How Non-Linear Scaling Relations Unify Dwarf and Giant Elliptical Galaxies

Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia

e-mail: AGraham@swin.edu.au

Abstract

Dwarf elliptical galaxies are frequently excluded from bright galaxy samples because they do not follow the same linear relations in diagrams involving effective half light radii, R_e, or mean effective surface brightnesses, ⟨μ⟩_e. However, using two linear relations which unite dwarf and bright elliptical galaxies we explain how these lead to curved relations when one introduces either R_e or ⟨μ⟩_e. In particular, the curved ⟨μ⟩_e − R_e relation is derived here. This and other previously misunderstood curved relations, once heralded as evidence for a discontinuity between faint and bright elliptical galaxies at M_B ≈ −18 mag, actually support the unification of such galaxies as a single population whose structure (i.e. stellar concentration) varies continuously with stellar luminosity and mass.