Galactic Dark Matter Halo Research Articles

THE PROLATE DARK MATTER HALO OF THE ANDROMEDA GALAXY

We present new limits on the global shape of the dark matter halo in the Andromeda galaxy using and generalizing non-spherical mass models developed by Hayashi & Chiba and compare our results with theoretical predictions of cold dark matter (CDM) models. This is motivated by the fact that CDM models predict non-spherical virialized dark halos, which reflect the process of mass assembly in the galactic scale. Applying our models to the latest kinematic data of globular clusters and dwarf spheroidal galaxies in the Andromeda halo, we find that the most plausible cases for Andromeda yield a prolate shape for its dark halo, irrespective of assumed density profiles. We also find that this prolate dark halo in Andromeda is consistent with theoretical predictions in which the satellites are distributed anisotropically and preferentially located along major axes of their host halos. It is a reflection of the intimate connection between galactic dark matter halos and the cosmic web. Therefore, our result is profound in understanding internal dynamics of halo tracers in Andromeda, such as orbital evolutions of tidal stellar streams, which play important roles in extracting the abundance of CDM subhalos through their dynamical effects on stream structures.

USING GAPS INN-BODY TIDAL STREAMS TO PROBE MISSING SATELLITES

We use N-body simulations to model the tidal disruption of a star cluster in a Milky-Way-sized dark matter halo, which results in a narrow stream comparable to (but slightly wider than) Pal-5 or GD-1. The mean Galactic dark matter halo is modeled by a spherical Navarro-Frenk-White (NFW) potential with subhalos predicted by the LCDM cosmological model. The distribution and mass function of the subhalos follow the results from the Aquarius simulation. We use a matched filter approach to look for "gaps" in tidal streams at 12 length scales from 0.1 kpc to 5 kpc, which appear as characteristic dips in the linear densities along the streams. We find that, in addition to the subhalos' perturbations, the epicyclic overdensities (EOs) due to the coherent epicyclic motions of particles in a stream also produce gap-like signals near the progenitor. We measure the gap spectra - the gap formation rates as functions of gap length - due to both subhalo perturbations and EOs, which have not been accounted for together by previous studies. Finally, we project the simulated streams onto the sky to investigate issues when interpreting gap spectra in observations. In particular, we find that gap spectra from low signal-to-noise observations can be biased by the orbital phase of the stream. This indicates that the study of stream gaps will benefit greatly from high-quality data from future missions.

Dark matter density profile and galactic metric in Eddington-inspired Born–Infeld gravity

We consider the density profile of pressureless dark matter in Eddington-inspired Born–Infeld (EiBI) gravity. The gravitational field equations are investigated for a spherically symmetric dark matter galactic halo, by adopting a phenomenological tangential velocity profile for test particles moving in stable circular orbits around the galactic center. The density profile and the mass distribution, as well as the general form of the metric tensor is obtained by numerically integrating the gravitational field equations, and in an approximate analytical form by using the Newtonian limit of the theory. In the weak field limit, the dark matter density distribution is described by the Lane–Emden equation with polytropic index n = 1, and is nonsingular at the galactic center. The parameter κ of the theory is determined so that the theory could provide a realistic description of the dark matter halos. The gravitational properties of the dark matter halos are also briefly discussed in the Newtonian approximation.

Rotation curves of rotating Galactic Bose-Einstein condensate dark matter halos

We present the dynamics of rotating Bose condensate Galactic dark matter halos, made of an ultralight spinless boson. We restrict to the case of adding axisymmetric rigid rotation to initially spherically symmetric structures and show there are three regimes: (i) small angular momentum, which basically retains the drawbacks of spherically symmetric halos related to compactness and failure at explaining Galactic rotation curves; (ii) an intermediate range of values of angular momentum that allow the existence of long-lived structures with acceptable rotation curve profiles; and (iii) high angular momentum, in which the structure is dispersed away by rotation. We also present in detail the new code used to solve the Gross--Pitaevskii Poisson system of equations in three dimensions.

Discussion on the energy content of the galactic dark matter Bose-Einstein condensate halo in the Thomas-Fermi approximation

We show that the galactic dark matter halo, considered composed of anaxionlike particles Bose-Einstein condensate [6] trapped by aself-graviting potential [5], may be stable in theThomas-Fermi approximation since appropriate choices for the darkmatter particle mass and scattering length are made. The demonstrationis performed by means of the calculation of the potential, kinetic andself-interaction energy terms of a galactic halo described by aBoehmer-Harko density profile. We discuss the validity of theThomas-Fermi approximation for the halo system, and show that thekinetic energy contribution is indeed negligible.

