Internal Dynamics, Structure, and Formation of Dwarf Elliptical Galaxies. I. A Keck/Hubble Space Telescope Study of Six Virgo Cluster Dwarf Galaxies

Spectroscopy with the Keck II 10 m telescope and Echelle Spectrograph and Imager is presented for six Virgo Cluster dwarf elliptical (dE) galaxies in the absolute magnitude range -15.7<=M_V<=-17.2. The mean line-of-sight velocity and velocity dispersion are resolved as a function of radius along the major axis of each galaxy, nearly doubling the total number of dEs with spatially resolved stellar kinematics. None of the observed objects shows evidence of strong rotation; upper limits on v_rot/σ, the ratio of the maximum rotational velocity to the mean velocity dispersion, are well below those expected for rotationally flattened objects. Such limits place strong constraints on dE galaxy formation models. Although these galaxies continue the trend of low rotation velocities observed in Local Group dEs, they are in contrast to recent observations of large rotation velocities in slightly brighter cluster dEs. Using surface photometry from Hubble Space Telescope Wide Field Planetary Camera 2 images and spherically symmetric dynamical models, we determine global mass-to-light ratios 3<=Υ_V<=6. These ratios are comparable to those expected for an old to intermediate-age stellar population and are broadly consistent with the observed V-I colors of the galaxies. These dE galaxies therefore do not require a significant dark matter component inside an effective radius. We are able to rule out central black holes more massive than ~10⁷ M_solar. For the five nucleated dEs in our sample, kinematic and photometric properties were determined for the central nucleus separately from the underlying host dE galaxy. These nuclei are as bright or brighter than the most luminous Galactic globular clusters and lie near the region of fundamental plane space occupied by globular clusters. In this space, the Virgo dE galaxies lie in the same general region as Local Group and other nearby dEs, although nonrotating dEs appear to have a slightly higher mean mass and mass-to-light ratio than rotating dEs; the dE galaxies occupy a plane parallel to, but offset from, that occupied by normal elliptical galaxies. 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.