}$ for RL AGN. The 610 MHz radio AGN population thus comprises two differently evolving populations whose radio emission is mostly SF-driven or AGN-driven respectively.