On the evolution of the black hole: spheroid mass ratio

We present the results of a study which uses the 3C RR sample of radio-loud active galactic nuclei to investigate the evolution of the black hole: spheroid mass ratio in the most massive early-type galaxies from 0 < z < 2. Radio-loud unification is exploited to obtain virial (linewidth) black hole mass estimates from the 3C RR quasars, and stellar mass estimates from the 3C RR radio galaxies, thereby providing black hole and stellar mass estimates for a single population of early-type galaxies. At low redshift (z less than or similar to 1), the 3C RR sample is consistent with a black hole: spheroid mass ratio of M-bh/M-sph similar or equal to 0.002, in good agreement with that observed locally for quiescent galaxies of similar stellar mass (M-sph similar or equal to 5 x 10(11) M-circle dot). However, over the redshift interval 0 < z < 2 the 3C RR black hole: spheroid mass ratio is found to evolve as M-bh/M-sph proportional to (1 + z)(2.07 +/- 0.76), reaching M-bh/M-sph similar or equal to 0.008 by redshift z similar or equal to 2. This evolution is found to be inconsistent with the local black hole: spheroid mass ratio remaining constant at a moderately significant level ( 98 per cent). If confirmed, the detection of evolution in the 3C RR black hole: spheroid mass ratio further strengthens the evidence that, at least for massive early-type galaxies, the growth of the central supermassive black hole may be completed before that of the host spheroid.