Annihilation of Dark Matter usually produces together with gamma rays comparable amounts of electrons and positrons. The e+e- gyrating in the galactic magnetic field then produce secondary synchrotron radiation which thus provides an indirect mean to constrain the DM signal itself. To this purpose, we calculate the radio emission from the galactic halo as well as from its expected substructures and we then compare it with the measured diffuse radio background. We employ a multi-frequency approach using data in the relevant frequency range 100 MHz-100 GHz, as well as the WMAP Haze data at 23 GHz. The derived constraints are of the order <sigma_A*v>=10^{-24} cm3 s^{-1} for a DM mass m_chi=100 GeV sensibly depending however on the astrophysical uncertainties, in particular on the assumption on the galactic magnetic field model. The signal from single bright clumps is instead largely attenuated by diffusion effects and offers only poor detection perspectives.
The GLAST LAT Collaboration is one among several experimental groups, covering a wide range of approaches, pursuing the search for the nature of dark matter. The GLAST LAT has the unique ability to find new sources of high energy gamma radiation emanating directly from WIMP annihilations in situ in the universe. Using it's wide band spectral and full sky spatial capabilities, the GLAST LAT can form "images" in high energy gamma-rays of dark matter substructures in the gamma-ray sky. We describe a preliminary feasibility study for indirect detection of Milky Way dark matter satellites using the…
We estimate the probability of detecting gamma-rays from the annihilation of neutralino dark matter in the substructure of the Milky Way. We characterize substructure statistically based on Monte Carlo realizations of the formation of a Milky Way-like halo using semi-analytic methods that have been calibrated against N-body simulations. We find that it may be possible for the upcoming GLAST and VERITAS experiments, working in concert, to detect gamma-rays from dark matter substructure if $\Mchi \lsim 100 \GeV$, while for $\Mchi \gsim 500 \GeV$ such a detection seems unlikely. We investigate th…
We estimate the two-point correlation function of dark matter haloes, with masses >10^{13} h^{-1} Mo, that have or not significant substructure. The haloes are identified with a friends of friends algorithm in a large LCDM simulation at two redshift snapshots (z=0 and 1), while halo substructure is determined using an observationally driven method. We find in both epochs a clear and significant signal by which haloes with substructure are more clustered than those with no-substructure. This is true for all the considered halo mass ranges, although for the highest halo masses the signal is nois…
Observations of multiple-image gravitational lens systems suggest that the projected mass distributions of galaxy haloes may contain substantial inhomogeneities. The fraction of the halo mass in dense substructure is still highly uncertain, but could be as large as a few percent. While halo substructure is seen in numerical simulations of CDM haloes, little of this substructure survives in the innermost regions of haloes, and thus the observational claims for substructure at small projected radii are slightly surprising. There is evidence, however, that even the highest-resolution simulations …
One of the targets of the recently launched Fermi Gamma-ray Space Telescope is a diffuse gamma-ray background from dark-matter annihilation or decay in the Galactic halo. N-body simulations and theoretical arguments suggest that the dark matter in the Galactic halo may be clumped into substructure, rather than smoothly distributed. Here we propose the gamma-ray-flux probability distribution function (PDF) as a probe of substructure in the Galactic halo. We calculate this PDF for a phenomenological model of halo substructure and determine the regions of the substructure parameter space in which…
We investigate the intrinsic alignments of dark halo substructures with their host halo major-axis orientations both analytically and numerically. Analytically, we derive the probability density distribution of the angles between the minor axes of the substructures and the major axes of their host halos from the physical principles, under the assumption that the substructure alignment on galaxy scale is a consequence of the tidal fields of the host halo gravitational potential. Numerically, we use a sample of four cluster-scale halos and their galaxy-scale substructures from recent high-resolu…
We examine recent suggestions that substructure in cold dark matter (CDM) halos may be in conflict with the presence of thin, dynamically fragile stellar disks. N-body simulations of an isolated disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities and orbits derived from high-resolution cosmological CDM simulations indicate that substructure at $z=0$ plays only a minor dynamical role in the heating of the disk over several Gyrs. This is because the orbits of satellites in present-day CDM halos seldom take them near the disk, where t…
We analyse the properties of substructures within dark matter halos (subhalos) using a set of high-resolution numerical simulations of the formation of structure in a Lambda-CDM Universe. Our simulation set includes 11 high-resolution simulations of massive clusters as well as a region of mean density, allowing us to study the spatial and mass distribution of substructures down to a mass resolution limit of 10^9 h^(-1)Mo. We also investigate how the properties of substructures vary as a function of the mass of the `parent' halo in which they are located. We find that the substructure mass func…
High-resolution cosmological N-body simulations were performed in order to study the substructure of Milky Way-like galactic halos and the density profiles of halos in a warm dark matter scenario. The results favor this scenario with respect to the cold dark matter one.