Dark-matter harmonics beyond annual modulation

The count rate at dark-matter direct-detection experiments should modulate annually due to the motion of the Earth around the Sun. We show that higher-frequency modulations, including daily modulation, are also present and in some cases are nearly as strong as the annual modulation. These higher-order modes are particularly relevant if (i) the dark matter is light, O(10) GeV, (ii) the scattering is inelastic, or (iii) velocity substructure is present; for these cases, the higher-frequency modes are potentially observable at current and ton-scale detectors. We derive simple expressions for the harmonic modes as functions of the astrophysical and geophysical parameters describing the Earth's orbit, using an updated expression for the Earth's velocity that corrects a common error in the literature. For an isotropic halo velocity distribution, certain ratios of the modes are approximately constant as a function of nuclear recoil energy. Anisotropic distributions can also leave observable features in the harmonic spectrum. Consequently, the higher-order harmonic modes are a powerful tool for identifying a potential signal from interactions with the Galactic dark-matter halo.

MIMAC: A micro-tpc matrix for dark matter directional detection

The dark matter directional detection opens a new field in cosmology bringing the possibility to build a map of nuclear recoils that would be able to explore the galactic dark matter halo giving access to a particle characterization of such matter and the shape of the halo. The MIMAC (MIcro-tpc MAtrix of Chambers) collaboration has developed in the last years an original prototype detector based on the direct coupling of large pixelized micromegas with a devoted fast self-triggered electronics showing the feasibility of a new generation of directional detectors. The discovery potential of this search strategy is discussed and illustrated. In June 2012, the first bi-chamber prototype has been installed at Modane Underground Laboratory (LSM) and the first underground background events, the gain stability and calibration are shown.

BLACK HOLES IN GAMMA RAY BURSTS AND GALACTIC NUCLEI

Current research marks a clear success in identifying the moment of formation of a Black Hole of ~ 10M⊙, with the emission of a Gamma Ray Burst. This explains in terms of the 'Blackholic Energy' the source of the energy of these astrophysical systems. Their energetics up to 1054 erg, make them detectable all over our Universe. Concurrently a new problematic has been arising related to: (a) The evidence of Dark Matter in galactic halos; (b) The origin of the Super Massive Black Holes in active galactic nuclei and Quasars and (c) The purported existence of a Black Hole in the Center of our Galaxy. These three aspects of this new problematic have been traditionally approached independently. We propose an unified approach to all three of them based on a system of massive self-gravitating neutrinos in General Relativity. Perspectives of future research are presented.

Stability of BEC galactic dark matter halos

In this paper we show that spherically symmetric BEC dark matter halos, with the sin r/r density profile, that accurately fit galactic rotation curves and represent a potential solution to the cusp-core problem are unstable. We do this by introducing back the density profiles into the fully time-dependent Gross-Pitaevskii-Poisson system of equations. Using numerical methods to track the evolution of the system, we found that these galactic halos lose mass at an approximate rate of half of its mass in a time scale of dozens of Myr. We consider this time scale is enough as to consider these halos are unstable and unlikely to be formed. We provide some arguments to show that this behavior is general and discuss some other drawbacks of the model that restrict its viability.

Analysis of single-photon and linear amplifier detectors for microwave cavity dark matter axion searches

We show that at higher frequencies, and thus higher axion masses, single-photon detectors become competitive and ultimately favored, when compared to quantum-limited linear amplifiers, as the detector technology in microwave cavity experimental searches for galactic halo dark matter axions. The cross-over point in this comparison is of order 10 GHz ($\sim 40\ \mu$eV), not far above the frequencies of current searches.

Gamma ray and neutrino flux from the annihilation of neutralino dark matter at the Galactic halo region in the mAMSB model

We consider the lightest supersymmetric particle, the neutralino, in a minimal anomaly mediated supersymmetry breaking model to be a possible candidate for weakly interacting massive particles (WIMP) or cold dark matter, and we investigate its direct and indirect detections. The theoretically allowed supersymmetric parametric space for such a model, along with the recent bounds from the LHC, is constrained by the WMAP results for relic densities. The spin-independent and spin-dependent scattering cross sections for dark matter off nucleons are thus constrained from the WMAP results. They are found to be within the allowed regions of different ongoing direct detection experiments. The annihilation of such dark-matter candidates at the Galactic centre produces different standard model particles such as γ-rays and neutrinos. In this work, we calculate the possible fluxes of these γ-rays and neutrinos coming from the direction of the Galactic centre (and its neighbourhood) at terrestrial or satellite-borne detectors. The calculated γ-ray flux is compared with the observational results of the HESS experiment. The neutrino fluxes of different flavours from the Galactic centre and at different locations away from the Galactic centre produced by WIMP annihilation in this model are also obtained for four types of Galactic dark-matter halo profiles. The detection prospects of such νμ coming from the direction of the Galactic centre at the ANTARES under-sea detector are discussed in terms of the muon signal yield from these muon neutrinos. Both the γ-ray and neutrino signals are estimated for four different dark-matter halo profiles.

