The Structure of Galaxies: III. Two Structural Families of Ellipticals

TL;DR

This paper identifies two structural families of elliptical galaxies based on their surface brightness profiles, suggesting different formation histories involving hierarchical merging and accretion processes.

Contribution

It introduces a new classification of ellipticals into two families based on profile shapes and links these to their formation mechanisms.

Findings

Ellipticals can be grouped into two families with distinct profile slopes.

D ellipticals show signs of recent equal mass dry mergers.

Normal ellipticals likely grow via accretion of low mass companions.

Abstract

Using isophotal radius correlations for a sample of 2MASS 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 non-homologous 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 these 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.