Colours of the Envelopes of cD Galaxies

We have carried out BVR surface photometry of a sample of 6 cD and cD-like galaxies using the 2.1 m telescope at San Pedro Martir, in general reaching surface brightnesses well into the cD envelope. In the majority of cases no appreciable colour gradient was detected, and the envelope has the same red colour as the central galaxy. In the two cD-like galaxies a redward gradient was observed, although in both these cases no extended envelope was detected, possibly due to contamination from neighbouring galaxies. The centre of NGC 1275 (one of the cD-like galaxies) is bluer due to many regions of active star formation. In two cDs with multiple nuclei (A193 and A2634), no colour gradients or regions of current active star formation were found. These results favour an early, 'primordial origin' for the cD envelope, the stellar populations of the envelope being coeval with those of the central elliptical galaxy.