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 ∼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( M BH /M ⊙ ) RL = 8.57(±0.06) − 0.27(±0.06) ( M B + 24.5) and log( M BH /M ⊙ ) RQ = 8.43(±0.05) − 0.32(±0.06) ( M B + 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.