We have carried out deep BVR surface photometry of 6 cD and cD-like galaxies using the 2.1-m telescope at San Pedro Mártir, cD galaxies are supergiant galaxies (M>1013M⊙) with enormous halos (>100 kpc in radius) surrounding a giant elliptical galaxy, found generally at the centre of rich clusters (Oemler 1976, Schombert 1988). The surface brightness profiles of their halos (envelopes) break from the r1/4 law, containing more light at large radii (Schombert (1988)), although a detailed 1 and 2 dimensional analysis of their morphology has yet to be carried out. There have been four main theories as to the origin of cD envelopes (Schombert (1988) and references within), a) stripping of stars from other cluster member galaxies, b) formation of galaxy and envelope at the same time during the formation of the cluster, c) mergers of cluster members, which do not easily explain the high velocity dispersions in the envelopes (~ 1000 km s−1), d) cooling flows: accumulation of cooling X-ray-emitting ICM gas around the central galaxy. Very red envelopes have been found around some cD's, and star formation biased towards lowmass stars in cooling flows were invoked to explain these red halos, but the expected very bright near-IR halos were not detected (Joy et al. 1995 and references within). Previous detailed studies of cD galaxies (e.g. Mackie 1992) found a range of colour gradients.
STRUCTURE AND FORMATION OF cD GALAXIES: NGC 6166 IN ABELL 2199