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Elliptical galaxies are characterized by a narrow range of properties which provide important constraints on their origin. Structural and kinematic differences between real ellipticals and remnants of simulations of galaxy pair mergers indicate that many ellipticals may have had rather complex evolutionary histories. The remnants analyzed here suggest that a significant fraction of elliptical galaxies may form through the merger of several, rather than two, progenitor galaxies.
Instead of two disk galaxies, the progenitors in our self-consistent simulations are dense, multi-member clusters with low velocity dispersions which are representative of conditions in some groups or cores of larger clusters. Initially, the systems consist of six disk galaxies each with bulge:disk:halo mass and particle number ratios of .333:1:5.8 and 16384:65536:65536, respectively. Remnants of these multiple mergers exhibit many characteristics of elliptical galaxies. In contrast to the mostly prolate remnants produced by galaxy pair mergers, those analyzed here have small intrinsic triaxialities, with mostly oblate shapes, perhaps accounting for the observed peak of Hubble types at E0-E2. Luminosity profiles are fit by an $R^{1/4}$ law over most of their extent. However, we find that the central density is not highly enhanced in mergers of several galaxies with bulges; like mergers of two stellar disks, multiple mergers produce remnants with a diffuse core. The remnants are supported by velocity dispersion in the inner regions; however, they also exhibit a rotational velocity which peaks near their effective radii. The angular momentum vectors and minor axes align as appears true for a majority of real ellipticals.
While it is premature to conclude that mergers of pairs of spirals could not have produced most elliptical galaxies, we consider additional, more complex formation paths in depth. We investigate one such possibility: the formation of remnants by repeated stellar dynamical merging in dense galactic environments. These objects share many properties in common with giant ellipticals. Whether or not this scenario, which is a logical extension of the original merger hypothesis, can account for all attributes of elliptical galaxies remains to be seen. Models such as the ones presented here can be combined with studies of both the cores and outer structure of ellipticals and surveys of galaxy environments in order to elucidate the role of mergers in galactic evolution.
