Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA - I. The nature of the sub-mJy radio population

In the run up to routine observations with the upcoming generation of radio facilities, the nature of sub-mJy radio population has been hotly debated. Here, we describe multi-frequency data designed to probe the emission mechanism that dominates in these faint radio sources. Our analysis is based on observations of the Lockman Hole using the Giant Metrewave Radio Telescope (GMRT) - the deepest 610-MHz imaging yet reported - together with 1.4-GHz imaging from the Very Large Array (VLA), well matched in resolution and sensitivity to the GMRT data: σ_610MHz ~ 15μJybeam^-1, σ_1.4GHz ~ 6μJybeam^-1, full width at half-maximum (FWHM) ~ 5arcsec. The GMRT and VLA data are cross-matched to obtain the radio spectral indices for the faint radio emitters. Statistical analyses show no clear evolution for the median spectral index, α^610MHz_1.4GHz (where S_ν ~ ν^α), as a function of flux density. α^610MHz_1.4GHz is found to be approximately -0.6 to -0.7, based on an almost unbiased 10σ criterion, down to a flux level of S_1.4GHz >~100μJy. The fraction of inverted spectrum sources (α^610MHz_1.4GHz > 0) is less than 10 per cent. The results suggest that the most prevalent emission mechanism in the sub-mJy regime is optically thin synchrotron, ruling out a dominant flat spectrum or ultra-steep spectrum radio population. The spectral index distribution has a significant scatter, Δα ~ 0.4-0.5, which suggests a mixture of different populations at all flux levels. Spectroscopic classification of radio sources with X-ray emission has allowed us to estimate that the fraction of radio-quiet active galactic nuclei (AGN) at 30μJy <~ S_1.4GHz <300μJy is roughly 25 +/- 10 per cent, suggesting that star-forming galaxies dominate the sub-mJy regime.