Jürg Diemand

Quantifying the heart of darkness with GHALO – a multibillion particle simulation of a galactic halo

We perform a series of simulations of a Galactic mass dark matter halo at different resolutions: our largest uses over 3 billion particles and has a mass resolution of 1000xa0M⊙. We quantify the structural properties of the inner dark matter distribution and study how they depend on numerical resolution. We can measure the density profile to a distance of 120 pc (0.05 per cent of Rvir), where the logarithmic slope is −0.8 and −1.4 at (0.5 per cent of Rvir). We propose a new two-parameter fitting function that has a linearly varying logarithmic density gradient over the resolved radii which fits the GHALO and VL2 density profiles extremely well. Convergence in the halo shape is achieved at roughly three times the convergence radius for the density profile at which point the halo becomes more spherical due to numerical resolution. The six-dimensional phase-space profile is dominated by the presence of the substructures and does not follow a power law, except in the central few kpc which is devoid of substructure even at this resolution. The quantity, ρ/σ3, which is often used as a proxy for the six-dimensional phase-space density should be used with caution.

Density Profiles of Cold Dark Matter Substructure: Implications for the Missing-Satellites Problem

The structural evolution of substructure in cold dark matter (CDM) models is investigated combining low-resolution satellites from cosmological N-body simulations of parent halos with N = 107 particles with high-resolution individual subhalos orbiting within a static host potential. We show that, as a result of mass loss, convergence in the central density profiles requires the initial satellites to be resolved with N = 107 particles and parsec-scale force resolution. We find that the density profiles of substructure halos can be well fitted with a power-law central slope that is unmodified by tidal forces even after the tidal stripping of over 99% of the initial mass and an exponential cutoff in the outer parts. The solution to the missing-satellites problem advocated by Stoehr et al. in 2002 relied on the flattening of the dark matter halo central density cusps by gravitational tides, enabling the observed satellites to be embedded within dark halos with maximum circular velocities as large as 60 km s-1. In contrast, our results suggest that tidal interactions do not provide the mechanism for associating the dwarf spheroidal (dSph) satellites of the Milky Way with the most massive substructure halos expected in a CDM universe. Motivated by the structure of our stripped satellites, we compare the predicted velocity dispersion profiles of Fornax and Draco to observations, assuming that they are embedded in CDM halos. We demonstrate that models with isotropic and tangentially anisotropic velocity distributions for the stellar component fit the data only if the surrounding dark matter halos have maximum circular velocities in the range 20-35 km s-1. If the dSphs are embedded within halos this large, then the overabundance of satellites within the concordance ΛCDM cosmological model is significantly alleviated, but this still does not provide the entire solution.

Haloes gone MAD: The Halo-Finder Comparison Project

We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We

THE DARK MATTER ANNIHILATION SIGNAL FROM GALACTIC SUBSTRUCTURE : PREDICTIONS FOR GLAST

Wepresent quantitative predictions for the detectability of individual Galactic dark matter subhalos in gamma rays from dark matter pair annihilations in their centers. Our method is based on a hybrid approach, employing the highest resolution numerical simulations available (including the recently completed 1 billion particle Via Lactea II simulation),aswellasanalyticalmodels,forextrapolatingbeyondthesimulations’resolutionlimit.Weincludeaselfconsistent treatment of subhalo boost factors, motivated by our numerical results, and a realistic treatment of the expected backgrounds that individual subhalos must outshine. We show that for reasonable values of the dark matter

Dark matter direct detection with non-Maxwellian velocity structure

The velocity distribution function of dark matter particles is expected to show significant departures from a Maxwell-Boltzmann distribution. This can have profound effects on the predicted dark matter - nucleon scattering rates in direct detection experiments, especially for dark matter models in which the scattering is sensitive to the high velocity tail of the distribution, such as inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for experiments that require high energy recoil events, such as many directionally sensitive experiments. Here we determine the velocity distribution functions from two of the highest resolution numerical simulations of Galactic dark matter structure (Via Lactea II and GHALO), and study the effects for these scenarios. For directional detection, we find that the observed departures from Maxwell-Boltzmann increase the contrast of the signal and change the typical direction of incoming DM particles. For iDM, the expected signals at direct detection experiments are changed dramatically: the annual modulation can be enhanced by more than a factor two, and the relative rates of DAMA compared to CDMS can change by an order of magnitude, while those compared to CRESST can change by a factor of two. The spectrum of the signal can also change dramatically, with many features arising due to substructure. For LDM the spectral effects are smaller, but changes do arise that improve the compatibility with existing experiments. We find that the phase of the modulation can depend upon energy, which would help discriminate against background should it be found.

