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

Abstract

Hobby–Eberly Telescope (HET) spectroscopy is used to measure the velocity dispersion profile of the nearest prototypical cD galaxy, NGC 6166 in the cluster Abell 2199. We also present composite surface photometry from many telescopes. We confirm the defining feature of a cD galaxy; i.e., (we suggest), a halo of stars that fills the cluster center and that is controlled dynamically by cluster gravity, not by the central galaxy. Our HET spectroscopy shows that the velocity dispersion of NGC 6166 rises from σ ≃ 300 km s[superscript −1] in the inner r ~ 10" to σ = 865 ± 58 km s[superscript −1] at r ~ 100'' in the cD halo. This extends published observations of an outward σ increase and shows for the first time that σ rises all the way to the cluster velocity dispersion of 819 ± 32 km s[superscript −1]. We also observe that the main body of NGC 6166 moves at +206 ± 39 km s[superscript −1] with respect to the cluster mean velocity, but the velocity of the inner cD halo is ~70 km s[superscript −1] closer to the cluster velocity. These results support our picture that cD halos consist of stars that were stripped from individual cluster galaxies by fast tidal encounters. However, our photometry does not confirm the widespread view that cD halos are identifiable as an extra, low-surface-brightness component that is photometrically distinct from the inner, steep-Sérsic-function main body of an otherwise-normal giant elliptical galaxy. Instead, all of the brightness profile of NGC 6166 outside its core is described to ±0.037 V mag arcsec[superscript −2] by a single Sérsic function with index n ≃ 8.3. The cD halo is not recognizable from photometry alone. This blurs the distinction between cluster-dominated cD halos and the similarly-large-Sérsic-index halos of giant, core-boxy-nonrotating ellipticals. These halos are believed to be accreted onto compact, high-redshift progenitors ("red nuggets") by large numbers of minor mergers. They belong dynamically to their central galaxies. Still, cDs and core-boxy-nonrotating Es may be more similar than we think: both may have outer halos made largely via minor mergers and the accumulation of tidal debris. We construct a main-body+cD-halo decomposition that fits both the brightness and dispersion profiles. To fit σ(r), we need to force the component Sérsic indices to be smaller than a minimum-x[superscript 2] photometric decomposition would suggest. The main body has M[subscript V] ≃ -22.8 ≃ 30% of the total galaxy light. The cD halo has M[subscript V] ≃ -23.7, ~1/2 mag brighter than the brightest galaxy in the Virgo cluster. A mass model based on published cluster dynamics and X-ray observations fits our observations if the tangential dispersion is larger than the radial dispersion at r ≃ 20"–60". The cD halo is as enhanced in α element abundances as the main body of NGC 6166. Quenching of star formation in [< over ~]1 Gyr suggests that the center of Abell 2199 has been special for a long time during which dynamical evolution has liberated a large mass of now-intracluster stars.