On the unification of dwarf and giant elliptical galaxies

TL;DR

This paper argues that dwarf and giant elliptical galaxies are part of a continuous spectrum rather than two distinct classes, based on linear relationships in their structural properties, despite some differences in core features.

Contribution

It demonstrates that linear trends in surface brightness and other properties support a unified formation mechanism for dE and E galaxies, challenging the notion of two separate processes.

Findings

Linear relationships connect dE and E galaxies across the boundary.

Differences in core structures indicate separate processes in luminous ellipticals.

dEs are a continuous extension of E galaxies in structural properties.

Abstract

The near orthogonal distributions of dwarf elliptical (dE) and giant elliptical (E) galaxies in the mu_e-Mag and mu_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 those between central surface brightness (mu_0) and (i) galaxy magnitude and (ii) light-profile shape (n) - suggest a similar, governing formation mechanism. Here we explain how these latter two linear trends necessitate a different behavior for dE and E galaxies, exactly as observed, in diagrams involving mu_e (and also <mu>_e). A natural consequence is that the distribution of dEs and Es in Fundamental Plane type analyses that use the associated intensity I_e, or <I>_e, are expected to appear different. 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.