Calibrated B, V surface photometry of X-ray cD galaxies

Carefully calibrated B, V, and B-V radial surface brightness profiles of 7 cD galaxies which are known to be centered on extended X-ray sources are presented. Pronounced color gradients between the galaxy centers and a 50 kpc radius are found. All observed cD galaxies have blue haloes. The global color profile properties of the observed galaxies are described. No systematic trend in the ellipticity parameters between B and V images is found, suggesting a similar kinematic origin for the stars that dominate both bands. No isophotal twisting greater than 10 deg is observed, but the eccentricity of the isophotes increases with increasing radii. It is argued that gas accretion from the cluster reservoir onto cD galaxies by means of radiative cooling is a very general phenomenon. The optical observations reported here are interpreted in terms of this process. The color gradients are interpreted as the result of ongoing star formation which is fueled by the accreted cluster gas.