We develop the formalism to include substructure in the halo model of clustering. Real halos are not likely to be perfectly smooth, but have substructure which has so far been neglected in the halo model -- our formalism allows one to estimate the effects of this substructure on measures of clustering. We derive expressions for the two-point correlation function, the power-spectrum, the cross-correlation between galaxies and mass, as well as higher order correlation functions. Simple forms of the formulae are obtained for the limit in which the size of the subclumps and mass fraction in them i…
In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of dark matter halo formation based on merger trees. In this paper, we present the basic predictions of our full model of halo formation. We first describe the overall properties of substructure in galaxy, group or cluster haloes at the present day. We then discuss the evolution of substructure, and the effect of the mass accretion history of an individual halo on the mass function and orbital grouping of its subhalo population. We show, in particular, that the shape of …
Models of structure formation based on cold dark matter predict that most of the small dark matter haloes that first formed at high redshift would have merged into larger systems by the present epoch. Substructure in present-day haloes preserves the remains of these ancient systems, providing the only direct information we may ever have about the low-mass end of the power spectrum. I describe some recent attempts to model halo substructure down to very small masses, using a semi-analytic model of halo formation. I make a preliminary comparison between the model predictions, observations of sub…
High resolution simulations reveal that in the cold dark matter scenario the structures form hierarchically and a large number of substructures survive in the galactic halos. The substructures can be probed if they emit gamma rays via dark matter annihilation. We calculated the gamma ray fluxes from the dark matter annihilations in the substructures of our Galaxy within the frame of the minimal supersymmetric extension of the standard model. The uncertainties of the prediction from both the low energy supersymmetry and especially from the density profiles of dark matter in the substructures ar…
Numerical simulations have revealed the presence of long-lived substructure in Cold Dark Matter (CDM) halos. These surviving cores of past merger and accretion events vastly outnumber the known satellites of the Milky Way. This finding has prompted suggestions that substructure in cold dark matter (CDM) halos may be incompatible with observation and in conflict with the presence of thin, dynamically fragile stellar disks. N-body simulations of a disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities and orbits derived from high-resolu…
A survey of 380 sq. deg. of the sky with the 1m Schmidt telescope at the Observatorio Nacional de Llano del Halo and the QUEST camera has found 498 RR Lyrae variables lying from 4 to 60 kpc from the Sun. We describe the halo substructure revealed by these data and the results of measuring some of the stars' radial velocities and metal abundances.