The Shapes, Orientation, and Alignment of Galactic Dark Matter Subhalos

We present a study of the shapes, orientations, and alignments of Galactic dark matter subhalos in the Via Lactea simulationofaMilkyWay-sizeCDMhosthalo.Whereasisolateddarkmatterhalostendtobeprolate,subhalosare predominantlytriaxial.Overallsubhalosaremoresphericalthanthehosthalo,withminor-to-majorandintermediate- to-major axis ratios of 0.68 and 0.83, respectively. Like isolated halos, subhalos tend to be less spherical in their central regions. The principal axis ratios are independent of subhalo mass when the shapes are measured within a physical scale such as rVmax , the radius of the peak of the circular velocity curve. Subhalos tend to be slightly more sphericalclosertothehosthalocenter.Thespatialdistributionof thesubhalostracestheprolateshapeof thehosthalo when they are selected by the largest Vmax they ever had, i.e., before they experienced strong tidal mass loss. The subhalos orientation is not random: the major axis tends to align with the direction toward the host halo center. This alignment disappears for halos beyond 3r200 and is more pronounced when the shapes are measured in the outer regions of the subhalos. The radial alignment is preserved during asubhalos orbit and they become elongated during pericenter passage, indicating that the alignment is likely caused by the host halos tidal forces. These tidal interactions with the host halo act to make subhalos rounder over time. Subject headingg cosmology: theory — dark matter — galaxies: dwarf — galaxies: formation — galaxies: halos — methods: numerical

The structure and evolution of cold dark matter halos

In the standard cosmological model a mysterious cold dark matter (CDM) component dominates the formation of structures. Numerical studies of the formation of CDM halos have produced several robust results that allow unique tests of the hierarchical clustering paradigm. Universal properties of halos, including their mass profiles and substructure properties are roughly consistent with observational data from the scales of dwarf galaxies to galaxy clusters. Resolving the fine grained structure of halos has enabled us to make predictions for ongoing and planned direct and indirect dark matter detection experiments. nWhile simulations of pure CDM halos are now very accurate and in good agreement (recently claimed discrepancies are addressed in detail in this review), we are still unable to make robust, quantitative predictions about galaxy formation and about how the dark matter distribution changes in the process. Whilst discrepancies between observations and simulations have been the subject of much debate in the literature, galaxy formation and evolution needs to be understood in more detail in order to fully test the CDM paradigm. Whatever the true nature of the dark matter particle is, its clustering properties must not be too different from a cold neutralino like particle to maintain all the successes of the model in matching large scale structure data and the global properties of halos which are mostly in good agreement with observations.

Warm dark matter does not do better than cold dark matter in solving small-scale inconsistencies

Over the last decade, warm dark matter (WDM) has been repeatedly proposed as an alternative scenario to the standard cold dark matter (CDM) one, potentially resolving several disagreements between the CDM model and observations on small scales. Here, we reconsider the most important CDM small-scale discrepancies in the light of recent observational constraints on WDM. As a result, we find that a conventional thermal (or thermal-like) WDM cosmology with a particle mass in agreement with Lyman-

Probing the epoch of reionization with Milky Way satellites

alpha

The graininess of dark matter haloes

is nearly indistinguishable from CDM on the relevant scales and therefore fails to alleviate any of the small-scale problems. The reason for this failure is that the power spectrum of conventional WDM falls off too rapidly. To maintain WDM as a significantly different alternative to CDM, more evolved production mechanisms leading to multiple dark matter components or a gradually decreasing small-scale power spectrum have to be considered.