Dissipationless Merging and the Assembly of Central Galaxies

We reanalyze the galaxy-mass correlation function measured by the SloanDigital Sky Survey to obtain host dark matter halo masses at galaxy and galaxygroup scales. We extend the data to galaxy clusters in the 2MASS catalog andstudy the relation between central galaxy luminosity and halo mass. While thecentral galaxy luminosity scales as ~M^{0.7-0.8} at low masses, the relationflattens to ~M^{<0.3} above ~4x10^{13} M_sun. The total luminosity of galaxiesin the halo, however, continues to grow as a power-law ~M^{0.8-0.9}. Startingfrom the hypothesis that the central galaxies grow by merging ("galacticcannibalism"), we develop a simple model for the evolution of theirluminosities as a consequence of the accretion of satellite galaxies. Theluminosity-mass relation flattens when the time scale on which dynamicalfriction induces orbital decay in the satellite galaxies exceeds the age of thedark matter halo. Then, the growth of the central galaxy is suppressed as itcan cannibalize only the rare, massive satellite galaxies. The model takes thedependence of the total luminosity of galaxies in a halo on its mass and theglobal galaxy luminosity function as input, and reproduces the observed centralgalaxy luminosity-mass relation over three decades in halo mass,(10^{12}-10^{15}) M_sun. The success of the model suggests that gas cooling and subsequent starformation did not play an important role in the final assembly of centralgalaxies from sub-L_star precursors.Comment: 4 pages, 2 figures, submitte