We estimate the probability of detecting gamma-rays from the annihilation of neutralino dark matter in dense, central regions of Milky Way substructure. We characterize Galactic substructure statistically based on Monte Carlo realizations of the formation of a Milky Way-like halo using a semi-analytic method that has been calibrated against the results of high-resolution N-body simulations. We find that it may be possible for the upcoming experiments GLAST and VERITAS, working in concert, to detect gamma-rays from dark matter substructure if the neutralino is relatively light ($\Mchi \lsim 100…
We study the effects of halo substructure and a distribution in the concentration parameter of haloes on large-scale structure statistics. The effects on the power spectrum and bispectrum are studied on the smallest scales accessible from future surveys. We compare halo-model predictions with results based on N-body simulations, but also extend our predictions to 10-kpc scales which will be probed by future simulations. We find that weak-lensing surveys proposed for the coming decade can probe the power spectrum on small enough scales to detect substructure in massive haloes. We discuss the pr…
We develop an algorithm to reconstruct the triaxial shapes of dark matter halos from the anisotropic spatial distributions of their substructures for the concordance background cosmology. First, we construct an analytic model for the anisotropic spatial distribution of dark halo substructures under the assumption that the tidal field with non-zero trace in the triaxial mass distribution of the host halos generates the substructure bulk motions toward the major principal axes of the hosts. Our analytic model implies that the degree of anisotropy depends sensitively on the triaxiality of the hos…
Gravitational lensing of background compact objects like active galactic nuclei and quasars, by extended intermediate mass lenses such as globular clusters and and dark matter clumps with masses 10^5 - 10^8 M_sun, is considered. It is shown that observational study of the galaxy-quasar's associations is a powerful direct observational test of the nature of massive galaxy haloes. Optical interferometric observations with VLTI and Keck instruments are able to constrain masses and number of substructure halo objects. Evidence of gravitational lensing by globular clusters in haloes of spiral and e…
We discuss the possibility of GLAST detecting gamma-rays from the annihilation of neutralino dark matter in the Galactic halo. We have used "Via Lactea", currently the highest resolution simulation of Galactic cold dark matter substructure, to quantify the contribution of subhalos to the annihilation signal. We present a simulated allsky map of the expected gamma-ray counts from dark matter annihilation, assuming standard values of particle mass and cross section. In this case GLAST should be able to detect the Galactic center and several individual subhalos.
Multiresolution analysis is applied to the problem of halo identification in cosmological N-body simulations. The procedure makes use of a discrete wavelet transform known as the algorithme a trous and segmentation analysis. It has the ability to find subhalos in the dense regions of a parent halo and can discern the multiple levels of substructure expected in the hierarchical clustering scenario. As an illustration, a 500,000 particle dark matter halo is analyzed and over 600 subhalos are found. Statistical properties of the subhalo population are discussed.
We construct a realistic model for the dark halo substructures, and derive analytically their spatial, mass, and velocity distributions. Basically, our model is a modification of the Press-Schechter theory to account for the dominant dynamical processes that mark the evolution of dark halo substructures such as the tidal stripping and dynamical friction. Our analytic model successfully reproduces all the well known behaviors of the substructure distributions that have been found in recent numerical simulations: the weak dependence of the mass distributions on the host halo mass; the anti-bias …
We describe our recent attempts to model substructure in dark matter halos down to very small masses, using a semi-analytic model of halo formation. The results suggest that numerical simulations of halo formation may still be missing substructure in the central regions of halos due to purely numerical effects. If confirmed, this central 'overmerging' problem will have important consequences for the interpretation of lensing measurements of substructure. We also show that the spatial distribution of subhalos relative to the satellite companions of the Milky Way rules out at least one simple mo…
We use numerical simulations to examine the substructure within galactic and cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialised extent of the Milky Way's halo should contain about 500 satellites with circular velocities larger than Draco and U…
We use large cosmological N-body simulations to study the subhalo population in galaxy group sized halos. In particular, we look for fossil group candidates with typical masses ~10-25% of Virgo cluster but with an order of magnitude less substructure. We examine recent claims that the earliest systems to form are deficient enough in substructure to explain the luminosity function found in fossil groups. Although our simulations show a correlation between the halo formation time and the number of subhalos, the maximum suppression of subhalos is a factor of 2-2.5, whereas a factor of 6 is requir…
In this proceeding, we briefly review three recent results. First, we show that halos formed in simulations with gas cooling are significantly rounder than halos formed in dissipationless $N$-body simulations. The increase in principle axis ratios is $δ(c/a) ~ 0.2 - 0.4$ in the inner halo and remains significant at large radii. Second, we discuss the CDM substructure crisis and demonstrate the sensitivity of the crisis to the spectrum of primordial density fluctuations on small scales. Third, we assess the ability of experiments like VERITAS and GLAST to detect $γ$-rays from neutralino dark ma…
I develop a model for the abundance and spatial distribution of dark matter subclumps. The model shows that subclumps of massive parent halos formed at earlier times than subclumps of the same mass in lower mass parents; equivalently, halos in dense regions at a given time formed earlier than halos of the same mass in less dense regions. This may provide the basis for interpreting recent observations which indicate that the stellar populations of the most massive elliptical galaxies are also the oldest.
