Near-infraredK-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 mean 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 the 7CRS of Willott et al., 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 cut-off 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.

Imaging and spectroscopy of ultrasteep spectrum radio sources

“…5 we show the K ‐band magnitude distribution of USS sources, measured in an 8‐arcsec diameter aperture. We compare our results with those obtained in the SUMSS–NVSS (De Breuck et al 2004), 6C** (Cruz et al 2007) catalogues and data selected at 74 MHz taken from Jarvis et al (2004). The mean K ‐band magnitudes in an 8‐arcsec diameter aperture of the USS is , which is 1 mag fainter than those obtained by De Breuck et al (2004) in the SUMSS–NVSS survey () and by Cruz et al (2006) in the 6C** survey ().…”

“…The vertical dashed line represent the expected K ‐band magnitude of a z ∼ 3 source, indicating that high‐redshift sources have compact radio morphologies. Several authors have applied the small angular size criterion in order to find high‐redshift radio galaxies (Blundell et al 1998; Jarvis et al 2004; Cruz et al 2006). There are a few cases of radio galaxies at high redshifts with large angular sizes (4C 23.56 at z = 2.483 has LAS = 53 arcsec).…”

“…Solid and dashed lines represent the distribution of K ‐band magnitude, measured in an 8‐arcsec diameter aperture for sources detected in the SUMSS–NVSS (De Breuck et al 2004) and 6C** (Cruz et al 2007) catalogues. Dotted histogram represents USS sources selected at 74 MHz taken from Jarvis et al (2004).…”

A search for distant radio galaxies from SUMSS and NVSS - III. Radio spectral energy distributions and the z- correlation

“…It has also been suggested that the SEDs of HzRGs will be intrinsically steeper thanks to enhanced inverse‐Compton (IC) losses of the relativistic electron population against the cosmic microwave background (CMB) photons whose energy density increases as (1 + z ) 4 (Krolik & Chen 1991). These two explanations have been adopted in the literature over recent years as the main contributors to the correlation (De Breuck et al 2000, 2002a; Pedani 2003; Cohen et al 2004; Jarvis et al 2004). The third explanation holds that the z –α correlation itself is indirect, reflecting an intrinsic correlation between radio luminosity and spectral index coupled to a Malmquist bias (Blundell, Rawlings & Willott 1999).…”

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

“…Although an ultra-steep (radio) spectrum (USS; α −1 with S ∝ ν α ) does not guarantee a high redshift, and the USS selection may actually miss a large fraction of HzRGs (e.g. Waddington et al 1999;Jarvis et al 2001cJarvis et al , 2009), a higher fraction of high-redshift sources can be found among those with the steepest radio spectra, and most of the radio galaxies known at z > 3.5 have been found using the USS criterion (Blundell et al 1998;De Breuck et al 1998Jarvis et al 2001a,b;De Breuck et al 2002a;Jarvis et al 2004;Cruz et al 2006;Miley & De Breuck 2008). A satisfactory explanation for this trend is still missing.…”