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Astron. Astrophys. 334, 87-95 (1998)

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1. Introduction

The main discussion about the formation of early-type galaxies centers on a 'nature' versus 'nurture' dichotomy. Sandage, Freeman & Stokes (1970) explained the 'nature' scheme as follows: A disk or a spheroidal configuration is produced by the collapse of a rapidly rotating or slowly rotating primordial gas cloud. The 'nurture' hypothesis, also referred to as the hierarchical scheme (Toomre 1977, Kauffmann 1996, Baugh et al. 1996, Mihos & Hernquist 1996, Walker et al. 1996) is: Mergers between disk galaxies where their mass ratio ([FORMULA]) exceeds a value (of order [FORMULA] in some simulations, but almost certainly a function of orbital parameters) form ellipticals or bulges in spiral galaxies; starbursts and nuclear activity can be triggered probably by the merger event (Fritze-v. Alvensleben & Gerhard 1994).

It seems that the 'nurture' scheme, which models the observed properties of ellipticals/bulges in a scenario where they are formed by the merging of disk galaxies in a hierarchical universe, is more supported by recent numerical simulations and observations (Barnes 1988, Kauffman et al. 1993), and it also, for the first time, placed the theory of the formation and evolution of elliptical galaxies in a proper cosmological context.

Massive compact objects appear to be ubiquitous components of galactic nuclei. Particularly attractive is the possibility that we are observing the black holes (BHs) that once powered quasars and that still provide the energy source of Active Galactic Nuclei (AGN). This hypothesis is at least crudely consistent with the observed properties of quasars and AGN. Current black hole candidates have masses ranging from [FORMULA] in the case of Milky Way to [FORMULA] in the case of M87 (Kormendy & Richstone 1995). The compelling AGN theory is that these massive BHs are fuelled by gaseous accretion disks (Lüst 1952, Lynden-Bell 1969, Shakura & Sunyaev 1973, Rees 1984).

Starbursts and AGN are two fascinating phenomena after a merger. They are usually treated as independent activities in numerical simulations (Sargent et al. 1991, Barnes 1992). Recent observations of Ultraluminous Infrared Galaxies (ULIGs) by Very Long Baseline Interferometry (VLBI), the Very Large Array (VLA), infrared and optical wavelengths start to allow to consider a comprehensive picture for a starburst-AGN coexistence scenario (Smith et al. 1997, Heckman 1997). Also, HST photometry of ellipticals and spiral bulges, and Near-IR images of quasar hosts shows us that the central properties strongly depend on their host galaxies. A 'nuclear luminosity/host-mass limitation' in the most luminous quasars probably represents a physical limit on the mass of a black hole that can exist in a given galaxy spheroid mass. A mass ratio constraint of black hole mass and its host spheroid mass [FORMULA] within a factor of three has been deduced from these observations and dynamical models (Faber et al. 1997, McLeod & Rieke 1994a, 1994b, 1995a, 1995b, Magorrian et al. 1996, Kormendy & Richstone 1995). The interesting point is whether this correlation is due to a statistical sampling bias, or a substantial evolutionary result.

In this paper, we try to use a simplified model to describe the steps of the elliptical/bulge formation, starburst and the central engine evolution in a hierarchical scheme by assuming that star formation and central object evolution are coexistent, and fed by a viscous accretion disk. In our scheme a disk galaxy is entirely modelled as an accretion disk. Mergers can trigger a starburst in the central region, and induce more turbulence there. They thus drive turbulent accretion to feed both the AGN and starburst, grow a massive BH (Linden et al. 1993, Tacconi et al. 1994). The competition and feedback interaction between these two activities can lead to a constraint, and limit the ratio of BH mass and its spheroidal stellar mass ([FORMULA]) to a certain range of values.

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998