In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of halo formation based on merger trees. In this paper, we compare the predictions of our model with results from self-consistent numerical simulations. We find that in general the two methods agree extremely well, particularly once numerical effects and selection effects in the choice of haloes are taken into account. As expected from the original analyses of the simulations, we see some evidence for artificial overmerging in the innermost regions of the simulated haloe…
Multiply-imaged quasar lenses can be used to constrain the substructure mass fraction in galaxy-sized dark matter halos via anomalous flux ratios in lensed images. The flux ratios, however, can be affected by both the substructure in the lens halo and by isolated small-mass halos along the entire line-of-sight to the lensed source. While lensing by dark matter clumps near the lens galaxy is more efficient than elsewhere, the cumulative effect of all objects along the line-of-sight could be significant. Here we estimate the potential contribution of isolated clumps to the substructure lensing s…
This is a summary of the discussion in Panel I of IAU Symposium 220 on Dark Matter in Galaxies. The panel topics and panellists were as follows -- Cusps Theory: Julio Navarro; Cusps Observations: Erwin de Blok, Rob Swaters; Substructure Observations: Mario Mateo; Substructure Theory: James Taylor, Shude Mao. In addition to the talks by the panellists and the discussion among them, there was a good deal of discussion with the audience. Despite the differences of opinion expressed I think a consensus emerged, and I try to summarize it here.
We present initial results from ``Via Lactea'', the highest resolution simulation to date of Galactic CDM substructure. It follows the formation of a Milky Way-size halo with Mvir=1.8x10^12 Msun in a WMAP 3-year cosmology, using 234 million particles. Over 10,000 subhalos can be identified at z=0: Their cumulative mass function is well-fit by N(>Msub)= 0.0064 (Msub/Mvir)^(-1) down to Msun=4x10^6 Msun. The total mass fraction in subhalos is 5.3%, while the fraction of surface mass density in substructure within a projected distance of 10 kpc from the halo center is 0.3%. Because of the signific…
Models for the formation and growth of structure in a cold dark matter dominated universe predict that galaxy halos should contain significant substructure. Studies of the Milky Way, however, have yet to identify the expected few hundred sub-halos with masses greater than about 10^6 Msun. Here we propose a test for the presence of sub-halos in the halos of galaxies. We show that the structure of the tidal tails of ancient globular clusters is very sensitive to heating by repeated close encounters with the massive dark sub-halos. We discuss the detection of such an effect in the context of the …
We use the Millennium Simulation, a large, high resolution N-body simulation of the evolution of structure in a LambdaCDM cosmology, to study the properties and fate of substructures within a large sample of dark matter haloes. We find that the subhalo mass function departs significantly from a power law at the high mass end. We also find that the radial and angular distributions of substructures depend on subhalo mass. In particular, high mass subhaloes tend to be less radially concentrated and to have angular distributions closer to the direction perpendicular to the spin of the host halo th…
The hierarchical mergers that form the haloes of dark matter surrounding galaxies, groups and clusters are not entirely efficient, leaving substantial amounts of dense substructure, in the form of stripped halo cores or `subhaloes', orbiting within these systems. Using a semi-analytic model of satellite dynamics, we study the evolution of haloes as they merge hierarchically, to determine how much substructure survives merging and how the properties of individual subhaloes change over time. We find that subhaloes evolve, due to mass loss, orbital decay, and tidal disruption, on a characteristic…
We focus on how astrometric surveys might be exploited for the study of Galactic halo substructure originating from the disruption of satellite systems. The Sagittarius (Sgr) dwarf spheroidal tidal stream provides a useful template to explore present and future capabilities of proper motion surveys for finding and studying substructure as well as for using individual tidal streams to probe the global properties of the Milky Way. The configuration of the Sgr stream is particularly well suited for measuring the flattening of the halo and for providing an independent means for measuring the speed…
We use group size haloes identified with a ``friends of friends'' (FOF) algorithm in a concordance $Λ\rm{CDM}$ GADGET2 (dark matter only) simulation to investigate the dependence of halo properties on the environment at $z=0$. The study is carried out using samples of haloes at different distances from their nearest massive {\em cluster} halo. We find that the fraction of haloes with substructure typically increases in high density regions. The halo mean axial ratio $<c/a>$ also increases in overdense regions, a fact which is true for the whole range of halo mass studied. This can be explained…
