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

TL;DR

This study compares the colour-magnitude and colour gradient properties of Brightest Cluster Galaxies (BCGs) and non-BCG E/S0 galaxies, revealing differences in their stellar populations and formation histories using SDSS data.

Contribution

It provides the first detailed comparison of colour gradients and relations between BCGs and E/S0 galaxies, highlighting distinct formation processes.

Findings

BCGs have flatter colour-magnitude relations than non-BCG E/S0s.

BCGs tend to be slightly redder and have flatter colour gradients.

Colour gradients in E/S0s increase with radius and decrease with stellar age.

Abstract

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.