THE STRUCTURE OF GALAXIES. III. TWO STRUCTURAL FAMILIES OF ELLIPTICALS

Abstract

Using isophotal radius correlations for a sample of Two Micron All Sky Survey ellipticals, we have constructed a series of template surface brightness profiles to describe the profile shapes of ellipticals as a function of luminosity. The templates are a smooth function of luminosity, yet are not adequately matched to any fitting function supporting the view that ellipticals are weakly nonhomologous with respect to structure. Through comparison to the templates, it is discovered that ellipticals are divided into two families: those well matched to the templates, and a second class of ellipticals with distinctly shallower profile slopes. We refer to this second type of ellipticals as D class, an old morphological designation acknowledging diffuse appearance on photographic material. D ellipticals cover the same range of luminosity, size, and kinematics as normal ellipticals, but maintain a signature of recent equal-mass dry mergers. We propose that normal ellipticals grow after an initial dissipation formation era by accretion of low-mass companions as outlined in hierarchical formation scenarios, while D ellipticals are the result of later equal-mass mergers producing shallow luminosity profiles.