Recent reverberation mapping of a sample of Seyfert 1 galaxies by Wandel (1999) suggests that the black hole to bulge mass correlation in Seyferts has a significant dispersion with a mean value of , which is about one magnitude lower than in case of QSOs. Considering a simple unified formation scheme for AGNs, we demonstrated a possible black hole evolution scheme where we assume black hole accretes gas coming within its influence by a uniform Bondi flow. This scenario could interpret not only the statistical mass correlation in AGNs and normal galaxies, but a large dispersion of the black hole to bulge mass ratio in QSOs and Seyferts. We found the black hole to bulge mass correlation in such evolution scheme strongly depends on the velocity dispersion of the accreting gas, which might be an important environmental parameter for the observed correlation besides the relation between the star formation and accretion in the central accretion disk during galaxy interactions (Wang & Biermann 1998, 2000a,b). Thus, the results of Wang & Biermann (1998) represent a limiting case for the black hole evolution in AGN, where the accretion is close to Eddington rate. The black hole to bulge mass ratio versus velocity dispersion of accreting gas to the central region is plotted in Fig. 1, which gives a rough proportionality of for a fixed bulge system in our model. If we consider a relation of bulge mass with the galaxy velocity dispersion in the virial equilibrium (), we could estimate a correlation of the nuclear black hole mass with the velocity dispersion of the accreting gas as , within the slope suggested by recent work of Ferrarese & Merritt (2000) and Gebhardt et al. (2000). The square and the star in Fig. 1 correspond to the mean value of the black hole to bulge mass ratio in two typical systems (Seyferts and QSOs), which shows the velocity dispersion of the accreting gas to form a QSO phase is much higher than in case of a Seyfert, almost close to the virial velocity of the system. We present a possible interpretation for such scenario, where we think the intense starburst during violent mergers could heat or shock the interstellar medium (ISM) to a higher sound speed more efficiently than by tidal interactions, result in a higher accretion rate close to Eddington limit and a higher black hole to bulge mass ratio finally in QSOs and early type galaxies than in case of Seyferts.
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000