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

TL;DR

This study uses deep multi-frequency radio observations of the Lockman Hole to analyze the emission mechanisms of faint radio sources, finding that optically-thin synchrotron emission dominates and star-forming galaxies are prevalent at sub-mJy flux levels.

Contribution

It provides the deepest 610-MHz imaging combined with 1.4-GHz data to analyze spectral indices and source populations in the faint radio regime.

Findings

Median spectral index is -0.6 to -0.7 down to 100 μJy.

Less than 10% of sources have inverted spectra.

Star-forming galaxies likely dominate the sub-mJy radio population.

Abstract

We describe multi-frequency data designed to probe the emission mechanism that dominates in faint radio sources. Our analysis is based on observations of the Lockman Hole using the GMRT - the deepest 610-MHz imaging yet reported - together with 1.4-GHz imaging from the VLA, well matched in resolution and sensitivity to the GMRT data. 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, as a function of flux density. alpha is found to be -0.6 to -0.7, based on an almost unbiased 10-sigma criterion, down to a flux level of S(1.4GHz) >~ 100uJy. The fraction of inverted spectrum sources (alpha > 0) is less than 10%. 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, 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 AGN at 30 <~ S(1.4GHz) < 300uJy is roughly 25 +/- 10%, suggesting that star-forming galaxies dominate the sub-mJy regime.