Dark Matter Halo Models Research Articles

What X-rays tell us about dark matter halos

We present a detailed non-spherical modeling of dark matter halos on the basis of a careful analysis of state-of-the art N-body simulations. the fitting formulae presented here form a complete and accurate description of the triaxial density profiles of halos in Cold Dark Matter (CDM) models. This modeling allows us to quantitatively discuss implications for shape observations of galactic and cluster halos. the predictions of the concordance ACDM model are confronted with the shape observations from the Milky Way to X-ray clusters.

Gravitational Lens Time Delays in Cold Dark Matter

In standard cold dark matter (CDM) halo models, the time delay of a gravitational lens is determined by the cold baryon mass fraction, fb ¼ � b; cold=� 0, of the visible galaxy relative to the overall halo. The observed time delays in PG 1115+080, SBS 1520+530, B1600+434, and HE 2149� 2745 give Hubble constants consistent with the Hubble Space Telescope Key Project value of H0 ¼ 72 � 8k m s � 1 Mpc � 1 only if fbe0:2 (onesided 68% confidence), which is larger than the upper bound of fb; max ¼ � b=� 0 ¼ 0:15 � 0:05 estimated from the cosmic microwave background. If all available baryons cool and fb ¼ fb; max, then the time delays imply H0 ¼ 65 � 6k m s � 1 Mpc � 1 (95% confidence). If local inventories of cold baryons, fb ’ 0:013=h70, are correct, then H0 ¼ 52 � 6k m s � 1 Mpc � 1 and the halo parameters closely match isothermal mass models. Isothermal models are also consistent with strong and weak lens studies, stellar dynamics, and X-ray observations on these scales, while significantly more centrally concentrated models are not. There is a conflict between gravitational lens time delays, the local distance scale, and standard CDM halo models. Subject headings: cosmological parameters — dark matter — distance scale — gravitational lensing

Triaxial Modeling of Halo Density Profiles with High‐ResolutionN‐Body Simulations

We present a detailed non-spherical modeling of dark matter halos on the basis of a combined analysis of the high-resolution halo simulations (12 halos with $N\sim 10^6$ particles within their virial radius) and the large cosmological simulations (5 realizations with $N=512^3$ particles in a $100h^{-1}$Mpc boxsize). The density profiles of those simulated halos are well approximated by a sequence of the concentric triaxial distribution with their axis directions being fairly aligned. We characterize the triaxial model quantitatively by generalizing the universal density profile which has previously been discussed only in the framework of the spherical model. We obtain a series of practically useful fitting formulae in applying the triaxial model; the mass and redshift dependence of the axis ratio, the mean of the concentration parameter, and the probability distribution functions of the the axis ratio and the concentration parameter. These accurate fitting formulae form a complete description of the triaxial density profiles of halos in Cold Dark Matter models. Our current description of the dark halos will be particularly useful in predicting a variety of nonsphericity effects, to a reasonably reliable degree, including the weak and strong lens statistics, the orbital evolution of galactic satellites and triaxiality of galactic halos, and the non-linear clustering of dark matter. In addition, this provides a useful framework for the non-spherical modeling of the intra-cluster gas, which is crucial in discussing the gas and temperature profiles of X-ray clusters and the Hubble constant estimated via the Sunyaev -- Zel'dovich effect.

Dark Baryons and the Profile Crisis in Cold Dark Matter Halos

This Letter proposes that the crisis in cold dark matter halo models is partly due to a weakly dissipative dark baryonic component. This component is built by rescaling the rotation curves of faint galaxies onto those of bright systems, and we argue that the collapse factor of the baryonic disk relative to the halo must be a constant. If this component is made of molecular gas in a frozen form, a large hole is expected in the central region that is due to UV heating by the stellar component. This hole, combined with a weakly peaked distribution of halo particles, may mimic an isothermal halo core that is wrongly attributed to the particles alone. Halos made of cold particles with or without a finite-scattering cross section are considered in this context. For faint objects, a concordance model is achieved with roughly half the mass in baryons, a mass concentration cM ≈ 6.2 ± 0.5, and a inner profile ρ ∝ r-0.5±0.3 at r = 0 for the halos.

