Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?

Abstract

New observations of 16 dwarf elliptical galaxies (dEs) in the Virgo Cluster indicate that at least seven dEs have significant velocity gradients along their optical major axis, with typical rotation amplitudes of 20-30 km s À1. Of the remaining nine galaxies in this sample, six have velocity gradients of less than 20 km s À1 kpc À1 , while the other three observations had too low a signal-to-noise ratio to determine an accurate velocity gradient. Typical velocity dispersions for these galaxies are $44 AE 5 km s À1 , indicating that rotation can be a significant component of the stellar dynamics of Virgo dEs. When corrected for the limited spatial extent of the spectral data, the rotation amplitudes of the rotating dEs are comparable to those of similar-brightness dwarf irregular galaxies (dIs). Evidence of a relationship between the rotation amplitude and galaxy luminosity is found and, in fact, agrees well with the Tully-Fisher relation. The similarity in the scaling relations of dIs and dEs implies that it is unlikely that dEs evolve from significantly more luminous galaxies. These observations reaffirm the possibility that some cluster dEs may be formed when the neutral gaseous medium is stripped from dIs in the cluster environment. We hypothesize that several different mechanisms are involved in the creation of the overall population of dEs and that stripping of infalling dIs may be the dominant process in the creation of dEs in clusters like Virgo.