Brightest members of clusters of galaxies

Abstract

THERE are two opposing interpretations of the small dispersion σ₁∼0.35 mag in the absolute magnitudes M₁ of the brightest members of rich clusters of galaxies measured by Sandage¹. In one view, advanced, among others, by Peebles^2,3, it is assumed that there is a universal cluster luminosity function, and that the value of σ₁ and of the variation of M₁ with cluster richness can be explained as the statistical result of drawing the brightest member from a set of clusters obeying this function. There is therefore nothing physically remarkable about the brightest member, and its luminosity is determined by the same mechanism as that which determines the luminosity of the fainter members. The opposing view, which has been advanced by Peach⁴, Sandage¹, and Tremaine and Richstone⁵, claims that the small dispersion is due to special conditions governing the formation or evolution of the brightest member, and is therefore entirely or in part independent of the luminosity function of the other cluster members. On this view one evokes either special conditions at the epoch of galaxy formation¹, or a mechanism, such as galactic accretion⁶, which governs the size and luminosity of the most massive galaxy in such a way as to give a small dispersion in the metric magnitude. The weakness of the statistical model, according to its opponents, is its inability to account for σ₁ without postulating a luminosity function steeper than those directly observed. The opposing view is open to the criticism that it is either ad hoc or that the mechanisms of galaxy evolution are too uncertain to lead to reliable conclusions. Here, we discuss the two interpretations in the light of new results and suggest that the case against the statistical model may not be as strong as sometimes suggested.