Pure Dark Matter Research Articles

Limits in late time conversion of cold dark matter into dark radiation

Structure formation creates high temperature and density regions in the Universe that allow the conversion of matter into more stable states, with a corresponding emission of relativistic matter and radiation. An example of such a mechanism is the supernova event, that releases relativistic neutrinos corresponding to 99% of the binding energy of remnant neutron star. We take this phenomena as a starting point for an assumption that similar processes could occur in the dark sector, where structure formation would generate a late time conversion of cold dark matter into a relativistic form of dark matter. We performed a phenomenological study about the limits of this conversion, where we assumed a transition profile that is a generalized version of the neutrino production in supernovae events. With this assumption, we obtained an interesting modification for the constraint over the cold dark matter density. We show that when comparing with the standard ΛCDM cosmology, there is no preference for conversion, although the best fit is within 1σ from the standard model best fit. The methodology and the results obtained qualify this conversion hypothesis, from the large scale structure point of view, as a viable and interesting model to be tested in the future with small scale data, and mitigate discrepancies between observations at this scale and the pure cold dark matter model.

The abundance of (not just) dark matter haloes

We study the effect of baryons on the abundance of structures and substructures in a Lambda-CDM cosmology, using a pair of high resolution cosmological simulations from the GIMIC project. Both simulations use identical initial conditions, but while one contains only dark matter, the other also includes baryons. We find that gas pressure, reionisation, supernova feedback, stripping, and truncated accretion systematically reduce the total mass and the abundance of structures below ~10^12 solar masses compared to the pure dark matter simulation. Taking this into account and adopting an appropriate detection threshold lowers the abundance of observed galaxies with maximum circular velocities below 100 km/s, significantly reducing the reported discrepancy between Lambda-CDM and the measured HI velocity function of the ALFALFA survey. We also show that the stellar-to-total mass ratios of galaxies with stellar masses of ~10^5 - 10^7 solar masses inferred from abundance matching of the (sub)halo mass function to the observed galaxy mass function increase by a factor of ~2. In addition, we find that an important fraction of low-mass subhaloes are completely devoid of stars. Accounting for the presence of dark subhaloes below 10^10 solar masses further reduces the abundance of observable objects, and leads to an additional increase in the inferred stellar-to-total mass ratio by factors of 2 - 10 for galaxies in haloes of 10^9 - 10^10 solar masses. This largely reconciles the abundance matching results with the kinematics of individual dwarf galaxies in Lambda-CDM. We propose approximate corrections to the masses of objects derived from pure dark matter calculations to account for baryonic effects.

A BARYONIC SOLUTION TO THE MISSING SATELLITES PROBLEM

It has been demonstrated that the inclusion of baryonic physics can alter the dark matter densities in the centers of low-mass galaxies, making the central dark matter slope more shallow than predicted in pure cold dark matter simulations. This flattening of the dark matter profile can occur in the most luminous subhalos around Milky Way-mass galaxies. Zolotov et al. (2012) have suggested a correction to be applied to the central masses of dark matter-only satellites in order to mimic the affect of (1) the flattening of the dark matter cusp due to supernova feedback in luminous satellites, and (2) enhanced tidal stripping due to the presence of a baryonic disk. In this paper, we apply this correction to the z=0 subhalo masses from the high resolution, dark matter-only Via Lactea II (VL2) simulation, and find that the number of massive subhalos is dramatically reduced. After adopting a stellar mass to halo mass relationship for the VL2 halos, and identifying subhalos that are (1) likely to be destroyed by stripping and (2) likely to have star formation suppressed by photo-heating, we find that the number of massive, luminous satellites around a Milky Way-mass galaxy is in agreement with the number of observed satellites around the Milky Way or M31. We conclude that baryonic processes have the potential to solve the missing satellites problem.

