Stellar population explosions in proto-elliptical galaxies

A model is proposed for the formation of elliptical galaxies by a hierarchical sequence of mergers among subsystems. Stars are assumed to form in bursts induced by the collisions. The mergers build up systems of increasing total mass, with an increasing mass fraction in stars and increasing metallicities of stars and residual gas. The empirical correlation between metallicity and mass for elliptical galaxies can be explained if the efficiency of star formation increases with the mass of the merging systems, as is plausible dynamically. Star formation in bursts eventually ceases, either because there are no more mergers or because the gas has become too hot to cool between collisions. A disk may then form by accretion of diffuse out-lying gas after the violent mergers have stopped, whereas an elliptical galaxy remains if the residual gas is swept away before forming a disk. The model thus accounts for the relative paucity of disk galaxies in dense clusters and for metal-rich intracluster gas; it also predicts that the spheroidal components of spiral and SO galaxies are identical to elliptical galaxies. (author)