A sample of 6C radio sources designed to find objects at redshift z > 4 - II. Spectrophotometry and emission-line properties

Abstract

This is the second in a series of three papers that present and interpret basic observational data on the 6C* 151-MHz radio sample: a low-frequency selected sample that exploits filtering criteria based on radio properties (steep spectral index and small angular size) to find radio sources at redshift within a 0.133-sr patch of sky. We present results of a programme of optical spectroscopy that has yielded redshifts in the range for the 29 sources in the sample, all but six of which are secure. We find that the filtering criteria used for 6C* are very effective in excluding the low-redshift, low-luminosity radio sources: the median redshift of 6C* is compared with for a complete sample matched in 151-MHz flux density. By combining the emission-line data set for the 6C* radio sources with those for the 3CRR, 6CE and 7CRS samples we establish that radio galaxies follow a rough proportionality between Lyα and 151-MHz luminosity, which, like similar correlations seen in samples of lower redshift radio sources, is indicative of a primary link between the power in the source of the photoionizing photons (most likely a hidden quasar nucleus) and the power carried by the radio jets. We argue that radio sources modify their environments and that the range of emission-line properties seen is determined more by the range of source age than by the range in ambient environment. The smallest radio galaxies have all the properties expected if the size distribution of luminous high-redshift steep-spectrum radio sources reflects a broad range (∼2 dex) of source ages with a narrower range (≲ 1.5 dex) of environmental densities, namely: (1) high-ionization lines, e.g. Lyα, of relatively low luminosity; (2) boosted low-ionization lines, e.g. C ii]; (3) spatially compact emission-line regions; and (4) H i-absorbed Lyα profiles. This is in accord with the idea that all high-redshift, high-luminosity radio sources are triggered in similar environments, presumably recently collapsed massive structures.