The Black Hole–to–Bulge Mass Relation in Active Galactic Nuclei

The masses of the central black holes in Active Galactic Nuclei (AGNs) can be estimated using the broad emission-lines as a probe of the virial mass. Using reverberation mapping to determine the size of the Broad Line Region (BLR) and the width of the variable component of the line profile Hβ line it is possible to find quite accurate virial mass estimates for AGNs with adequate data. Compiling a sample of AGNs with reliable central masses and bulge magnitudes we find an average black-hole-to-bulge mass ratio of 0.0003, a factor of 20 less than the value found for normal galaxies and for bright quasars. This lower ratio is more consistent with the black hole mass density predicted from quasar light, and is similar to the central black hole/bulge mass ratio in our Galaxy. We argue that the black hole/bulge mass ratio actually has a significantly larger range than indicated by MBHs detected in normal galaxies (using stellar dynamics) and in bright quasars, which may be biased towards large black holes. We derive a scenario of black hole growth that explains the observed distribution.