Influence of baryons on the spatial distribution of matter: higher order correlation functions

Physical processes involving baryons could leave a non-negligible imprint on the distribution of cosmic matter. A series of simulated data sets at high resolution with identical initial conditions are employed for count-in-cell analysis, including one N-body pure dark matter run, one with only adiabatic gas and one with dissipative processes. Variances and higher order cumulants Sn of dark matter and gas are estimated. It is found that physical processes with baryons mainly affect distributions of dark matter at scales less than 1 h−1 Mpc. In comparison with the pure dark matter run, adiabatic processes alone strengthen the variance of dark matter by ∼ 10% at a scale of 0.1 h−1 Mpc, while the Sn parameters of dark matter only mildly deviate by a few percent. The dissipative gas run does not differ much from the adiabatic run in terms of variance for dark matter, but renders significantly different Sn parameters describing the dark matter, bringing about a more than 10% enhancement to S3 at 0.1 h−1 Mpc and z = 0 and being even larger at a higher redshift. Distribution patterns of gas in two hydrodynamical simulations are quite different. Variance of gas at z = 0 decreases by ∼ 30% in the adiabatic simulation but by ∼ 60% in the nonadiabatic simulation at 0.1 h−1 Mpc. The attenuation is weaker at larger scales but is still obvious at ∼ 10 h−1 Mpc. Sn parameters of gas are biased upward at scales < ∼ 4 h−1 Mpc, and dissipative processes show an ∼ 84% promotion at z = 0 to S3 at 0.1 h−1 Mpc in contrast with the ∼ 7% change in the adiabatic run. The segregation in clustering between gas and dark matter could have dramatic implications on modeling distributions of galaxies and relevant cosmological applications demanding fine details of matter distribution in a strongly nonlinear regime.

Dark matter assimilation into the baryon asymmetry

Pure singlets are typically disfavored as dark matter candidates, since they generically have a thermal relic abundance larger than the observed value. In this paper, we propose a new dark matter mechanism called “assimilation”, which takes advantage of the baryon asymmetry of the universe to generate the correct relic abundance of singlet dark matter. Through assimilation, dark matter itself is efficiently destroyed, but dark matter number is stored in new quasi-stable heavy states which carry the baryon asymmetry. The subsequent annihilation and late-time decay of these heavy states yields (symmetric) dark matter as well as (asymmetric) standard model baryons. We study in detail the case of pure bino dark matter by augmenting the minimal supersymmetric standard model with vector-like chiral multiplets. In the parameter range where this mechanism is effective, the LHC can discover long-lived charged particles which were responsible for assimilating dark matter.

INTERACTION BETWEEN DARK MATTER SUB-HALOS AND A GALACTIC GASEOUS DISK

We investigate the idea that the interaction of Dark Matter (DM) sub-halos with the gaseous disk of galaxies can be the origin for the observed holes and shells found in their neutral hydrogen (HI) distributions. We use high resolution hydrodynamic simulations to show that pure DM sub-halos impacting a galactic disk are not able to produce holes; on the contrary, they result in high density regions in the disk. However, sub-halos containing a small amount of gas (a few percent of the total DM mass of the sub-halo) are able to displace the gas in the disk and form holes and shells. The size and lifetime of these holes depend on the sub-halo gas mass, density and impact velocity. A DM sub-halo of mass $10^8$ $M_{\odot}$ and a gas mass fraction of $\sim 3$%, is able to create a kpc scale hole, with a lifetime similar to those observed in nearby galaxies. We also register an increase in the star formation rate at the rim of the hole, again in agreement with observations. Even though the properties off these simulated structures resemble those found in observations we find that the number of predicted holes (based on mass and orbital distributions of DM halos derived from cosmological N-body simulations) falls short compared to the observations. Only a handful of holes are produced per Giga year. This leads us to conclude that DM halo impact is not the major channel through which these holes are formed.

The baryons in the Milky Way satellites

We investigate the formation and evolution of satellite galaxies using smoothed particle hydrodynamics (SPH) simulations of a Milky Way (MW) like system, focusing on the best resolved examples, analogous to the classical MW satellites. Comparing with a pure dark matter simulation, we find that the condensation of baryons has had a relatively minor effect on the structure of the satellites’ dark matter haloes. The stellar mass that forms in each satellite agrees relatively well over three levels of resolution (a factor of ∼64 in particle mass) and scales with (sub)halo mass in a similar way in an independent semi-analytical model. Our model provides a relatively good match to the average luminosity function of the MW and M31. To establish whether the potential wells of our satellites are realistic, we measure their masses within observationally determined half-light radii, finding that they have somewhat higher mass-to-light ratios than those derived for the MW dSphs from stellar kinematic data; the most massive examples are most discrepant. A statistical test yields an ∼6 per cent probability that the simulated and observationally derived distributions of masses are consistent. If the satellite population of the MW is typical, our results could imply that feedback processes not properly captured by our simulations have reduced the central densities of subhaloes, or that they initially formed with lower concentrations, as would be the case, for example, if the dark matter were made of warm, rather than cold particles.

