The velocity dispersion slope against The central velocity dispersion

The slope η of the velocity dispersion profile is plotted against the central velocity dispersion σ₀ for galaxies in multiple different samples. The blue points represent brightest galaxies in groups (BGGs) of high (square) and low (circles) density, while the green, red, and yellow points represent brightest cluster galaxies (BCGs) in various samples of galaxy clusters. The grey points indicate generic “early-type galaxies” (ETGs). The slope η is negative if the velocity dispersion decreases with radius and positive if it rises. Thus, massive BCGs tend to have rising profiles, with the stellar velocities responding to the cluster potential at larger radii. [Reproduced from Loubser et al. 2018, MNRAS, in press.]