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 b asic observational data on the 6C* 151-MHz radio sample: a low-frequency se lected sample that exploits filtering criteria based on radio properties (s teep spectral index and small angular size) to find radio sources at redshi ft 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 less th an or similar to z less than or similar to 4.4 for the 29 sources in the sa mple, 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-luminosi ty radio sources: the median redshift of 6C* is z approximate to 1.9 compar ed with z approximate to 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 greater t han or equal to 1.75 radio galaxies follow a rough proportionality between Ly alpha and 151-MHz luminosity, which, like similar correlations seen in s amples of lower redshift radio sources, is indicative of a primary link bet ween the power in the source of the photoionizing photons (most likely a hi dden quasar nucleus) and the power carried by the radio jets. We argue that radio sources modify their environments and that the range of emission-lin e 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 al l the properties expected if the size distribution of luminous high-redshif t steep-spectrum radio sources reflects a broad range (similar to2 dex) of source ages with a narrower range (less than or similar to1.5 dex) of envir onmental densities, namely: (1) high-ionization lines, e.g. Ly alpha of rel atively low luminosity; (2) boosted low-ionization lines, e.g. C II]; (3) s patially compact emission-line regions; and (4) HI-absorbed Ly alpha profil es. This is in accord with the idea that all high-redshift, high-luminosity radio sources are triggered in similar environments, presumably recently c ollapsed massive structures.