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

4. Summary

In this paper, we presented an model of formation and evolution of early-type galaxies by combination of the viscous accretion disk model, the star formation and nuclear activity within the hierarchical galaxy formation scheme.

The basic ideas are:

1) An elliptical/bulge is formed by the merger of two disk galaxies in a hierarchical universe.

2) Star formation and AGN evolution coexist in the disk evolution stage (pre-merger) and also post-merger stage. They compete for the gas supply and interact with each other.

3) The violent interaction between two disk galaxies by a merger can not only form the spheroidal component of the elliptical or bulge, but also drive the cool outer gas to the center, trigger a starburst and a central AGN there. The merger triggered starburst probably can stir up a turbulent accretion to feed both the starburst and AGN in the center. These two activities interact with each other, feedback, thus speed up the BH growth, drain the gas in the molecular disk quickly and constrain the final mass ratio .

From our calculation, we found:

1) Mergers can help to grow a massive BH in a very short time, and shorten the convergence time for the mass ratio () to a limited region.

2) The final mass ratio limitation does not depend on the mass of the protogalaxy.

3) The exact time, when the merger happens after the disk evolution quiets down, will not influence the mass ratio limitation.

4) Whether the ellipticals/bulges are formed by the major mergers or minor mergers between two disk galaxies, or by multimergers is not very critical for our final result.

5) The final mass ratio limitation does depend moderately on the inner radius of the molecular disk , and the mass return rate from star formation.

6) The exact number of the parameters for the star formation and accretion time scale has no influence on the final mass ratio limitation, but it is obvious that the correlation of these two time scales (or ) is very critical to the final mass ratio limitation. It seems that the mass ratio () can finally converge to a limited region near 0.006 only when the starburst time scale and the turbulent accretion time scale can be approximately equal.

The conclusion 6) probably indicates a physical relationship between starburst and central AGN as we discussed already in the previous section. In the hierarchical universe, mergers will destroy the quiescent disk galaxies, transfer the stars in the disks completely to form a spheroidal component. At the same time, it will also drive a large amount of gas into the central region and trigger a starburst. The kinetic energy input to the ISM from the young massive stars and supernovae can heat and shock the ISM, and probably induce turbulent viscous accretion there. This turbulent viscous accretion can feed both the starburst and central AGN, and grow a massive BH very quickly. The two activities, starburst and central AGN can drain the gas in the disk in a very short time, and also when the power of the central engine reaches some degree, it probably can start a wind and help the starburst to blow out the hot gas completely out of the disk, thus stop the accretion and star formation there.

In this paper, we did not include the kinetic energy term in our viscosity directly, but from this simple model and calculation, it seems that the feedback relationship between the starburst and central engine can be a very interesting physical process. To do this, we need more elaborated physical models and more numerical work.

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998

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