The Australia Telescope Large Area Survey: 2.3 GHz observations of ELAIS-S1 and CDF-S: Spectral index properties of the faint radio sky

Context. The Australia Telescope Large Area Survey (ATLAS) aims to image a 7 deg² region centred on the European Large Area ISO Survey – South 1 (ELAIS-S1) field and the Chandra Deep Field South (CDF-S) at 1.4 GHz with high sensitivity (up to σ ~ 10   μJy) to study the evolution of star-forming galaxies (SFGs) and Active Galactic Nuclei (AGN) over a wide range of cosmic time.

Aims. We present here ancillary radio observations at a frequency of 2.3 GHz obtained with the Australia Telescope Compact Array (ATCA). The main goal of this is to study the radio spectra of an unprecedented large sample of sources (~2000 observed,  ~600 detected in both frequencies).

Methods. With this paper, we provide 2.3 GHz source catalogues for both ATLAS fields, with a detection limit of 300 μJy (equivalent to 4.5σ in the ELAIS-S1 field and 4.0σ in the CDF-S). We compute spectral indices between 1.4 GHz and 2.3 GHz using matched-resolution images and investigate various properties of our source sample in dependence of their spectral indices.

Results. We find the entire source sample to have a median spectral index α_med =  −0.74, in good agreement with both the canonical value of  −0.7 for optically thin synchrotron radiation and other spectral index studies conducted by various groups. Regarding the radio spectral index as indicator for source type, we find only marginal correlations so that flat or inverted spectrum sources are usually powered by AGN and hence conclude that at least for the faint population the spectral index is not a strong discriminator. We investigate the z–α relation for our source sample and find no such correlation between spectral index and redshift at all. We do find a significant correlation between redshift and radio to near-infrared flux ratio, making this a much stronger tracer of high-z radio sources. We also find no evidence for a dependence of the radio-IR correlation on spectral index.