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: Sigma(610 MHz) similar to 15 mu Jy beam(-1), Sigma(1.4 GHz) similar to 6 mu Jy beam(-1), full width at half-maximum (FWHM) similar to 5 arcsec. 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, alpha(610 MHz)(1.4 GHz) (where S(nu) proportional to nu(alpha)), as a function of flux density. alpha(610 MHz)(1.4 GHz) is found to be approximately -0.6 to -0.7, based on an almost unbiased 10 Sigma criterion, down to a flux level of S(1.4 GHz) greater than or similar to 100 mu Jy. The fraction of inverted spectrum sources (alpha(610 MHz)(1.4 GHz) > 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, delta alpha approximate to 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 30 mu Jy less than or similar to S(1.4 GHz) < 300 mu Jy is roughly 25 +/- 10 per cent, suggesting that star-forming galaxies dominate the sub-mJy regime.