LoCuSS: connecting the dominance and shape of brightest cluster galaxies with the assembly history of massive clusters

We study the luminosity gap, m12, between the firstand second-ranked galaxies in a sample of 59 massive (∼1015 M ) galaxy clusters, using data from the Hale Telescope, the Hubble Space Telescope, Chandra and Spitzer. We find that the m12 distribution, p( m12), is a declining function of m12 to which we fitted a straight line: p( m12) ∝−(0.13 ± 0.02) m12. The fraction of clusters with ‘large’ luminosity gaps is p( m12 ≥ 1) = 0.37 ± 0.08, which represents a 3σ excess over that obtained from Monte Carlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with ‘extreme’ luminosity gaps, m12 ≥ 2, giving a fraction of p( m12 ≥ 2) = 0.07+0.05 −0.03. More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/discy brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/discy BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch and the recent infall history of the cluster. ‘BCG dominance’ is therefore a phase that a cluster may evolve through and is not an evolutionary ‘cul-de-sac’. We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models is able to reproduce all of the observational results on m12, underlining the inability of the current generation of models to match the empirical properties of BCGs. We identify the strength of active galactic nucleus feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies.