The stellar content of central dominant galaxies. II - Colors of cD envelopes

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Abstract
Deep g- and r-band CCD imaging of three rich clusters, A2589, A2634, and A407 containing cD galaxies or structures possibly relating to cD envelopes is presented. Observations and data reduction have been done in a manner to achieve maximum sky flatness and minimize potential contamination of cD envelope light from scattered light due to foreground stars and other cluster galaxies. Scattered light r^-2^ halos have been modeled and subtracted from the images. The (g - r) color profiles of the cDs NGC 7647 (A2589) and NGC 7720 (A2634) down to the start of the envelopes are quite flat. There is no evidence of a change in color at the surface brightness break that signifies the start of the cD envelope, nor are the colors of the envelope dramatically blue. In fact, the envelope of NGC 7647 may become redder by 0.15 mag at very large radii. In comparison, the color profile of the D galaxy NGC 7728 also in A2634 displays a steeper blueward color gradient than NGC 7720. A407 G1, which has nine nuclei in a common envelope may also possess a red envelope in comparison to its higher surface brightness region, and thus may evolve into a cD similar to NGC 7647. Overall the results are consistent with cDs being enhanced by mergers, with their precursor color gradients being weakened. The envelopes of the cDs observed are not overtly blue and do not have a unique optical color. The implications for cD envelope formation and evolution are discussed.

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