Internal Dynamics, Structure, and Formation of Dwarf Elliptical Galaxies. II. Rotating versus Nonrotating Dwarfs

We present spatially resolved internal kinematics and stellar chemical abundances for a sample of dwarf elliptical (dE) galaxies in the Virgo Cluster observed with the Keck telescope and Echelle Spectrograph and Imager. In combination with previous measurements, we find that four out of 17 dE's have major-axis rotation velocities consistent with rotational flattening, while the remaining dE's have no detectable major-axis rotation. Despite this difference in internal kinematics, rotating and nonrotating dE's are remarkably similar in terms of their position in the fundamental plane, morphological details, stellar populations, and local environment. We present evidence for (or confirm the presence of) faint underlying disks and/or weak substructure in a fraction of both rotating and nonrotating dE's, but a comparable number of counterexamples exist for both types that show no evidence of such structure. Absorption line strengths were determined based on the Lick/IDS system (Hβ, Mg b, Fe5270, and Fe5335) for the central region of each galaxy. We find no difference in the line-strength indices, and hence stellar populations, between rotating and nonrotating dE galaxies. The best-fitting mean age and metallicity for our same of 17 dE's are 5 Gyr and [Fe/H]=-0.3 dex, respectively, with rms spreads of 3 Gyr and 0.1 dex. The majority of dE's are consistent with solar [α/Fe] abundance ratios. By contrast, the stellar populations of classical elliptical galaxies are, on average, older, more metal-rich, and α-enhanced relative to our dE sample. The line strengths of our dE's are consistent with the extrapolation of the line strength versus velocity dispersion trend seen in classical elliptical galaxies. Finally, the local environments of both rotating and nonrotating dE's appear to be diverse in terms of their proximity to larger galaxies in real or velocity space within the Virgo Cluster. Thus, rotating and nonrotating dE's are remarkably similar in terms of their structure, stellar content, and local environments, presenting a significant challenge to theoretical models of their formation.

Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.