On the Unification of Dwarf and Giant Elliptical Galaxies

Abstract

The near orthogonal distributions of dwarf elliptical (dE) and giant elliptical (E) galaxies in the µ _e −M and µ _e −log R _e diagrams have been interpreted as evidence for two distinct galaxy formation processes. However, continuous, linear relationships across the alleged dE/E boundary at M _B = −18 mag — such as the relationships between central surface brightness (µ ₀) and: a) galaxy magnitude (M); and b) light-profile shape (n) — suggest a similar initial formation mechanism. Here we explain how these latter two trends in fact necessitate a different behavior for dE and E galaxies, exactly as observed, in diagrams involving µ _e (and also <µ>_e). Together with other linear trends across the alleged dE/E boundary, such as those between luminosity and color, metallicity, and velocity dispersion, it appears that the dEs form a continuous extension to the E galaxies. The presence of partially depleted cores in luminous (M _B < −20.5 mag) Es does however signify the action of a different physical process at the centers (<~300 pc) of these galaxies.