DEFORMED DISTANCE DUALITY RELATIONS AND SUPERNOVA DIMMING

The basic cosmological distances are linked by the Etherington cosmic distance duality relation, $\eta (z) = D_{L}(z)(1+z)^{-2}/D_{A}(z) \equiv 1$, where $D_{L}$ and $D_{A}$ are, respectively, the luminosity and angular diameter distances. In order to test its validity, some authors have proposed phenomenological expressions for $\eta(z)$ thereby deforming the original Etherington's relation and comparing the resulting expressions with the available and future cosmological data. The relevance of such studies is unquestionable since any violation of the cosmic distance duality relation could be the signal of new physics or non-negligible astrophysical effects in the usually assumed perfectly transparent Universe. In this letter, we show that under certain conditions such expressions can be derived from a more fundamental approach with the parameters appearing in the $\eta(z)$ expression defining the cosmic absorption parameter as recently discussed by Chen and Kantowski. Explicit examples involving four different parametrizations of the deformation function are given. Based on such an approach, it is also found that the latest Supernova data can also be explained in the framework of a pure cold dark matter model (Einstein-de Sitter). Two different scenarios with cosmic absorption are discussed. Only if the cosmic opacity is fully negligible, the description of an accelerating Universe powered by dark energy or some alternative gravity theory must be invoked.

The effects of BCGs on the statistics of large-separation lensed quasars by clusters

We study the statistics of large-separation multiply-imaged quasars lensed by clusters of galaxies. In particular, we examine how the observed brightest cluster galaxies (BCGs) affect the predicted numbers of wide-separation lenses. We model the lens as an NFW-profiled dark matter halo with a truncated singular isothermal sphere to represent the BCG in its center. We mainly make predictions for the Sloan Digital Sky Survey Quasar Lens Search (SQLS) sample from the Data Release 5 (DR5) in two standard ΛCDM cosmological models: a model with matter density ΩM = 0.3 and σ8 = 0.9, as is usually adopted in the literature (ΛCDM1), and a model suggested by the WMAP seven-year (WMAP7) data with ΩM = 0.266 and σ8 = 0.801. We also study the lensing properties for the WMAP3 cosmology in order to compare with the previous work. We find that BCGs in the centers of clusters significantly enhance the lensing efficiency by a factor of 2 ∼ 3 compared with that of NFW-profiled pure dark matter halos. In addition, the dependence of mass ratios of BCGs to their host halos on the host halo masses reduces the lensing rate by ∼ 20% from assuming a constant ratio as in previous studies, but considering the evolution of this ratio with redshift out to z ∼ 1 would reduce it by ∼ 3%. Moreover, we predict that the numbers of lensed quasars with image separations larger than 10″ in the statistical sample of SQLS from DR5 are 1.22 and 0.47, respectively for ΛCDM1 and WMAP7 and 0.73 and 0.33 for separations between 10″ and 20″, which are consistent with the only observed cluster lens with such a large separation in the complete SQLS sample.

Comparing galactic satellite properties in hydrodynamical and N-body simulations

In this work, we examine the different properties of galactic satellites in hydrodynamical and pure dark matter simulations. We use three pairs of simulations (collisional and collision-less) starting from identical initial conditions. We concentrate our analysis on pairs of satellites in the hydro and Nbody runs that form from the same Lagrangian region. We look at the radial positions, mass loss as a function of time and orbital parameters of these "twin" satellites. We confirm an overall higher radial density of satellites in the hydrodynamical runs, but find that trends in the mass loss and radial position of these satellites in the inner and outer region of the parent halo differ from the pure dark matter case. In the outskirts of the halo (~70% of the virial radius) satellites experience a stronger mass loss and higher dynamical friction in pure dark matter runs. The situation is reversed in the central region of the halo, where hydrodynamical satellites have smaller apocenter distances and suffer higher mass stripping. We partially ascribe this bimodal behaviour to the delayed infall time for hydro satellites, which on average cross the virial radius of the parent halo 0.7 Gyrs after their dark matter twins. Finally, we briefly discuss the implications of the different set of satellite orbital parameters and mass loss rates in hydrodynamical simulations within the context of thin discs heating and destruction.

