Mass accretion histories & density profiles of LCDM clusters

Abstract

We analyze the mass accretion histories (MAHs) and density profiles of cluster-size halos in a flat LCDM cosmology.To fit the MAHs of all systems we need to generalize the MAH fit found in previous systematic studies of predominantly galactic halos.We find that the concentration of the density distribution is tightly correlated with the halo MAH and its formation redshift. During the early period of fast mass growth the concentration remains roughly constant and low (~3-4), while during the slow accretion phase it increases with decreasing redshift as 1/(1+z).We consider fits of three widely discussed analytic density profiles.We find that there is no unique best fit for all clusters.At the same time, if a cluster is best fit by a particular analytic profile at z=0, the same is usually true at earlier epochs out to z ~ 1-2.The local logarithmic slope of the density profiles at 3% of the virial radius ranges from -1.2 to -2.0.In addition, the logarithmic slope becomes shallower with decreasing radius without reaching an asymptotic value down to the smallest resolved scale (<1% of the virial radius).During the early MAH period of rapid mass growth the density profiles can be well described by a single power law rho(r) ~ r^-a with a ~ 1.5-2.The relatively shallow power law slopes result in low concentrations at these stages of evolution, as the scale radius where the density profiles reaches the slope of -2 is at large radii.This indicates that the inner power law like density distribution of halos is built up during the periods of rapid mass accretion and active merging, while the outer steeper profile is formed when the mass accretion slows down.