The 6C** sample of steep-spectrum radio sources: II - Redshift distribution and the space density of high-redshift radio galaxies

摘要

We use the 6C** sample to investigate the co-moving space density ofpowerful, steep-spectrum radio sources. This sample, consisting of 68 objects,has virtually complete K-band photometry and spectroscopic redshifts for 32 percent of the sources. In order to find its complete redshift distribution, wedevelop a method of redshift estimation based on the K-z diagram of the 3CRR,6CE, 6C* and 7CRS radio galaxies. Based on this method, we derive redshiftprobability density functions for all the optically identified sources in the6C** sample. Using a combination of spectroscopic and estimated redshifts, weselect the most radio luminous sources in the sample. Their redshiftdistribution is then compared with the predictions of the radio luminosityfunction of Jarvis et al. We find that, within the uncertainties associatedwith the estimation method, the data are consistent with a constant co-movingspace density of steep-spectrum radio sources beyond z > 2.5, and rule out asteep decline.