Pure leptonic gauge symmetry, neutrino masses and dark matter

A possible extension of the Standard Model to include lepton number as local gauge symmetry is investigated. In such a model, anomalies are canceled by two extra fermions doublet. After leptonic gauge symmetry spontaneously broken, three active neutrinos may acquire non-zero Majorana masses through the modified Type-II seesaw mechanism. Constraints on the model from electro-weak precision measurements are studied. Due to the Z2 discrete flavor symmetry, right-handed Majorana neutrinos can serve as cold dark matter candidate of the Universe. Constraint from dark matter relic abundance is calculated.

On the universality of density profiles

We use the secondary infall model described in Del Popolo (2009), which takes into account the effect of dynamical friction, ordered and random angular momentum, baryons adiabatic contraction and dark matter baryons interplay, to study how in- ner slopes of relaxed LCDM dark matter (DM) halos with and without baryons (baryons+DM, and pure DM) depend on redshift and on halo mass. We apply the quoted method to structures on galactic scales and clusters of galaxies scales. We find that the inner logarithmic density slope, of dark matter halos with baryons has a significant dependence on halo mass and redshift with slopes ranging from 0 for dwarf galaxies to 0.4 for objects of M = 10^13M_solar and 0.94 for M = 10^15M_solar clusters of galaxies. Structures slopes increase with increasing redshift and this trend reduces going from galaxies to clusters. In the case of density profiles constituted just of dark matter the mass and redshift dependence of slope is very slight. In this last case, we used the Merrit et al. (2006) analysis who compared N-body density profiles with various parametric models finding systematic variation in profile shape with halo mass. This last analysis suggests that the galaxy-sized halos obtained with our model have a different shape parameter, i.e. a different mass distribution, than the cluster-sized halos, obtained with the same model. The results of the present paper argue against universality of density profiles constituted by dark matter and baryons and confirm claims of a systematic variation in profile shape with halo mass, for dark matter halos.

On the coldness of the local Hubble flow: the role of baryons

(Abridged) Our aim is to investigate whether the presence of baryons can have any significant influence on the properties of the local Hubble flow which has proved to be "cold". We use two cosmological zoom simulations in the standard LCDM cosmology with the same set of initial conditions to study the formation of a local group-like system within a sphere of ~7 Mpc/h. The first one is a pure dark matter simulation (runDM) while a complete treatment of the physics of baryons is introduced in the second one (runB). We found that galaxies identified in runB and their corresponding dark matter haloes in runDM have very similar spatial distributions and dynamical properties on large scales. Then, when analyzing the velocity field and the deviation from a pure Hubble flow in both simulations, namely when computing the dispersion of peculiar velocities of galaxies \sigma*(R) and those of their corresponding dark matter haloes in runDM, we found no particular differences for distances R=1 to 8 Mpc from the local group mass center. The results indicate that the "true" \sigma*(R) values can be estimated from the pure dark matter simulation with a mean error of 3 km/s when dark matter haloes are selected with maximum circular velocities of Vc$\ge$30 km/s, corresponding to a population of dark matter haloes in runB that host galaxies. By investigating the properties of the Hubble flow at distances R~0.7 to 3 Mpc, we also found that the estimation of the total mass enclosed at the radius of the zero-velocity surface R0, using the spherical infall model adapted to LCDM, can be underestimated by at least 50%.

DISSECTING GALAXY FORMATION. II. COMPARING SUBSTRUCTURE IN PURE DARK MATTER AND BARYONIC MODELS

We compare the substructure evolution in pure dark matter (DM) halos with those in the presence of baryons, hereafter PDM and BDM models, respectively. The prime halos have been analyzed in the previous work. Models have been evolved from identical initial conditions which have been constructed by means of the constrained realization method. The BDM model includes star formation and feedback from stellar evolution onto the gas. A comprehensive catalog of subhalo populations has been compiled and individual and statistical properties of subhalos analyzed, including their orbital differences. We find that subhalo population mass functions in PDM and BDM are consistent with a single power law, Mαsbh, for each of the models in the mass range of ∼2 × 108 M☉–2 × 1011 M☉. However, we detect a nonnegligible shift between these functions, the time-averaged α ∼ −0.86 for the PDM and −0.98 for the BDM models. Overall, α appears to be a nearly constant in time, with variations of ±15%. Second, we find that the radial mass distribution of subhalo populations can be approximated by a power law, with a steepening that occurs at the radius of a maximal circular velocity, Rvmax, in the prime halos. Here we find that γsbh ∼ −1.5 for the PDM and −1 for the BDM models, when averaged over time inside Rvmax. The slope is steeper outside this region and approaches −3. We detect little spatial bias (less than 10%) between the subhalo populations and the DM distribution of the main halos. Also, the subhalo population exhibits much less triaxiality in the presence of baryons, in tandem with the shape of the prime halo. Finally, we find that, counter-intuitively, the BDM population is depleted at a faster rate than the PDM one within the central 30 kpc of the prime halo. The reason for this is that although the baryons provide a substantial glue to the subhalos, the main halo exhibits the same trend. This assures a more efficient tidal disruption of the BDM subhalo population. However, this effect can be reversed for a more efficient feedback from stellar evolution and the central supermassive black holes, which will expel baryons from the center and decrease the central concentration of the prime halo. We compare our results with via Lactea and Aquarius simulations and other published results.

