Dark Matter Halos from the Inside Out

James E Taylor

doi:10.1155/2011/604898

Abstract

The balance of evidence indicates that individual galaxies and groups or clusters of galaxies are embedded in enormous distributions of cold, weakly interacting dark matter. These dark matter “halos” provide the scaffolding for all luminous structures in the universe, and their properties comprise an essential part of the current cosmological model. I review the internal properties of dark matter halos, focussing on the simple universal trends predicted by numerical simulations of structure formation. Simulations indicate that halos should all have roughly the same spherically averaged density profile and kinematic structure and predict simple distributions of shape, formation history, and substructure in density and kinematics, over an enormous range of halo mass and for all common variants of the concordance cosmology. I describe observational progress towards testing these predictions by measuring masses, shapes, profiles, and substructure in real halos using baryonic tracers or gravitational lensing. An important property of simulated halos (possibly the most important property) is their dynamical “age”, or degree of internal relaxation. I review recent gravitational lensing studies of galaxy clusters which will measure substructure and relaxation in a large sample of individual cluster halos, producing quantitative measures of age that are well matched to theoretical predictions.

Highlights

Everywhere in the universe, on scales comparable to the size of galaxies or larger, the effects of gravity appear to be anomalously strong
The standard cosmological model of structure formation posits that the universe contains known particles—baryons, photons, a small contribution from warm or hot neutrinos— and two dominant dark components, weakly interacting cold dark matter and a cosmological constant or some similar form of “dark energy”. Given these ingredients and starting from an inflationary power spectrum at early times, the CDM model predicts the subsequent growth of fluctuations in the matter distribution, the 3D power spectrum of these fluctuations after radiation-matter equality, the angular power spectrum of temperature fluctuations in the cosmic microwave background (CMB) at the time of last scattering, and the properties of large scale structure post CMB
Several early analytic models of this kind, notably [25, 34, 35], predicted that matter would cluster around a point to produce a radial density profile that was a steep power law with a constant slope

Summary

Introduction

Everywhere in the universe, on scales comparable to the size of galaxies or larger, the effects of gravity appear to be anomalously strong. The standard cosmological model of structure formation (which I will refer to loosely as the “CDM model” it contains many other ingredients) posits that the universe contains known particles—baryons, photons, a small contribution from warm or hot neutrinos— and two dominant dark components, weakly interacting cold dark matter and a cosmological constant or some similar form of “dark energy” Given these ingredients and starting from an inflationary power spectrum at early times, the CDM model predicts the subsequent growth of fluctuations in the matter distribution, the 3D power spectrum of these fluctuations after radiation-matter equality, the angular power spectrum of temperature fluctuations in the CMB at the time of last scattering, and the properties of large scale structure post CMB. I do not discuss specific dark matter candidates or their properties in detail in this paper, since these have been extensively reviewed elsewhere (see, e.g., [9]), but I summarize in Section 5 how measurements of halo properties can help constrain these candidates and other aspects of fundamental physics

From Linear to Nonlinear Structure Formation

The Universality of Halo Properties

Defining and Measuring Halo Age

Findings

Prospects