The nature of the faint low-frequency radio source population

Abstract

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 µq24 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 farIR luminosities. We conclude that radio emission from host galaxies of RQ AGN in this flux density regime results primarily from star formation activity.