|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
1.4. Preconception Number 3: Bulges Are Similar to Elliptical Galaxies
Bulges and ellipticals have traditionally been fit by the same surface brightness profiles, the de Vaucouleurs R1/4 law; for simplicity, one is tempted to assume that bulges are simply scaled-down ellipticals and that they formed the same way. N-body simulations (e.g. van Albada 1982), together with analytic considerations of "maximum entropy" end states (Tremaine et al 1986), suggested that this was through violent relaxation of a dissipationless, perhaps lumpy, system. These ideas incorporate the proposition (e.g. Toomre 1977, Barnes & Hernquist 1992) that equal-mass mergers destroy preexisting stellar disks and form bulges and ellipticals, of which these latter two are distinguished only by mass.
Furthermore, the stellar kinematics of ellipticals and bulges of the same luminosity are similar, in that each rotates approximately as rapidly as predicted by isotropic oblate models (Davies et al 1983). However, the two general categories of "bulges" and "ellipticals" are becoming clear to be somewhat heterogeneous and may cover systems that formed in a variety of ways.
The above preconceptions may be tested against modern data. We proceed with the systems for which the most detailed data may be obtained, the galaxies in the Local Group, and then outward in distance.