Near-infrared K-band imaging of a sample of Ultra-Steep-Spectrum radio sources selected at 74 MHz

Abstract

In this paper we present near-infrared K-band imaging of a sample of ultra-steep spectrum (USS) radio sources selected at 74 MHz. The dual selection criteria of low frequency and USS means that we should be sensitive to the highest redshift (z > 5) radio galaxies. We have obtained $K-$band magnitudes for all of the objects in our sample of 26 and discuss the properties of each. There is a pronounced bias in this sample towards fainter magnitudes and thus higher redshifts when compared to complete unfiltered samples such as 7CRS (Willott et al. 2002), implying that the steep-spectrum technique is still viable at 74 MHz. However, there are more bright (K < 17) sources in the 74 MHz sample than in a similar sample selected at 151 MHz, namely 6C*. This is principally due to the additional selection criterion of a small angular size for the radio sources in 6C*, four of the six sources in the 74 MHz USS sample with K < 17 have angular sizes > 15 arcsec (the angular size cutoff of 6C*). We find that the distribution of K-band magnitudes from a sample selected at 74 MHz is statistically indistinguishable from the 6C* sample, when similar angular size filtering is applied to the 74 MHz sample.

arXiv comment7 pages, 28 figures in .png format. Accepted for publication in MNRAS. Full resolution figures can be found at http://www-astro.physics.ox.ac.uk/~mjj/Jarvis74MHz.ps.gz

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