Formation of Dark Matter Halos

TL;DR

This paper investigates the formation, structure, abundance, and merger rates of dark matter halos, providing new insights into their density profiles, the accuracy of the Press-Schechter approximation, and implications for high-redshift galaxy formation.

Contribution

It presents higher resolution simulation results on halo density profiles, compares Press-Schechter predictions with simulations, and discusses implications for galaxy formation and starburst activity.

Findings

Dark matter halos have a central density profile p(r) ∝ 1/r^g with g ≈ 0.2.

Press-Schechter overpredicts the number of galaxy-mass halos at z=0.

High merger rates at high redshift may explain properties of Lyman-break galaxies.

Abstract

This article concerns the formation and structure of dark matter halos, including (1) their radial density profiles, (2) their abundance, and (3) their merger rates. The last topic may be relevant to the nature of the small, bright, high-redshift galaxies discovered by the Lyman break technique. (1) Study of a statistical sample of galaxy-mass dark halos in high-resolution Adaptive Refinement Tree simulations shows that they have a central density profile p(r) \propto 1/r^g with g \approx 0.2, in agreement with data on dark-matter-dominated disk galaxies. We present recent, higher resolution results on this. (2) Another important new result is that the Press-Schechter approximation predicts about twice as many galaxy-mass halos at z=0 as are present in large dissipationless N-body simulations; more generally, PS overpredicts the abundance of M ~< 0.1M_* halos at all redshifts. (3) Finally, we discuss the assembly of these halos, in particular the merger rate of (sub-)halos at high redshift and the distribution of the starbursts that these mergers are likely to trigger. If most of the Lyman-break galaxies are such starbursts, this perhaps resolves the apparent paradox that these galaxies appear to cluster like massive halos (~10^{12} M_\odot), while their relatively low linewidths and their spectral energy distributions suggest that they have relatively low mass (few X 10^{10} M_\odot) and young ages (few X 10^8 yr). It also predicts much more star formation at high redshift in CDM-type hierarchical models for structure formation than if only quiescent star formation is included.