The redshift distribution of infrared-faint radio sources

Infrared-faint radio sources (IFRSs) are an important class of high-redshift active galaxy, and potentially important as a means of discovering more high-redshift radio sources, but only 25 IFRSs had redshifts prior to this paper. Here we increase the number of IFRSs with known spectroscopic redshifts, by a factor of about 5, to 131, with redshifts up to zz = 4.387, and a median redshift of zz = 2.68. The IFRS redshift distribution overlaps with the high-zz radio galaxy (HzRG) redshift distribution but is significantly narrower, suggesting that the IFRSs are a subset of the larger class of HzRGs. We also confirm and measure the proposed correlation between redshift and 3.6 μμm flux density, making it possible to use this correlation to find even higher redshift radio sources. Many more high-redshift sources are probably present in existing radio survey catalogues.