The role of black hole mass in quasar radio activity

We use a homogeneous sample of ∼ 300, 0 . 3 < ∼ z < ∼ 3, radio-loud quasars drawn from the FIRST and 2dF QSO surveys to investigate a possible dependence of radio activity on black-hole mass. By analyzing composite spectra for the populations of radio-quiet and radio-loud QSOs – chosen to have the same redshift and luminosity distribution – we find with high statistical significance that radio-loud quasars are on average associated with black holes of masses ∼ 10 8 . 6 M ⊙ , about twice as large as those measured for radio-quiet quasars ( ∼ 10 8 . 3 M ⊙ ). We also find a clear dependence of black hole mass on optical luminosity of the form log (cid:16) M BH M ⊙ RL = 8 . 57( ± 0 . 06) − 0 . 27( ± 0 . 06)(M B + 24 . 5) and log (cid:16) (cid:17) RQ 8 . 43( ± 0 . 05) − 0 . 32( ± 0 . 06)(M B + 24 . 5), respectively for the case 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.