Cusp-core problem and strong gravitational lensing

Cosmological numerical simulations of galaxy formation have led to the cuspy density profile of a pure cold dark matter halo toward the center, which is in sharp contradiction with the observations of the rotation curves of cold dark matter-dominated dwarf and low surface brightness disk galaxies, with the latter tending to favor mass profiles with a flat central core. Many efforts have been devoted to resolving this cusp-core problem in recent years, among them, baryon-cold dark matter interactions are considered to be the main physical mechanisms erasing the cold dark matter (CDM) cusp into a flat core in the centers of all CDM halos. Clearly, baryon-cold dark matter interactions are not customized only for CDM-dominated disk galaxies, but for all types, including giant ellipticals. We first fit the most recent high resolution observations of rotation curves with the Burkert profile, then use the constrained core size-halo mass relation to calculate the lensing frequency, and compare the predicted results with strong lensing observations. Unfortunately, it turns out that the core size constrained from rotation curves of disk galaxies cannot be extrapolated to giant ellipticals. We conclude that, in the standard cosmological paradigm, baryon-cold dark matter interactions are not universal mechanisms for galaxy formation, and therefore, they cannot be true solutions to the cusp-core problem.

Substructures in hydrodynamical cluster simulations

The abundance and structure of dark matter subhalos has been analyzed extensively in recent studies of dark matter-only simulations, but comparatively little is known about the impact of baryonic physics on halo substructures. We here extend the SUBFIND algorithm for substructure identification such that it can be reliably applied to dissipative hydrodynamical simulations that include star formation. This allows, in particular, the identification of galaxies as substructures in simulations of clusters of galaxies, and a determination of their content of gravitationally bound stars, dark matter, and hot and cold gas. Using a large set of cosmological cluster simulations, we present a detailed analysis of halo substructures in hydrodynamical simulations of galaxy clusters, focusing in particular on the influence both of radiative and non-radiative gas physics, and of non-standard physics such as thermal conduction and feedback by galactic outflows. We also examine the impact of numerical nuisance parameters such as artificial viscosity parameterizations. We find that diffuse hot gas is efficiently stripped from subhalos when they enter the highly pressurized cluster atmosphere. This has the effect of decreasing the subhalo mass function relative to a corresponding dark matter-only simulation. These effects are mitigated in radiative runs, where baryons condense in the central subhalo regions and form compact stellar cores. However, in all cases, only a very small fraction, of the order of one percent, of subhalos within the cluster virial radii preserve a gravitationally bound hot gaseous atmosphere. (abridged)

Tidal evolution of discy dwarf galaxies in the Milky Way potential: the formation of dwarf spheroidals

We conduct high-resolution collisionless N-body simulations to investigate the tidal evolution of dwarf galaxies on an eccentric orbit in the Milky Way (MW) potential. The dwarfs originally consist of a low surface brightness stellar disk embedded in a cosmologically motivated dark matter halo. During 10 Gyr of dynamical evolution and after 5 pericentre passages the dwarfs suffer substantial mass loss and their stellar component undergoes a major morphological transformation from a disk to a bar and finally to a spheroid. The bar is preserved for most of the time as the angular momentum is transferred outside the galaxy. A dwarf spheroidal (dSph) galaxy is formed via gradual shortening of the bar. This work thus provides a comprehensive quantitative explanation of a potentially crucial morphological transformation mechanism for dwarf galaxies that operates in groups as well as in clusters. We compare three cases with different initial inclinations of the disk and find that the evolution is fastest when the disk is coplanar with the orbit. Despite the strong tidal perturbations and mass loss the dwarfs remain dark matter dominated. For most of the time the 1D stellar velocity dispersion, \sigma, follows the maximum circular velocity, V_{\rm max}, and they are both good tracers of the bound mass. Specifically, we find that M_{\rm bound} \propto V_{\rm max}^{3.5} and V_{\rm max} \sim \sqrt{3} \sigma in agreement with earlier studies based on pure dark matter simulations. The latter relation is based on directly measuring the stellar kinematics of the simulated dwarf and may thus be reliably used to map the observed stellar velocity dispersions of dSphs to halo circular velocities when addressing the missing satellites problem.

