Cannibalism in Clusters’ Cusps

Abstract

Galactic cannibalism has long been assumed to play a role in the luminosity evolution of the brightest galaxies in clusters, but whether is it important enough to grow the luminous (L ~ 12L*) first-ranked galaxies completely over the age of the universe, or is rather just a modest source of luminosity growth has been a point of contention. I have been attempting to answer this question observationally through imaging investigations of multiple-nucleus first ranked galaxies. My work (Lauer 1986, 1988, and 1989) leads me to conclude that cannibalism may well be important for luminosity growth of first-ranked galaxies, but is probably insufficient to grow them completely — my best guess is that typical luminosity growth by cannibalism is ~ 4L*. The high prevalence of multiple nucleus first-ranked cluster galaxies is often sited as strong evidence for cannibalism. Fully 48% of first-ranked galaxies in Abell clusters have a companion galaxy brighter and M B = -17.5 within 20kpc of their centers; and if first-ranked galaxies are built by steady cannibalism, then it can be shown that a large fraction of these “secondary galaxies” are currently being cannibalized as we watch. For cluster galaxies initially distributed as r -γ in space density, the present epoch cannibalism rate is $$\frac{{dLt}}{{dt}}\left| {_{{t = T}}} \right. = {\mkern 1mu} (\frac{{3{\mkern 1mu} - {\mkern 1mu} \gamma }}{{2T}}){\mkern 1mu} {{L}_{{T,}}} $$ (1) where L T is the total luminosity cannibalized, and T is the time since cluster formation. For typical first-ranked galaxy luminosity, L 1 ≈ 12L*, T = 1010 yr, and γ = 2, dL T /dt ≈ 0.6L*/109 yr. Absorption of the brightest secondary galaxies will take place in about an orbital period or typically Δt ≈ 3 x 108 yr. This gives the average luminosity actively being cannibalized within the metric radius at any time as L C ≈ dL T /dtΔt ≈ 0.2L*. This number is close to the typical total luminosity of secondary galaxies within the metric radius averaged over all Abell clusters regardless of multiplicity, but the radial velocities of the secondary galaxies are high (Tonry 1984, 1985, Smith et al 1985); the majority of these galaxies do not appear to be bound to the central galaxy in a merging event.