Implications for the origin of early-type dwarf galaxies - the discovery of rotation in isolated, low-mass early-type galaxies

Abstract

We present the discovery of rotation in quenched, low-mass early-type galaxies that are isolated. This finding challenges the claim that (all) rotating dwarf early-type galaxies in clusters were once spiral galaxies that have since been harassed and transformed into earlytype galaxies. Our search of the Sloan Digital Sky Survey data within the Local Volume (z < 0.02) has yielded a sample of 46 galaxies with a stellar mass M* ≲ 5 × 109M⊙ (median M* ~ 9.29 × 108M⊙), a low Hα equivalent width EWHα < 2 Å, and no massive neighbour (M* ≳ 3 × 1010 M⊙) within a velocity interval of δV = 500 km s-1 and a projected distance of ~1 Mpc. Nine of these galaxies were subsequently observed with Keck Echellette Spectrograph and Imager and their radial kinematics are presented here. These extend out to the half-light radius Re in the best cases, and beyond Re/2 for all. They reveal a variety of behaviours similar to those of a comparison sample of early-type dwarf galaxies in the Virgo cluster observed by Toloba et al. Both samples have similar frequencies of slow and fast rotators, as well as kinematically decoupled cores. This, and especially the finding of rotating quenched low-mass galaxies in isolation, reveals that the early-type dwarfs in galaxy clusters need not be harassed or tidally stirred spiral galaxies.