A sample of 6C radio sources designed to find objects at redshift<i>z&gt;4</i>- III. Imaging and the radio galaxy<i>K-z</i>relation

Abstract

In this paper, the third and final of a series, we present complete K-band\nimaging and some complementary I-band imaging of the filtered 6C* sample. We\nfind no systematic differences between the K-z relation of 6C* radio galaxies\nand those from complete samples, so the near-infrared properties of luminous\nradio galaxies are not obviously biased by the additional 6C* radio selection\ncriteria (steep spectral index and small angular size). The 6C* K-z data\nsignificantly improve delineation of the K-z relation for radio galaxies at\nhigh-redshift (z &gt;2). Accounting for non-stellar contamination, and for\ncorrelations between radio luminosity and stellar mass, we find little support\nfor previous claims that the underlying scatter in the stellar luminosity of\nradio galaxies increases significantly at z &gt;2. In a particular spatially-flat\nuniverse with a cosmological constant, the most luminous radio sources appear\nto be associated with galaxies with a luminosity distribution with a high mean\n(~5 Lstar), and a low dispersion (sigma ~ 0.5 mag) which formed their stars at\nepochs corresponding to z &gt;~2.5. This result is in line with recent sub-mm\nstudies of high-redshift radio galaxies and the inferred ages of extremely red\nobjects from faint radio samples.\n