Density Profiles Of Dark Matter Halos Research Articles

Dependence of the inner dark matter profile on the halo mass

I compare the density profile of dark matter (DM) haloes in cold dark matter (CDM) N-body simulations with 1, 32, 256 and 1024 h - 1 Mpc box sizes. I compare the profiles when the most massive haloes are composed of about 10 5 DM particles. The DM density profiles of the haloes in the 1 h - 1 Mpc box at redshift z ∼ 10 show systematically shallower cores with respect to the corresponding haloes in the 32 h - 1 Mpc simulation at z ∼ 3 that have masses, M d m , typical of the Milky Way and are fitted by a Navarro-Frenk-White (NFW) profile. The DM density profiles of the haloes in the 256 h - 1 Mpc box at z ∼ 0 are consistent with having steeper cores than the corresponding haloes in the 32 h - 1 Mpc simulation, but higher mass resolution simulations are needed to strengthen this result. Combined, these results suggest that the density profile of DM haloes is not universal, presenting shallower cores in dwarf galaxies and steeper cores in clusters. More work is needed to validate this finding at z = 0. Physically, the result sustains the hypothesis that the mass function of the accreting satellites determines the inner slope of the DM profile. However, the result can also be interpreted as a trend with the dynamical state in the assembly process of haloes of different mass. In comoving coordinates, r, the profile ρ d m with X = c Δ r/r Δ and a = (9 + 3n)/(5 + n) 1.3 + (M 1 / 6 dm,14 - 1)/(M 1 / 6 dm,14 + 1), provides a good fit to all the DM haloes from dwarf galaxies to clusters at any redshift with the same concentration parameter c Δ ∼ 7. Here, r Δ (M d m ) is the virial radius, n is the effective spectral index of the initial power spectrum of density perturbations and M d m , 1 4 = M d m /(3 × 10 1 4 M O .). The slope, y, of the outer parts of the halo appears to depend on the acceleration of the universe; when the scale parameter is a = (1+z) - 1 ≤ 1, the slope is γ 3 as in the NEW profile, but γ 4 at a > 1 when Ω Λ ∼ 1 and the universe is inflating. The shape of the DM profiles presents a significant scatter around the mean. It is therefore important to analyse a significant statistical sample of haloes in order to determine the mean profile. I compare the DM profiles in the 1 h - 1 Mpc box with the same simulation including stars, baryons and radiative transfer presented by Ricotti, Gnedin and Shull. Radiative feedback effects produce a larger scatter in the density profile shapes but, on average, do not affect the shape of the DM profiles significantly.

The Failure of Self‐Interacting Dark Matter to Solve the Overabundance of Dark Satellites and the Soft Core Question

Self-interacting dark matter was proposed by Spergel & Steinhardt (2000) to alleviate two conflicts between Cold Dark Matter (CDM) models and observations. Firstly, CDM N-body simulations predict dark matter halo density profiles that diverge at the centre in disagreement with the constant density cores observed in late-type dwarf and Low Surface Brightness (LSB) galaxies. Secondly, N-body simulations predict an overabundance of subhalos in the Galactic halo. Using a simple semi-analytical argument we show that weakly self-interacting dark matter models, which can produce halo cores of the sizes observed in dark matter dominated galaxies, are unable to reconcile the number of satellites in the Galactic halo with the observed number of dwarf galaxies in the Local Group.

Profiles of dark haloes: evolution, scatter and environment

We study dark-matter halo density profiles in a high-resolution N-body simulation of an LCDM cosmology. Our statistical sample contains ~5000 haloes in the range 10^{11}-10^{14} M_sun. The profiles are parameterized by an NFW form with two parameters, an inner radius r_s and a virial radius r_v, and we define the halo concentration c_v = r_v/r_s. First, we find that, for a given halo mass, the redshift dependence of the median concentration is c_v ~ 1/(1+z), corresponding to a roughly constant r_s with redshift. We present an improved analytic treatment of halo formation that fits the measured relations between halo parameters and their redshift dependence. The implications are that high-redshift galaxies are predicted to be more extended and dimmer than expected before. Second, we find that the scatter in log(c_v) is ~0.18, corresponding to a scatter in maximum rotation velocities of dV/V ~ 0.12. We discuss implications for modelling the Tully-Fisher relation, which has a smaller reported intrinsic scatter. Third, haloes in dense environments tend to be more concentrated than isolated haloes. These results suggest that c_v is an essential parameter for the theory of galaxy modelling, and we briefly discuss implications for the universality of the Tully-Fisher relation, the formation of low surface brightness galaxies, and the origin of the Hubble sequence.

The constant-density region of the dark haloes of spiral galaxies

We determine a crucial feature of the dark halo density distribution from the fact that the luminous matter dominates the gravitational potential at about one disc scalelength Rd, but at the optical edge the dark matter has already become the main component of the galaxy density. From the kinematics of 137 spirals we find that the dark matter halo density profiles are self-similar at least out to Ropt and show core radii much larger than the corresponding disc scalelengths. The luminous regions of spirals consist of stellar discs embedded in dark haloes with roughly constant density. This invariant dark matter profile is very difficult to reconcile with the fundamental properties of the density distribution of cold dark matter haloes. With respect to previous work, the present evidence is obtained by means of a robust method and for a large and complete sample of normal spirals.

Halo occupation numbers and galaxy bias

We propose a heuristic model that displays the main features of realistic theories for galaxy bias. We show that the low-order clustering statistics of the dark-matter distribution depend almost entirely on the locations and density profiles of dark-matter haloes. A hypothetical galaxy catalogue depends on (i) the efficiency of galaxy formation, as manifested by the halo occupation number -- the number of galaxies brighter than some sample limit contained in a halo of a given mass; (ii) the location of these galaxies within their halo. The first factor is constrained by the empirical luminosity function of groups. For the second factor, we assume that one galaxy marks the halo centre, with any remaining galaxies acting as satellites that trace the halo mass. These simple assumptions amount to a recipe for non-local bias, in which the probability of finding a galaxy is not a simple function of its local mass density. We have applied this prescription to some CDM models of current interest, and find that the predictions are close to the observed galaxy correlations for a flat $\Omega=0.3$ model ($\Lambda$CDM), but not for an $\Omega=1$ model with the same power spectrum ($\tau$CDM). This is an inevitable consequence of cluster normalization for the power spectra: cluster-scale haloes of given mass have smaller core radii for high $\Omega$, and hence display enhanced small-scale clustering. Finally, the pairwise velocity dispersion of galaxies in the $\Lambda$CDM model is lower than that of the mass, allowing cluster-normalized models to yield a realistic Mach number for the peculiar velocity field. This is largely due to the strong variation of galaxy-formation efficiency with halo mass that is required in this model.