STRUCTURE AND FORMATION OF cD GALAXIES: NGC 6166 IN ABELL 2199

Abstract

The Hobby-Eberly Telescope is used to measure the velocity dispersion profile of the nearest prototypical cD galaxy, NGC 6166 in cluster Abell 2199. We also present surface photometry from many telescopes. We confirm the defining feature of a cD -- a halo of stars that fills the cluster center and that is controlled by cluster gravity, not by the central galaxy. The velocity dispersion of NGC 6166 rises from 300 km/s at the center to 865 +- 58 km/s at 100 arcsec radius in the halo. This shows for the first time that the dispersion rises all the way to the cluster value, 819 +- 32 km/s. We find that the main body of NGC 6166 moves at 206 +- 39 km/s with respect to the cluster velocity, whereas the velocity of the cD halo is 70 km/s closer to the cluster velocity. These results support our picture that cD halos consist of stars that are stripped from cluster galaxies. But we do not confirm the view that cD halos are an extra, low-surface-brightness component that is distinct from the main body of a normal giant elliptical. Instead, all of the brightness profile of NGC 6166 outside its core is described to +- 0.037 mag/arcsec**2 by a single Sersic function with index n ~ 8.3. The cD halo is not recognizable from photometry alone. This blurs the distinction between cDs and similar-n core-boxy-nonrotating ellipticals. Both may have halos made largely via minor mergers and the accumulation of debris. However, the cD halo of NGC 6166 is as enhanced in alpha elements as the main body. Quenching of star formation in <~1 Gyr happened even to the galaxies that contributed the cD halo.