THE FREQUENCY OF TIDAL FEATURES ASSOCIATED WITH NEARBY LUMINOUS ELLIPTICAL GALAXIES FROM A STATISTICALLY COMPLETE SAMPLE

We present a deep broadband optical imaging study of a complete sample of luminous elliptical galaxies (M-B < -20) at distances 15-50 Mpc, selected from the Tully catalog of nearby galaxies. The images are flat to similar to 0.35% across the 20' field and reach a V-band depth of 27.7 mag arcsec-2. We derive an objective tidal interaction parameter for all galaxies and find that 73% of them show tidal disturbance signatures in their stellar bodies. This is the first time that such an analysis is done on a statistically complete sample and it confirms that tidal features in ellipticals are common even in the local universe. From the dynamical time of the sample galaxies at the innermost radius where tidal features are detected, we estimate the mass assembly rate of nearby ellipticals to be dM/M similar to 0.2 per Gyr with large uncertainty. We explore the relation between gravitational interaction signatures and the galaxy environment and find that galaxies in clusters are less disturbed than group and field galaxies. We also study how these interactions affect the broadband colors of ellipticals and find a moderate correlation, suggesting that the mergers are not accompanied by significant star formation. Finally, we find no correlation between active galactic nucleus activity, as measured by 6 cm radio emission, and large-scale tidal distortions. This implies that gravitational interactions are not the only, and perhaps not the most important, trigger of nuclear activity. In summary, we find that elliptical galaxies in groups and low-density environments continue to grow at the present day through mostly dry mergers involving little star formation.