Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: I. A Keck/HST Study of Six Virgo Cluster Dwarfs

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  • 10 June 2002
Abstract
Keck/ESI spectroscopy is presented for six Virgo Cluster dwarf elliptical (dE) galaxies. 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/sigma 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 WFPC2 surface photometry and spherically-symmetric dynamical models, we determine global mass-to-light ratios 3 < M/L_V < 6. These ratios are comparable to those expected for an intermediate-age stellar population and are broadly consistent with the (V-I) colors of the galaxies. This implies that these dEs do not have a significant dark matter component inside an effective radius. Central black holes more massive than 10^7 M_sun are ruled out at the 99.9% confidence level. For the five nucleated dEs in our sample, we determine kinematic and photometric properties 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 non-rotating dEs appear to have a slightly higher mean mass and mass-to-light ratio than their rotating counterparts.

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