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 similar to 5000 brightest cluster galaxies (BCGs) in the Sloan Digital Sky Survey (SDSS). The colour-magnitude and colour-velocity dispersion relations of the BCGs are flatter in slope, by factors of 2 or more, than those of non-BCG early-type galaxies of similar luminosity (M(r) < -22.5), while their g - r colours at the half-light radius are similar to 0.01 mag redder. We investigate and compare radial colour gradients (which are usually negative) in these BCGs and a sample of 37 000 early-type galaxies, using a gradient estimator based on the ratio of de Vaucouleurs effective radii in g and r passbands, . The mean colour gradient of BCGs is flatter (by 23 per cent) than for other E/S0 galaxies of similar luminosity. The colour gradients in early-type galaxies are stronger at intermediate luminosity (M(r) similar or equal to - 22) than at low or the highest luminosities, and tend to decrease with increasing velocity dispersion (Sigma). In non-BCG E/S0s, colour gradients increase for larger effective radii (up to 10-12 kpc), and are negatively correlated with 10 log Sigma + M(r), the mass density and stellar age. However, gradients can be reduced or inverted (positive) for post-starburst galaxies at the youngest ages. In BCGs, these trends are absent and the mean colour gradient remains at a relatively low level (similar to 0.08 by our measure) whatever the other properties of the galaxies. The redder half-light radius colours of the BCGs can be explained by their slightly greater ages combined with flatter radial colour gradients. We discuss possible explanations in terms of spheroidal galaxies initially having a colour gradient positively correlated with luminosity and positively correlated with large radius and/or low density. Subsequently, elliptical-elliptical 'dry' mergers progressively reduce colour gradients, towards a low but non-zero value. This has occurred a greater number of times during the formation histories of the most massive E/S0s, and to by far the greatest degree in the BCGs.