Galaxy hierarchical formation theories, numerical simulations, the discovery of the Sagittarius Dwarf Elliptical Galaxy (SagDEG) in 1994 and more recent investigations suggest that the dark halo of the Milky Way can have a rich phenomenology containing non thermalized substructures. In the present preliminary study, we investigate the case of the SagDEG (the best known satellite galaxy in the Milky Way crossing the solar neighbourhood) analyzing the consequences of its dark matter stream contribution to the galactic halo on the basis of the DAMA/NaI annual modulation data. The present analysis…
Two studies have recently reported the discovery of pronounced Halo substructure in the Sloan Digital Sky Survey (SDSS) commissioning data. Here we show that this Halo substructure is almost in its entirety due to the expected tidal stream torn off the Sagittarius dwarf galaxy during the course of its many close encounters with the Milky Way. This interpretation makes strong predictions on the kinematics and distances of these stream stars. Comparison of the structure in old horizontal branch stars, detected by the SDSS team, with the carbon star structure discovered in our own survey, indicat…
We examine the expected signal from annihilation events in realistic cold dark matter halos. If the WIMP is a neutralino, with an annihilation cross-section predicted in minimal SUSY models for the lightest stable relic particle, the central cusps and dense substructure seen in simulated halos may produce a substantial flux of energetic gamma rays. We derive expressions for the relative flux from such events in simple halos with various density profiles, and use these to calculate the relative flux produced within a large volume as a function of redshift. This flux peaks when the first halos c…
Dark matter (DM) annihilations in the Galaxy may produce high energy neutrinos, which can be detected by the neutrino telescopes, for example IceCube, ANTARES and Super-Kamiokande. The neutrinos can also arise from hadronic interaction between cosmic ray and atmosphere around the Earth, known as atmospheric neutrino. Current measurements on neutrino flux is consistent with theoretical prediction of atmospheric neutrino within the uncertainties. In this paper, by requiring that the DM annihilation neutrino flux is less than the current measurements, we obtain an upper bound on the cross section…
We study the impact of the halo shape and geometry on the expected weakly interacting massive particle (WIMP) dark matter annihilation signal from the galactic center. As the halo profile in the innermost region is still poorly constrained, we consider different density behaviors like flat cores, cusps and spikes, as well as geometrical distortions. We show that asphericity has a strong impact on the annihilation signal when the halo profile near the galactic center is flat, but becomes gradually less significant for cuspy profiles, and negligible in the presence of a central spike. However, t…
Upcoming $γ$-ray satellites will search for Dark Matter annihilations in Milky Way substructures (or 'clumps'). The prospects for detecting these objects strongly depend on the assumptions made on the distribution of Dark Matter in substructures, and on the distribution of substructures in the Milky Way halo. By adopting simplified, yet rather extreme, prescriptions for these quantities, we compute the number of sources that can be detected with upcoming experiments such as GLAST, and show that, for the most optimistic particle physics setup ($m_χ=40$ GeV and annihilation cross section $σv = 3…
N-body simulations that follow only a collisionless dark matter component have failed to produce galaxy halos or substructure within dense environments. We investigate the `over-merging' problem analytically and with numerical simulations, by calculating dissolution timescales of halos due to physical and artificial dynamical effects. The numerical resolution that has recently been attained is such that mass-loss from two-body relaxation is negligible. We demonstrate that substructure is destroyed in present simulations as a result of large force softening combined with the heating sources of …
The concordance Cold Dark Matter model for the formation of structure in the Universe, while remarkably successful at describing observations on large scales, has a number of problems on galaxy scales. The Milky Way and its satellite system provide a key laboratory for exploring dark matter (DM) in this regime, but some of the most definitive tests of local DM await microarcsecond astrometry, such as will be delivered by the Space Interferometry Mission (SIM Planetquest). I discuss several tests of Galactic DM enabled by future microarcsecond astrometry.
