The intrinsic colour of radio galaxies and photometric determination of redshift

Spectroscopic observations of 11 radio galaxies from the 5C12 survey are reported. The galaxies have redshifts in the range of |$0.12 \lt z \lt 0.31$| and radio luminosites |$\text P_{408}\enspace (\text {W/Hz}^{-1})$| in the range of |$24.4 \lt \text {log}_{10} \enspace P_{408} \lt 26.3 \enspace(H_0 = 50\enspace \text {km} \enspace \text s^{-1} \enspace \text {Mpc}^{-1})$|⁠. The spectra show no evidence of nuclear activity or of strong star formation. V – R colors in the rest-frame of the galaxies have been determined from the spectra and show an intrinsic scatter of < 0.02 mag. There is no significant change of rest-frame color with redshift, consistent with no evolution of the stellar population at z < 0.3, and with there having been a single short burst of star formation at high redshift. The spectroscopic redshifts agree with those predicted on the basis of broad-band photographic colors, with rms difference |$\sigma_z=0.025$|⁠. Given sufficiently accurate photometry, the small intrinsic range of V – R colors may allow redshifts of similar radio sources to be predicted from broad-band colors with |$\sigma_z$| as small as 0.01. The median absolute magnitude of the galaxies is |$M_\text v=-22.8$| mag, with rms scatter about this value of 0.4 mag.