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

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. 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 610 MHz ∼ 15 μJy beam −1 , σ 1.4 GHz ∼ 6 μJy beam −1 , full width at half-maximum (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, α 610 MHz 1.4 GHz (where S ν ∝ ν α ), as a function of flux density. α 610 MHz 1.4 GHz is found to be approximately −0.6 to −0.7, based on an almost unbiased 10σ criterion, down to a flux level of S 1.4 GHz 100 μJy. The fraction of inverted spectrum sources (α 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, α ≈ 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 μJy S 1.4 GHz < 300 μJy is roughly 25 ± 10 per cent, suggesting that star-forming galaxies dominate the sub-mJy regime.

The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals*

“…The Lockman field includes the deepest radio survey at 1.4 GHz to date, a single 40 0 × 40 0 pointing centered at 161:5 days; þ59:017 days, reaching a 5σ detection limit of 15 μJy near the center of the primary beam (Owen & Morrison 2008) and further deep coverage by Ibar et al (2009). Simpson et al (2006) have surveyed the Subaru Extragalactic Deep Survey region with the VLA to a detection limit of 100 μJy, and Bondi et al (2007) have surveyed the VVDS field in XMM-LSS at both 610 MHz with the GMRT and 1.4 GHz with the VLA to limits of ≈200 and 80 μJy, respectively.…”

Ultra Steep Spectrum Radio Sources in the Lockman Hole: SERVS Identifications and Redshift Distribution at the Faintest Radio Fluxes

“…In order to select USS sources we require coverage at both frequencies, and thus restricted our search to the smaller 1.4-GHz area. As indicated in Ibar et al (2009), the 610-MHz catalogue contains 1,236 components within the area covered by the 1.4-GHz observations. After component merging (Ibar et al 2009) we are left with 1,213 individual radio sources.…”

“…As indicated in Ibar et al (2009), the 610-MHz catalogue contains 1,236 components within the area covered by the 1.4-GHz observations. After component merging (Ibar et al 2009) we are left with 1,213 individual radio sources. In order to perform a more robust selection of USS sources we further restrict the sample in two ways: firstly, to avoid the unreliable spectral indices measurements of the faintest detected sources, we limit the 610 MHz radio sample to integrated flux densities of S 610 MHz ≥ 100 µJy; secondly, we reject all sources at the edge (within ∼ 20 ′′ ) of the 1.4-GHz map, where the rapidly varying noise can bias the measured 1.4-GHz flux.…”

“…The Lockman Hole radio coverage at 610 MHz and 1.4 GHz from Ibar et al (2009) was chosen for the selection of USS radio sources. Besides the depth and resolution of these surveys (reaching an r.m.s.…”

“…With the recent appearance of sensitive low-frequency radio facilities such as the Giant Metrewave Radio Telescope (GMRT), and with LOFAR being commissioned, it has become viable to extend the search for and subsequent study of USS sources to the µJy regime (e.g. Bondi et al 2007;Owen et al 2009;Afonso et al 2009;Ibar et al 2009). Detailed studies of these sources are fundamental, but still lacking, as the deepest possible supporting multi-wavelength data are an absolute necessity.…”