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

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 z >4 within a 0.133-sr patch of sky. We present results of a programme of optical spectroscopy that has yielded redshifts in the range 0.5≲ z ≲ 4.4 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 z ≈ 1.9 compared with z ≈ 1.1 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 z ≥ 1.75 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 z >1.75 radio galaxies have all the properties expected if the size distribution of luminous high-redshift steep-spectrum radio sources reflects a broad range (~2dex) of source ages with a narrower range (<~1.5dex) of environmental densities, namely: (1) high-ionization lines, e.g. Lyα, of relatively low luminosity; (2) boosted low-ionization lines, e.g. CII] (3) spatially compact emission-line regions; and (4) HI-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.