THE STRUCTURE AND DYNAMICS OF ELLIPTIC GALAXIES IN COMPACT-GROUPS

We present isophotal information (effective radius, surface brightness , color, and isophotal shape) for a sample of 32 elliptical galaxies a nd central velocity dispersions for a subset of 22 of these galaxies. Using this sample of 22 galaxies, we find that the ellipticals in comp act groups lie off the ''fundamental plane'' defined by ellipticals in other environments because they have low velocity dispersions. Althou gh the number of galaxies is small, the correlation of low-velocity di spersion with isophotal shape (i.e., lower velocity dispersions for '' diskier'' galaxies) supports the reality of the offset to low velocity dispersions. Moreover, compact group ellipticals have lower velocity dispersions than other ellipticals with the same colors. This comparis on is particularly persuasive, since both quantities are distance inde pendent. We also analyze the isophotal shapes of our larger sample of galaxies and find that the frequency of elliptical galaxies with perfe ctly elliptical isophotes is lower in compact groups than in other env ironments. In addition, we find that elliptical galaxies in compact gr oups are less likely to have an excess of light at 45-degrees to the m ajor axis (i.e., boxy isophotes) and more likely to have irregular iso photes (i.e., features that are either nonaxisymmetric or misaligned w ith respect to the major and minor axes) than comparable galaxies in o ther environments. We conclude that the environment of the compact gro ups, which is favorable for tidal interactions, has modified the struc ture and dynamics of the elliptical galaxies within the groups.