The role of black hole mass in quasar radio activity

Abstract

We use a homogeneous sample of about 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_sun, about twice as large as those measured for radio-quiet quasars (~10^{8.3} M_sun). We also find a clear dependence of black hole mass on optical luminosity of the form log (M_BH/M_sun)_{RL}= 8.57(\pm 0.06) - 0.27(\pm 0.06) (M_B + 24.5) and log (M_BH/M_sun)_{RQ}= 8.43(\pm 0.05) -0.32(\pm 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.