The role of black hole mass in quasar radio activity

We use a homogeneous sample of ∼300, 0.3 ≲z≲ 3, radio-loud quasars (QSOs) drawn from the FIRST and 2dF QSO surveys to investigate a possible dependence of radio activity on black hole mass. By analysing composite spectra for the populations of radio-quiet and radio-loud QSOs - chosen to have the same redshift and luminosity distributions - we find with high statistical significance that radio-loud quasars are on average associated with black holes of masses ∼108.6 M⊙, about twice as large as those measured for radio-quiet quasars (∼108.3 M ⊙). We also find a clear dependence of black hole mass on optical luminosity of the form log(MBH/M⊙) RL= 8.57(±0.06) - 0.27(±0.06) (MB+ 24.5) and log(MBH/M⊙) RQ= 8.43(±0.05) - 0.32(±0.06) (MB+ 24.5), respectively, for the cases of radio-loud and radio-quiet quasars. It is intriguing to note that these two trends run roughly parallel to each other, implying that radio-loud quasars are associated to black holes more massive than those producing the radio-quiet case at all sampled luminosities. On the other hand, in the case of radio-loud quasars, we find evidence for only a weak (if any) dependence of the black hole mass on radio power. The above findings seem to support the belief that there exists - at a given optical luminosity - a threshold black hole mass associated with the onset of significant radio activity such as that of radio-loud QSOs; however, once the activity is triggered, there appears to be very little connection between black hole mass and level of radio output. © 2005 RAS.