Colour Gradients and the Colour-Magnitude Relation: Different Properties of Brightest Cluster Galaxies and E/S0 Galaxies in the Sloan Digital Sky Survey

摘要

We examine the colour-magnitude relation of approximately 5000 BrightestCluster Galaxies (BCGs) in the Sloan Digital Sky Survey, and compare withnon-BCG E/S0 galaxies. The colour-magnitude and colour-sigma (velocitydispersion) relations are flatter in slope (by a factor of about 2) for BCGsthan for non-BCG E/S0s, and the BCGs also tend to be redder by 0.01 magnitudesin g-r. We investigate radial colour gradients in both samples, using the ratioof the de Vaucouleurs radii in the g and r bands. We find BCGs havesignificantly flatter (by 23%) mean colour gradients than other high luminosityE/S0s. In early-type galaxies, the colour gradients are strongest atintermediate luminosities of Mr=-22. Colour gradients in E/S0s increase withradius (up to 10kpc) and are negatively correlated with 10sigma + Mr (velocitydispersion relative to luminosity) and with mass density. The gradients alsotend to decrease with increasing stellar age. These trends are weak or not seenin BCGs, in which the mean colour gradient is low whatever the otherproperties. We discuss possible explanations, which involve a greater amount ofdry merging in the formation history of the BCGs.