We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to build up the entire stellar halo, in order to investigate what the next generation of astrometric satellites will reveal by observing the halo of the Milky Way. We generate artificial DIVA, FAME and GAIA halo catalogues, in which we look for the signatures left by the accreted satellites. We develop a method based on the standard Friends-of-Friends algorithm applied to the space of integrals of motion. We find this simple method can recover about 50% of the different accretion events, when the obser…
Recent observations on the cosmic microwave background by Wilkinson Microwave Anisotropy Probe(WMAP) strongly suggest that the reionization of the universe took place quite early (z \sim 17). On the other hand, it has been pointed out that the cold dark matter cosmology suffers from the substructure problem that subgalactic halos are overproduced than the observed dwarf galaxies in the Local Group. In this paper, as a potential mechanism to solve this problem, we consider the feedback effects of the early reionization on the formation of small-scale structures. For this purpose, we perform 3D …
Using high resolution cosmological N-body simulations, we investigate the survival of dark matter satellites falling into larger haloes. Satellites preserve their identity for some time after merging. We compute their loss of mass, energy and angular momentum as dynamical friction, tidal forces and collisions with other satellites dissolve them. We also analyse the evolution of their internal structure. Satellites with less than a few per cent the mass of the main halo may survive for several billion years, whereas larger satellites rapidly sink into the center of the main halo potential well …
We study the structure and substructure of halos obtained in N-body simulations for a Lambda Cold Dark Matter (LCDM) cosmology with non-Gaussian initial conditions (NGICs). The initial statistics are lognormal in the gravitational potential field with positive (LNp) and negative (LNn) skewness; the sign of the skewness is conserved by the density field, and the power spectrum is the same for all the simulations. Our aim is not to test a given non-Gaussian statistics, but to explore the generic effect of positive- and negative-skew statistics on halo properties. From our low-resolution simulati…
We present a detailed study of the real and integrals-of-motion space distributions of a satellite obtained from a self-consistent high-resolution simulation of a galaxy cluster and re-simulated using various analytical halo potentials. We found that the disrupted satellite appears as a coherent structure in integrals-of-motion space in all models (``live'' and analytical potential) although the distribution is significantly smeared for the live host halo. Further the primary mechanism for this smearing is the mass growth of the host, which changes both the energy and angular momentum of the s…
We describe our new "MLAPM-halo-finder" (MHF) which is based on the adaptive grid structure of the N-body code MLAPM. We then extend the MHF code in order to track the orbital evolution of gravitationally bound objects through any given cosmological N-body simulation - our so-called "MLAPM-halo-tracker" (MHT). The mode of operation of MHT is demonstrated using a series of eight high-resolution N-body simulations of galaxy clusters. Each of these halos hosts more than one million particles within their virial radii Rvir. We use MHT as well as MHF to follow the temporal evolution of hundreds of …
Although the currently favored cold dark matter plus cosmological constant model for structure formation assumes an n=1 scale-invariant initial power spectrum, most inflation models produce at least mild deviations from n=1. Because the lever arm from the CMB normalization to galaxy scales is long, even a small ``tilt'' can have important implications for galactic observations. Here we calculate the COBE-normalized power spectra for several well-motivated models of inflation and compute implications for the substructure content and central densities of galaxy halos. Using an analytic model, no…
We develop an explicit model for the formation of the stellar halo from tidally disrupted, accreted dwarf satellites in the cold dark matter (CDM) framework, focusing on predictions testable with the Sloan Digital Sky Survey (SDSS) and other wide-field surveys. Subhalo accretion and orbital evolution are calculated using a semi-analytic approach within the Press-Schechter formalism. Motivated by our previous work, we assume that low-mass subhalos (v < 30 km/s) can form significant populations of stars only if they accreted a substantial fraction of their mass before the epoch of reionization. …
