THE FIRST GENERATION OF VIRGO CLUSTER DWARF ELLIPTICAL GALAXIES?

Abstract

In the light of the question whether most early-type dwarf (dE) galaxies in\nclusters formed through infall and transformation of late-type progenitors, we\nsearch for an imprint of such an infall history in the oldest, most centrally\nconcentrated dE subclass of the Virgo cluster: the nucleated dEs that show no\nsignatures of disks or central residual star formation. We select dEs in a\n(projected) region around the central elliptical galaxies, and subdivide them\nby their line-of-sight velocity into fast-moving and slow-moving ones. These\nsubsamples turn out to have significantly different shapes: while the fast dEs\nare relatively flat objects, the slow dEs are nearly round. Likewise, when\nsubdividing the central dEs by their projected axial ratio into flat and round\nones, their distributions of line-of-sight velocities differ significantly: the\nflat dEs have a broad, possibly two-peaked distribution, whereas the round dEs\nshow a narrow single peak. We conclude that the round dEs probably are on\ncircularized orbits, while the flat dEs are still on more eccentric or radial\norbits typical for an infalling population. In this picture, the round dEs\nwould have resided in the cluster already for a long time, or would even be a\ncluster-born species, explaining their nearly circular orbits. They would thus\nbe the first generation of Virgo cluster dEs. Their shape could be caused by\ndynamical heating through repeated tidal interactions. Further investigations\nthrough stellar population measurements and studies of simulated galaxy\nclusters would be desirable to obtain definite conclusions on their origin.\n