Cosmic microwave background constraints on dark matter models of the Galactic center 511 keV signal

The high positron production rate required to explain the flux of 511 keV gamma rays from the galactic center has inspired many models in which dark matter creates positrons. These models include the annihilation of light dark matter and scattering of dark matter with excited states (exciting dark matter). We show that existing cosmic microwave background data robustly constrains such models when the annihilation or scattering cross section is not velocity suppressed depending on the model of the galactic dark matter halo. Upcoming data from the Planck mission can exclude the fiducial Via Lactea II halo model, which also provides a good fit to the 511 keV morphology. We additionally find combined constraints on exciting dark matter scattering and annihilation and update constraints on the lifetimes of dark matter excited states. Finally, we apply constraints to models of dark matter decay in which produced positrons fall into the galactic center and produce the 511 keV signal on their annihilation, demonstrating that most of the parameter space of interest is ruled out.

The global dark halo structure of the Andromeda galaxy

AbstractWe set new limits on the global shape of the dark halo in the Andromeda galaxy based on axisymmetric mass models constructed by Hayashi & Chiba (2012). This is motivated by the fact that CDM models predict non-spherical virialized dark halos, which reflect the process of mass assembly in the galactic scale. Based on the application of our models to latest kinematical data of globular clusters and dwarf spheroidal galaxies in the Andromeda halo, we find that the most plausible cases for Andromeda yield not a spherical but a prolate shape for its dark halo. We also find that the prolate dark halo is consistent with theoretical predictions in which the satellites are distributed anisotropically and preferentially located along major axes of their galactic host halos. It is a reflection of the intimate connection between galactic dark matter halos and the cosmic web.

Domain wall model in the galactic Bose-Einstein condensate halo

We assume that the galactic dark matter halo, considered composed ofan axionlike particles Bose-Einstein condensate [1], canpresent topological defects, namely domain walls, arising asthe dark soliton solution for the Gross-Pitaevskii equation in aself-graviting potential. We investigate the influence that suchsubstructures would have in the gravitational interactions within agalaxy. We find that, for the simple domain wall model proposed, theeffects are too small to be identified, either by means of a localmeasurement of the gradient of the gravitational field or by analysinggalaxy rotation curves. In the first case, the gradient of thegravitational field in the vicinity of the domain wall would be10−31 (m/s2)/m. In the second case, the ratio of thetangential velocity correction of a star due to the presence of thedomain wall to the velocity in the spherical symmetric case would be10−8.

Interaction between collisionless galactic discs and non-axisymmetric dark matter haloes

Using $N$-body simulations ($N\sim 10^6 - 10^7$), we examine how a non-axisymmetric dark halo affects the dynamical evolution of the structure in collisionless (stellar) discs. We demonstrate how the model parameters such as mass of the halo, initial conditions in the disc and the halo axes ratio affect morphology and kinematics of the stellar discs. We show that a non-axisymmetric halo can generate a large-scale spiral density pattern in the embedded stellar disc. The pattern is observed in the disc for many periods of its revolution, even if the disc is gravitationally over-stable. The growth of the spiral arms is not accompanied by significant dynamical heating of the disc, irrelevant to its initial parameters. We also investigate transformation of the dark halo's shape driven by the long-lived spiral pattern in the disc . We show that the analysis of the velocity field in the stellar disc and in the spiral pattern gives us a possibility to figure out the spatial orientation of the triaxial-shaped dark halo and to measure the triaxiality.

Search for Photon-Linelike Signatures from Dark Matter Annihilations with H.E.S.S.

Gamma-ray line signatures can be expected in the very-high-energy (E(γ)>100 GeV) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical γ-ray sources that in most cases produce continuous spectra that span over several orders of magnitude in energy. Using data collected with the H.E.S.S. γ-ray instrument, upper limits on linelike emission are obtained in the energy range between ∼ 500 GeV and ∼ 25 TeV for the central part of the Milky Way halo and for extragalactic observations, complementing recent limits obtained with the Fermi-LAT instrument at lower energies. No statistically significant signal could be found. For monochromatic γ-ray line emission, flux limits of (2 × 10(-7) -2 × 10(-5)) m(-2) s(-1) sr(-1) and (1 × 10(-8) -2 × 10(-6)) m(-2) s(-1)sr(-1) are obtained for the central part of the Milky Way halo and extragalactic observations, respectively. For a DM particle mass of 1 TeV, limits on the velocity-averaged DM annihilation cross section ⟨σv⟩(χχ → γγ) reach ∼ 10(-27) cm(3)s(-1), based on the Einasto parametrization of the Galactic DM halo density profile.

