On Quasar Masses and Quasar Host Galaxies

The mass of massive black holes in quasar cores can be deduced by using the typical velocities of Hβ-emitting clouds in the broad-line region and by the size of this region. However, this estimate depends on various assumptions and is susceptible to large systematic errors. The Hβ-deduced black hole mass in a sample of 14 bright quasars is found here to correlate with the quasar host galaxy luminosity, as determined with the Hubble Space Telescope (HST). This correlation is similar to the black hole mass versus bulge luminosity correlation found by Magorrian et al. in a sample of 32 nearby normal galaxies. The similarity of the two correlations is remarkable since the two samples involve apparently different types of objects and since the black hole mass estimates in quasars and in nearby galaxies are based on very different methods. This similarity provides a "calibration" of the Hβ-deduced black hole mass estimate, suggesting that it is accurate to ±0.5 on a log scale. The similarity of the two correlations also suggests that quasars reside in otherwise normal galaxies and that the luminosity of quasar hosts can be estimated to ±0.5 mag based on the quasar continuum luminosity and the Hβ line width. Future imaging observations of additional broad-line active galaxies with the HST are required in order to explore the extent, slope, and scatter of the black hole mass versus host bulge luminosity correlation in active galaxies.