The existence of supermassive black holes, at least in elliptical and bulge-dominated galaxies is suggested by various observations, such as the optical spectroscopy, VLBI water maser measurements and X-ray observations of AGNs (Nakai et al. 1993, Kormendy & Richstone 1995, Faber et al. 1997, Miyoshi et al. 1995, Greenhill et al. 1996, Pounds et al. 1990). An intensive discussion about the possible relationship between the central properties and their host galaxies was spurred on especially since the high resolution HST photometry and several ground-based CCD photometries of early type galaxies. Although there are significant scatters, a linear mass correlation of central black holes and their host spheroids has still a high probability.
The theoretical interpretation for such linear scale is discussed by several authors (Merritt 1998, Silk & Rees 1998, Wang & Biermann 1998). Considering various observations, Wang & Biermann (1998) proposed a possible formation scenario for elliptical and bulge-dominated galaxies, as well as the consequential active evolution phase where early type galaxies are the products of major mergers between two comparable disk galaxies; the violent collision between two galaxies could destroy the original stellar disks and form the spheroidal component of merging galaxies after the relaxation; help to release the angular moment of the cold gas outside and drive them inwards; a central starburst and QSO accretion in the central condensed gas disk will compete for the gas supply, feedback and drain the gas in the disk in a short time; possibly grow a supermassive black hole during the spheroid formation, blow up the gas left probably by the nuclear wind when the central engine gets to be powerful enough; thus restrict the central black hole mass and the mass of the stellar component to a ratio of .
The numerical simulation by Wang & Biermann (1998) shows the star formation approximately scales with the nuclear accretion during galaxy interactions, which can regulate the black hole to bulge mass ratio to the observed level within a factor of three. In this scenario, a quasar black hole is possibly formed at a cosmic time scale of , corresponding to in a flat Einstein-de Sitter world model with . Afterwards, when the quasar active phase has ceased and the spheroidal stellar system gets relaxed in a comparable relaxation time scale, an elliptical or spiral bulge harboring a massive black hole in the center with the mass in a factor of the host spheroid may appear in the universe.
We should mention the black hole evolution in this model is assumed to reach the Eddington accretion rate whenever there is sufficient gas supplied to the center. The question of whether this mass correlation is universal for all AGNs, i.e. whether Seyfert galaxies follow a similar black hole to bulge mass correlation as in early type galaxies and luminous QSOs, is not only important for the understanding of the correlation between the host galaxies and the evolution of active nuclei, but for the relation of the formation and evolution scheme between Seyferts and QSOs.
Recent reverberation mapping of a sample of Seyferts with reliable central masses and the bulge magnitudes by Wandel (1999), suggests that there is a broad distribution of black hole to bulge mass ratio with a mean of , about one magnitude lower than the value in early type galaxies and bright QSOs. Although this discrepancy may in part be explained by systematic errors and selection or orientation effects (Wandel 2000), a significant difference probably remains, in particular between Seyfert 1 galaxies and QSOs, at least a large dispersion for the medium-bulge system, with masses similar to those of Seyferts (Magorrian et al. 1998, Richstone et al. 1999, Ferrarese & Merritt 2000). Our purpose is to firstly present a model which could explain the black hole to bulge mass correlation in AGNs and the dependence on the environmental parameters of the host galaxies, such as the gas or stellar velocity dispersion, as well as the relation of the central starburst and accretion process during galactic interaction; Secondly, discuss the dispersion of black hole to bulge mass ratio in QSOs and Seyferts within a simple unified formation scheme, where the bulge formation and nuclear activity are triggered by galaxy mergers or tidal interactions. We found the variation of the velocity dispersion of accreting gas could cause a range of distribution for the mass ratio, leading to a correlation of the nuclear black hole mass and the gas velocity dispersion roughly as from our simulation, within the slope suggested by recent work of Ferrarese & Merritt (2000) and Gebhardt et al. (2000).
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000