Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA

Abstract

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. In this talk, we describe multi-frequency radio 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 Metre-wave 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 (σ610MHz ∼ 15μJy/beam, σ1.4GHz ∼ 6μJy/beam, FWHM∼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, α1.4GHz 610MHz (where Sν ∝ ν α ), which is found to be approximately −0.6 to −0.7 based on an almost unbiased 10-sigma criterion, down to a flux level of S1.4GHz ≥ 100μJy. The fraction of inverted spectrum sources, α1.4GHz 610MHz ≥ 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 an aged ultra-steep spectrum radio population. The spectral index distribution has a significant scatter, ∆α ∼ 0.4–0.5, suggesting a mixture of different populations at all flux levels. This is supported by spectroscopic classifications of radio sources with X-ray emission, which has allowed us to estimate that the fraction of radio-quiet (type 1 and 2) active galactic nuclei (AGN) at 30 μJy <S1.4GHz ≤ 300μJy is 15–35%, consistent with a dominant star-forming galaxy population in the sub-mJy regime.