Concentrations and Assembly Histories of Dark Matter Halos
Abstract
We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the LCDM cosmology. For each halo at z=0, we identify its merger-history tree, and determine concentration parameters cvir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch ac, defined when the log mass accretion rate dlog M/doga falls below a critical value. We find that late forming galaxies tend to be less concentrated, such that cvir ``observed'' at any epoch aobs is strongly correlated with ac via cvir=c1 aobs/ac. Scatter about this relation is mostly due to measurement errors in cvir and ac, implying that the actual spread in cvir for halos of a given mass can be mostly attributed to scatter in ac. Because of the direct connection between halo concentration and velocity rotation curves, and because of probable connections between halo mass assembly history and star formation history, the tight correlation between these properties provides an essential new ingredient for galaxy formation modeling.
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