We use a homogeneous sample of \sim 300 , 0.3 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } { z } \lower 2.15 pt \hbox { $% \buildrel < \over { \sim } $ } 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 \sim 10 ^ { 8.6 } { M _ { \odot } } , about twice as large as those measured for radio-quiet quasars ( \sim 10 ^ { 8.3 } { M _ { \odot } } ) .

We also find a clear dependence of black hole mass on optical luminosity of the form { log \left ( \frac { M _ { BH } } { M _ { \odot } } \right ) _ { RL } } = 8.57 ( \pm 0.06 ) -0.27 ( \pm 0. % 06 ) ( { M _ { B } } +24.5 ) and { log \left ( \frac { M _ { BH } } { M _ { \odot } } \right ) _ { 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 .