The existence of dark matter can be proved in an astrophysical context by the discovery of a system in which the observed baryons and the inferred dark matter are spatially segregated, such as the bullet cluster (1E0657-558). The full descriptions of the dark matter halo and X-ray gas substructure motions are necessary to forecast the location of the dark halo from X-ray maps, which can be confirmed by the detection of a galaxy concentration or by gravitational lensing. We present an analytical hydrodynamic model to determine the distance between the X-ray and dark-matter components and the Ma…
(Abridged) We use cosmological N-body simulations to study the properties of substructure halos in galaxy-sized dark matter halos. We extend prior work on the subject by considering the whole population of subhalos physically associated with the main system. These are defined as subhalos that have at some time in the past been within the virial radius of the halo's main progenitor and that have survived as self-bound entities to $z=0$. We find that this population extends beyond {\it three times} the virial radius, and contains objects on extreme orbits. We trace the origin of these unorthodox…
We show how observations of multiply-imaged quasars at high redshift can be used as a probe of dark matter clumps (subhalos with masses ~ 10^9 solar masses) within the virialized extent of more massive lensing halos. A large abundance of such satellites is predicted by numerical simulations of galaxy formation in cold dark matter (CDM) cosmogonies. Small-scale structure within galaxy halos affects the flux ratios of the images without appreciably changing their positions. We use numerical simulations to quantify the effect of dark matter substructure on the distribution of magnifications, and …
A sophisticated analysis of EGRET data has found evidence for gamma-ray emission from the galactic halo. I entertain the possibility that part of the EGRET signal is due to WIMP annihilations in the halo. I show that a viable candidate with the required properties exists in a model with an extended Higgs sector. The candidate has a mass of 2--4 GeV, a relic density $Ω\sim 0.1$ (for a Hubble constant of 60 km/s/Mpc), and a scattering cross section off nucleons in the range $10^{-5}$--$10^{-1}$ pb. The model satisfies present observational and experimental constraints, and makes strict predictio…
We performed cosmological simulations based upon both a cold dark matter (CDM) and a warm dark matter (WDM) model. The focus of our investigations lies with selected spatial and kinematic properties of substructure halos (subhalos) orbiting within host halos, that form in both dark-matter cosmologies. We aim at using the dynamics of the subhalos as a probe of the respective cosmology.
(ABRIDGED) We present a semi-analytic model to explore merger histories, destruction rates, and survival probabilities of substructure in dark matter halos and use it to study the substructure populations of galaxy-sized halos as a function of the power spectrum. We successfully reproduce the subhalo velocity function and radial distribution seen in N-body simulations for standard LCDM. We explore the implications of spectra with normalizations and tilts spanning sigma_8 = 0.65-1 and n = 0.8-1. We also study a running index (RI) model with dn/dlnk=-0.03, as discussed in the first year WMAP rep…
Within the framework of hierarchical clustering scenarios, we investigate the consequences for the properties of virialized halos of the constraints provided by numerical simulations on the first few correlation functions. Thus, we show that the density field cannot be described by a collection of smooth halos with a universal density profile. This implies that substructures within larger objects play an important role (but a mean spherically averaged density profile may exist). In particular, a possible interpretation is that collapsed objects can be divided into an infinite hierarchy of smal…
We demonstrate that the Sommerfeld correction to CDM annihilations can be appreciable if even a small component of the dark matter is extremely cold. Subhalo substructure provides such a possibility given that the smallest clumps are relatively cold and contain even colder substructure due to incomplete phase space mixing. Leptonic channels can be enhanced for plausible models and the solar neighbourhood boost required to account for PAMELA/ATIC data is plausibly obtained, especially in the case of a few TeV mass neutralino for which the Sommerfeld-corrected boost is found to be $\sim10^4-10^5…
