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 inside the broad-line region (BLR). Using reverberation mapping to determine the size of the 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 10 -3.5 , 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 agrees with the value found for our Galaxy. We argue that the black hole-to-bulge mass ratio actually has a significantly larger range than indicated by massive black holes detected in normal galaxies (using stellar dynamics) and in bright quasars—which may be biased toward larger black holes—and derive a scenario of black hole growth that explains the observed distribution.