Annu. Rev. Astron. Astrophys. 1991. 29: 239-274
Copyright © 1991 by . All rights reserved

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4.3 When Did Ellipticals Form?

The dynamics of equilibrium models do not give constraints on when ellipticals formed. Kinematically distinct cores may persist for at least a relaxation time, which is larger than a Hubble time. The original arguments for a very fast collapse and formation of ellipticals came from estimates of the free-fall time of galaxies with the assumption of a small collapse factor (146). Since the discovery of dark matter (and the resulting decrease in mean density and increase in free-fall time), and the realization that galaxy formation may proceed in a hierarchical way, the estimates of the formation epoch vary between redshifts of 30 to redshifts of 1 or less (21, 365). Furthermore, the formation process may be extended in time, even for spheroids.

The non-equilibrium structures in elliptical galaxies, such as shells, tidal tails, and asymmetric rotation curves, give independent information on the formation epoch, or on merger rates and mass accretion rates (11, 332). If measured accurately, these can constrain simulations of galaxy formation (57). Important information may come from studies which combine deep photometry with an analysis of the stellar populations (59, 310). This will shed light on star formation during merger events, and may help in detecting older merger remnants.