The Elliptical-Spheroidal and Elliptical-Elliptical Galaxy Dichotomies

TL;DR

This paper reviews galaxy classifications, highlighting the structural and formation differences between spheroidal, elliptical, and bulge galaxies, and introduces new insights into the properties and origins of elliptical galaxy subtypes.

Contribution

It clarifies the structural distinctions among galaxy types and introduces a new feature of the E-E dichotomy related to core properties and formation processes.

Findings

Spheroidals are structurally similar to late-type galaxies, not dwarf ellipticals.

Ellipticals exhibit two varieties: giant non-rotating and smaller rotating types.

Coreless ellipticals have extra light from starbursts, indicating wet merger origins.

Abstract

This paper summarizes Kormendy et al. (2009, ApJS, in press, arXiv:0810.1681). We confirm that spheroidal galaxies have fundamental plane correlations that are almost perpendicular to those for bulges and ellipticals. Spheroidals are not dwarf ellipticals. They are structurally similar to late-type galaxies. We suggest that they are defunct ("red and dead") late-type galaxies transformed by a variety of gas removal processes. Minus spheroidals, ellipticals come in two varieties: giant, non-rotating, boxy galaxies with cuspy cores and smaller, rotating, disky galaxies that lack cores. We find a new feature of this "E-E dichotomy": Coreless ellipticals have extra light at the center with respect to an inward extrapolation of the outer Sersic profile. We suggest that extra light is made in starbursts that swamp core scouring in wet mergers. In general, only giant, core ellipticals contain X-ray gas halos. We suggest that they formed in mergers that were kept dry by X-ray gas heated by active galactic nuclei.