Numerical simulation code for self-gravitating Bose–Einstein condensates

We completed the development of simulation code that is designed to study the behavior of a conjectured dark matter galactic halo that is in the form of a Bose–Einstein Condensate (BEC). The BEC is described by the Gross–Pitaevskii equation, which can be solved numerically using the Crank–Nicholson method. The gravitational potential, in turn, is described by Poisson’s equation, that can be solved using the relaxation method. Our code combines these two methods to study the time evolution of a self-gravitating BEC. The inefficiency of the relaxation method is balanced by the fact that in subsequent time iterations, previously computed values of the gravitational field serve as very good initial estimates. The code is robust (as evidenced by its stability on coarse grids) and efficient enough to simulate the evolution of a system over the course of 109 years using a finer (100×100×100) spatial grid, in less than a day of processor time on a contemporary desktop computer. Program summaryProgram title: bec3pCatalogue identifier: AEOR_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOR_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 5248No. of bytes in distributed program, including test data, etc.: 715402Distribution format: tar.gzProgramming language: C++ or FORTRAN.Computer: PCs or workstations.Operating system: Linux or Windows.Classification: 1.5.Nature of problem:Simulation of a self-gravitating Bose–Einstein condensate by simultaneous solution of the Gross–Pitaevskii and Poisson equations in three dimensions.Solution method:The Gross–Pitaevskii equation is solved numerically using the Crank–Nicholson method; Poisson’s equation is solved using the relaxation method. The time evolution of the system is governed by the Gross–Pitaevskii equation; the solution of Poisson’s equation at each time step is used as an initial estimate for the next time step, which dramatically increases the efficiency of the relaxation method.Running time:Depends on the chosen size of the problem. On a typical personal computer, a 100×100×100 grid can be solved with a time span of 10 Gyr in approx. a day of running time.

Physical properties underlying observed kinematics of satellite galaxies

We study the kinematics of satellites around isolated galaxies selected from the Sloan Digital Sky Survey (SDSS) spectroscopic catalog. Using a model of the phase-space density previously measured for the halos of LCDM dark matter cosmological simulations, we determine the properties of the halo mass distribution and the orbital anisotropy of the satellites as a function of the colour-based morphological type and the stellar mass of the central host galaxy. We place constraints on the halo mass and the concentration parameter of dark matter and the satellite number density profiles. We obtain a concentration-mass relation for galactic dark matter haloes that is consistent with predictions of a standard LCDM cosmological model. At given halo or stellar mass, red galaxies have more concentrated halos than their blue counterparts. The fraction of dark matter within a few effective radii is minimal for 11.25<log M_star<11.5. The number density profile of the satellites appears to be shallower than of dark matter, with the scale radius typically 60 per cent larger than of dark matter. The orbital anisotropy around red hosts exhibits a mild excess of radial motions, in agreement with the typical anisotropy profiles found in cosmological simulations, whereas blue galaxies are found to be consistent with an isotropic velocity distribution. Our new constraints on the halo masses of galaxies are used to provide analytic approximations of the halo-to-stellar mass relation for red and blue galaxies.

Astronomical constraints on quantum theories of cold dark matter – I. Einasto density profile for galactic haloes

Among different models of dark matter, the so-called quantum (or ‘fuzzy’) cold dark matter plays an important role as it allows removing cuspy halo profiles and an abundance of low mass haloes, which are predicted by cold dark matter models but have never been observed. The basic idea of the quantum cold dark matter is the existence of extremely light bosonic particles whose expected masses are of the order of 10−22 eV or even as small as 10−24 eV. With such a small mass, the particle's behaviour in galactic dark matter haloes is described by the Schrödinger equation with a gravitational potential term. In this paper, two previously introduced quantum models of dark matter are reconsidered and the probability densities predicted by these models are compared to the astronomically established density profiles of dark matter haloes of the Milky Way and a dwarf galaxy. By imposing these astronomical constraints on the quantum models, it is shown that the models do reproduce well the Einasto profile with index n ≈ 0.56. However, the models can neither account for the total masses and halo shapes of these two galaxies, nor other galaxies, using the same dark matter elementary particle. Possible improvements of the quantum models that could potentially remove this deficiency are discussed.

Direct detection of dark matter axions with directional sensitivity

We study the directional effect of the expected axion dark matter signal in a resonant cavity of an axion haloscope detector, for cavity geometries not satisfying the condition that the axion de Broglie wavelength λa is sufficiently larger than the cavity dimensions L for a fully coherent conversion, i.e. λa≳2πL. We focus on long thin cavities immersed in dipole magnets and find, for appropriately chosen cavity lengths, an O(1) modulation of the signal with the cavity orientation with respect the momentum distribution of the relic axion background predicted by the isothermal sphere model for the galactic dark matter halo. This effect can be exploited to design directional axion dark matter detectors, providing an unmistakable signature of the extraterrestrial origin of a possible positive detection. Moreover, the precise shape of the modulation may give information of the galactic halo distribution and, for specific halo models, give extra sensitivity for higher axion masses.