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The Hobby-Eberly Telescope is used to measure the velocity dispersion profileof the nearest prototypical cD galaxy, NGC 6166 in cluster Abell 2199. We alsopresent surface photometry from many telescopes. We confirm the definingfeature of a cD -- a halo of stars that fills the cluster center and that iscontrolled by cluster gravity, not by the central galaxy. The velocitydispersion of NGC 6166 rises from 300 km/s at the center to 865 +- 58 km/s at100 arcsec radius in the halo. This shows for the first time that thedispersion rises all the way to the cluster value, 819 +- 32 km/s. We find thatthe main body of NGC 6166 moves at 206 +- 39 km/s with respect to the clustervelocity, whereas the velocity of the cD halo is 70 km/s closer to the clustervelocity. These results support our picture that cD halos consist of stars thatare stripped from cluster galaxies. But we do not confirm the view that cDhalos are an extra, low-surface-brightness component that is distinct from themain body of a normal giant elliptical. Instead, all of the brightness profileof NGC 6166 outside its core is described to +- 0.037 mag/arcsec**2 by a singleSersic function with index n ~ 8.3. The cD halo is not recognizable fromphotometry alone. This blurs the distinction between cDs and similar-ncore-boxy-nonrotating ellipticals. Both may have halos made largely via minormergers and the accumulation of debris. However, the cD halo of NGC 6166 is asenhanced in alpha elements as the main body. Quenching of star formation in <~1Gyr happened even to the galaxies that contributed the cD halo.

The Cluster Velocity Dispersion of the Abell 2199 cD Halo of NGC 6166