A sample of radio galaxies spanning three decades in radio luminosity - I. The host-galaxy properties and black-hole masses
The results of analysis of HST I-band imaging of a sample of 41 radio galaxies spanning three orders of magnitude in radio luminosity at redshift z~0.5 are presented. The full sample is drawn from four complete, low-frequency selected radio samples with progressively fainter flux-density limits (3CRR, 6CE, 7CRS and the new TexOx-1000 sample). Modelling of the HST imaging data shows that the host galaxies have surface-brightness distributions consistent with those expected for classic ellipticals (Sersic parameter, beta~0.25), with beta in the range 0.17<beta<0.30, and a mean of <beta>=0.23\pm0.01. The host-galaxy luminosities are comparable with those of galaxies drawn from the bright end of the local cluster galaxy luminosity function, spanning the range 0.7L^{\star}<L<10L^{\star}, with a mean of 3.2\pm0.3 L^{\star}. In addition, the radio galaxies are shown to follow a Kormendy relation indistinguishable from that of powerful low-redshift radio galaxies. Combining our new results with those in the literature it is found that the scalelengths and Kormendy relations of 3C-class radio galaxies do not vary significantly over the redshift range 0.0<z<0.8, providing no evidence for dynamical evolution of this class of host galaxy within this redshift interval. Converting the host-galaxy luminosities into black-hole mass estimates predicts that the radio galaxies harbour central black holes with masses in the range 10^{8.1}\Msun <M_{bh}< 10^{9.5}\Msun, with a geometric mean of <M_{bh}>=10^{8.87\pm 0.04}\Msun. Finally, a significant (~3 sigma) correlation is found between black-hole mass and 151-MHz radio luminosity for those objects in the sample with either high-excitation nuclear spectra or classical double radio structures. (abridged)