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.