Colour Gradients and the Colour-Magnitude Relation: Different Properties
  of Brightest Cluster Galaxies and E/S0 Galaxies in the Sloan Digital Sky
  Survey
  We examine the colour-magnitude relation of approximately 5000 Brightest
Cluster Galaxies (BCGs) in the Sloan Digital Sky Survey, and compare with
non-BCG E/S0 galaxies. The colour-magnitude and colour-sigma (velocity
dispersion) relations are flatter in slope (by a factor of about 2) for BCGs
than for non-BCG E/S0s, and the BCGs also tend to be redder by 0.01 magnitudes
in g-r. We investigate radial colour gradients in both samples, using the ratio
of the de Vaucouleurs radii in the g and r bands. We find BCGs have
significantly flatter (by 23%) mean colour gradients than other high luminosity
E/S0s. In early-type galaxies, the colour gradients are strongest at
intermediate luminosities of Mr=-22. Colour gradients in E/S0s increase with
radius (up to 10kpc) and are negatively correlated with 10sigma + Mr (velocity
dispersion relative to luminosity) and with mass density. The gradients also
tend to decrease with increasing stellar age. These trends are weak or not seen
in BCGs, in which the mean colour gradient is low whatever the other
properties. We discuss possible explanations, which involve a greater amount of
dry merging in the formation history of the BCGs.
