Concentrations and Assembly Histories of Dark Matter Halos

TL;DR

This paper investigates how the density profiles and concentrations of dark matter halos relate to their formation histories using high-resolution simulations, revealing a strong correlation that aids galaxy formation models.

Contribution

It introduces a universal function to describe halo mass accretion histories and links halo concentration to formation epoch, enhancing understanding of galaxy formation processes.

Findings

Late forming halos are less concentrated.

Halo concentration correlates with formation epoch a_c.

Scatter in concentration mainly due to measurement errors.

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 c_vir 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 a_c, 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 c_vir ``observed'' at any epoch a_obs is strongly correlated with a_c via c_vir=c_1 a_obs/a_c. Scatter about this relation is mostly due to measurement errors in c_vir and a_c, implying that the actual spread in c_vir for halos of a given mass can be mostly attributed to scatter in a_c. 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.