Genus Statistics for Galaxy Clusters and Nonlinear Biasing of Dark Matter Halos

We derive an analytic formula of genus for dark matter halos assuming that the primordial mass density field obeys the random-Gaussian statistics. In particular, we for the first time take account of the nonlinear nature of the halo biasing, in addition to the nonlinear gravitational evolution of the underlying mass distribution. We examine in detail the extent to which the predicted genus for halos depends on the redshift, smoothing scale of the density field, and the mass of those halos in representative cold dark matter models. In addition to the full model predictions, we derive an explicit perturbation formula for the halo genus which can be applied to a wider class of biasing model for dark matter halos. With the empirical relation between the halo mass and the luminosity/temperature of galaxy clusters, our prediction can be compared with the upcoming cluster redshift catalogs in the optical and X-ray bands. Furthermore, we discuss the detectability of the biasing and cosmological model dependence from the future data.

Small‐Scale Anisotropy of Cosmic Rays above 1019eV Observed with the Akeno Giant Air Shower Array

With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 10^19eV, 47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998. Arrival direction distribution of these extremely high energy cosmic rays has been studied. While no significant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 x 10^19eV, there are one triplet and three doublets within separation angle of 2.5^o and the probability of observing these clusters by a chance coincidence under an isotropic distribution is smaller than 1 %. Especially the triplet is observed against expected 0.05 events. The cos(\theta_GC) distribution expected from the Dark Matter Halo model fits the data as well as an isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorer fit than isotropy above 10^19eV. Arrival direction distribution of seven 10^20eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of seven are members of doublets above about 4 x 10^19eV.

Ultra high energy cosmic rays and the Galactic halo

An analysis has been made of the fraction of ultra high energy cosmic rays (above 10 18 eV) which could be due to processes involved in two possible ‘Models’. The first is the Giant Magnetic Halo Model and the second is the Dark Matter Halo Model. We find that the former, in which heavy nuclei are trapped in a giant halo, fails for energies above about 3 × 10 19 eV. For the Dark Matter Halo Model, in which relic particles follow the “conventional” dark matter and whose decays give ultra high energy cosmic rays, the predicted anisotropies are much higher than those observed. The lack of observation of a finite flux from the Andromeda Galaxy means that the conclusion is insensitive to the spatial scale size of the assumed halo distribution. It is concluded that less than 10% of the ultra high energy cosmic rays come from relic particles in the Galactic halo.

Anisotropy of ultra high energy cosmic rays in the dark matter halo model

The harmonic analysis of anisotropy of Ultra High Energy Cosmic Rays is performed for the Dark Matter halo model. In this model the relic superheavy particles comprise part of the Dark Matter and are concentrated in the Galactic halo. The Ultra High Energy Cosmic Rays are produced by the decays of these particles. Anisotropy is caused by the non-central position of the Sun in the Galactic halo. The calculated anisotropy is in reasonable agreement with the AGASA data. For more precise test of the model a comparison of fluxes in the directions of the Galactic Center and Anticenter is needed.

Local anisotropy of gamma-bursts' angular distribution

Using COMPTON observatory data, we have found the existence of the local cluster of gamma-ray sources in the region of the celestial sphere corresponding to the halo of Andromeda (galaxy M31). It is in agreement with the giant dark matter halo (GDMH) model.

Anisotropy of the angular distribution of gamma-bursts

The analysis of the data of the COMPTON observatory [BATSE Burst catalog] indicates the existence of dipole asymmetry with amplitude a=0.036 in the direction b=-22°, l=30°. The observational data are shown to agree qualitatively with the giant dark matter halo (GDMH) model of Gurevich et al. [Phys. Lett. A 175 (1993) 397]. The method of observational verification of the GDHM model is suggested.