DISSECTING GALAXY FORMATION. I. COMPARISON BETWEEN PURE DARK MATTER AND BARYONIC MODELS

We compare assembly of dark matter (DM) halos with and without baryons from identical initial conditions, within the context of cosmological evolution in the {lambda}CDM WMAP3 Universe (baryons+DM, hereafter BDM model, and pure DM, PDM model). In representative PDM and BDM models, we find that baryons contribute decisively to the evolution of the central region, leading to an isothermal DM cusp, and thereafter to a flat DM density core-the result of heating by dynamical friction of the DM+baryon substructure during a quiescent evolution epoch. This process ablates the cold gas from an embedded disk, cutting the star formation rate by a factor of 10, and heats up the spheroidal gas and stellar components, triggering their expansion. The substructure is more resilient to the tidal disruption in the presence of baryons. The disk which formed from inside-out as gas-dominated is transformed into an intermediate Hubble type by z {approx} 2 and to an early type by z {approx} 0.5, based on its gas contents and spheroidal-to-disk stellar mass ratio. We find that only a relatively small {approx}20% fraction of DM particles in PDM and BDM models are bound within the radius of maximal circular velocity in the halo, slightly less somore » within halo characteristic radii-most of the DM particles perform larger radial excursions. The DM particles are unbound to the cusp region. We also find that the fraction of baryons within the halo virial radius somewhat increases during the major mergers and decreases during the minor mergers. The net effect appears to be negligible-an apparent result of our choice of feedback from stellar evolution. Furthermore, we find that the DM halos are only partially relaxed beyond their virialization. While the substructure is being tidally disrupted, mixing of its debris in the halo is not efficient and becomes even less so with z. The phase-space correlations (streamers) formed after z {approx} 1 will survive largely to the present time-an important implication for embedded disk evolution.« less

Neutralino dark matter stars can not exist

Motivated by the recent "Cosmos Project" observation of dark-matter concentrations with no ordinary matter in the same place, we study the question of the existence of compact objects made of pure dark matter. We assume that the dark matter is neutralino, and compare its elastic and annihilation cross sections. We find that the two cross sections are of the same order of magnitude. This result has a straightforward and important consequence that neutralinos comprising a compact object can not achieve thermal equilibrium. To substantiate our arguments, by solving Oppenheimer-Volkoff equation we constructed a model of the star made of pure neutralinos. We explicitly showed that the condition for the thermal equilibrium supported by the Fermi pressure is never fulfilled inside the star. This neutralino state can not be described by the Fermi-Dirac distribution. Thus, a stable neutralino star, which is supported by the Fermi pressure, can not exist. We also estimated that a stable star can not contain more than a few percents of neutralinos, most of the mass must be in the form of the standard model particles.

Relics of structure formation: extra-planar gas and high-velocity clouds around the Andromeda Galaxy

Using the 100-m radio telescope at Effelsberg, we mapped a large area around the Andromeda Galaxy in the 21 cm line emission of neutral hydrogen to search for high-velocity clouds (HVCs) out to large projected distances in excess of 100 kpc. Our 3σ H i mass sensitivity for the warm neutral medium is 8 × 104M⊙. We can confirm the existence of a population of HVCs near M31 with typical H i masses of a few times 105M⊙. However, we did not detect any HVCs beyond a projected distance of about 50 kpc from M31, suggesting that HVCs are generally found in proximity of large spiral galaxies at typical distances of a few 10 kpc. Comparison with cold dark matter (CDM)-based models and simulations suggests that only a few of the detected HVCs could be associated with primordial dark matter satellites, whereas others are most likely the result of tidal stripping. The lack of clouds beyond a projected distance of 50 kpc from M31 is also in conflict with the predictions of recent CDM structure formation simulations. A possible solution to this problem could be ionization of the HVCs as a result of decreasing pressure of the ambient coronal gas at larger distances from M31. A consequence of this scenario would be the presence of hundreds of mainly ionized or pure dark matter satellites around large spiral galaxies like the Milky Way and M31.