A search for distant radio galaxies from SUMSS and NVSS: I. Sample definition, radio and K-band imaging

Abstract

We present the first results from a pilot study to search for distant radio galaxies in the southern hemisphere (delta < -32). Within a 360 deg^2 region of sky, we define a sample of 76 ultra-steep spectrum (USS) radio sources from the 843 MHz Sydney University Molonglo Sky Survey (SUMSS) and 1.4 GHz NRAO VLA Sky Survey (NVSS) radio surveys with alpha_843^1400 < -1.3 and S_1400 > 15 mJy. We observed 71 sources without bright optical or near-infrared counterparts at 1.385 GHz with the ATCA, providing ~5" resolution images and sub-arcsec positional accuracy. To identify their host galaxies, we obtained near-IR K-band images with IRIS2 at the AAT and SofI at the NTT. We identify 92% of the USS sources down to K~20.5. The SUMSS-NVSS USS sources have a surface density more than 4 times higher than USS sources selected at lower frequencies. This is due to the higher effective selection frequency, and the well-matched resolutions of both surveys constructed using the same source fitting algorithm. The scattering of alpha >-1.3 sources into the USS sample due to spectral index uncertainties can account for only 35% of the observed USS sources. Since our sample appears to contain a similar fraction of very distant (z>3) galaxies, selecting USS sources from SUMSS-NVSS should allow us to identify large numbers of massive galaxies at high redshift.

arXiv comment17 Pages, including 76 PostScript figures. Accepted for publication in MNRAS. No changes to the text. Version with all figures available from http://www.eso.org/~cdebreuc/papers.html (updated web page adress)

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