2017MNRAS.468.1156O

We present a multiwavelength 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 deg² of the ELAIS N1 region. We detect 2800 sources above 50 µJy beam^–1. By matching to multiwavelength data, we obtain a redshift estimate for 63 per cent, with 29 per cent based on spectroscopy. For 1526 of the sources with redshifts, we use radio and X-ray luminosity, optical spectroscopy, mid-infrared colours and 24 µm and IR to radio flux ratios to search for the presence of an active galactic nucleus (AGN). The analysis reveals a rapid change in the population as flux density decreases from ∼500 µJy to ∼100 µJy. We find that 80.3 per cent of the objects show no evidence of AGN and have multiwavelength properties consistent with radio emission from star-forming galaxies (SFG). We classify 11.4 per cent as radio-quiet (RQ) AGN and the remaining 8.3 per cent as radio-loud (RL) AGN. The redshift of all populations extends to z > 3 with a median of ∼1. The median radio and far-IR luminosity increases systematically from SFG, to RQ AGN and RL AGN. The median q_24 µm_ for SFG, 0.89 ± 0.01, is slightly below that for RQ AGN, 1.05 ± 0.03, and both differ substantially from the value for RL AGN of -0.06 ± 0.07. 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 results primarily from star formation activity.