Dark Matter In Dwarf Galaxies Research Articles

The dual nature of the tidal tails of NGC 5904 (M5)

ABSTRACT The tangential velocity dispersion of stars belonging to the Milky Way globular cluster’s tidal tails has recently been found from N-body simulations to be a parameter that distinguishes between cored and cuspy profiles of low-mass dwarf galaxy dark matter subhaloes where that globular cluster formed, and the in situ formation scenario. In this context, we discovered that M5’s tidal tails are composed by stars at two different metallicity regimes ([Fe/H] ∼ −1.4 and −2.0 dex). The more metal-rich tidal tail stars are of the same metal content than M5’s members and have a tangential velocity dispersion that coincides with the predicted value for a cuspy formation scenario (subhalo mass ∼ 109 M⊙). The more metal-poor stars, that are found along the entire M5 tidal tails and have similar distributions to their more metal-rich counterparts in the M5 colour–magnitude diagram and orbit trajectory, have a tangential velocity dispersion that refers to a cored subhalo (mass ∼ 109 M⊙), or an in situ formation scenario. In order to reconcile the dual distribution of M5 tidal tail stars, in kinematics and chemistry, we propose that M5 collided with another more metal-poor and less-massive globular cluster anytime before or after it was accreted into the Milky Way.

Viability of ultralight bosonic dark matter in dwarf galaxies

The dark matter distribution in dwarf galaxies holds a wealth of information on the fundamental properties and interactions of the dark matter particle. In this paper, we study whether ultralight bosonic dark matter is consistent with the gravitational potential extracted from stellar kinematics. We use velocity dispersion measurements to constrain models for halo mass and particle mass. The posterior likelihood is multimodal. Particle masses of order $m\sim 10^{-22} {\rm{eV}}$ require halos of mass in excess of $\sim 10^{10} M_\odot$, while particle mass of order $m \gtrsim 10^{-20}{\rm{eV}}$ are favored by halos of mass $\sim [10^{8} - 10^{9}] M_\odot$, with a similar behavior to cold dark matter. Regardless of particle mass, the lower halo masses are allowed if stellar dynamics are influenced by the presence of a central black hole of mass at most $\sim 10^{-2}$ the host halo mass. We find no preference for models that contain a black hole over models that do not contain a black hole. Our main conclusion is that either the fuzzy dark matter particle mass must be $m \gtrsim 10^{-20}$ eV, or the Milky Way dwarfs must be unusually heavy given the expected hierarchical assembly of the Milky Way, or the Milky Way dwarfs must contain a central black hole. We find no evidence for either of the last two possibilities and consider them unlikely.

Characteristics of Effective Dark Matter in Nonlocal Gravity

Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation that has been constructed in close analogy with the nonlocal electrodynamics of media. According to NLG, what appears as dark matter in astrophysics and cosmology is in reality the nonlocal aspect of the universal gravitational interaction. We focus here on two main features of the effective dark matter in NLG, namely, (a) the density of effective dark matter in NLG is always finite and therefore cusp-free, and (b) there is less effective dark matter in dwarf galaxies than is generally assumed in the standard particle dark matter paradigm. The corresponding astrophysical implications of NLG in connection with three ultra-diffuse galaxies AGC 114905, 242019, and 219533 are discussed.

Dark matter density profiles in dwarf galaxies: linking Jeans modelling systematics and observation

ABSTRACT The distribution of dark matter in dwarf galaxies can have important implications on our understanding of galaxy formation as well as the particle physics properties of dark matter. However, accurately characterizing the dark matter content of dwarf galaxies is challenging due to limited data and complex dynamics that are difficult to accurately model. In this paper, we apply spherical Jeans modelling to simulated stellar kinematic data of spherical, isotropic dwarf galaxies with the goal of identifying the future observational directions that can improve the accuracy of the inferred dark matter distributions in the Milky Way dwarf galaxies. We explore how the dark matter inference is affected by the location and number of observed stars as well as the line-of-sight velocity measurement errors. We use mock observation to demonstrate the difficulty in constraining the inner core/cusp of the dark matter distribution with data sets of fewer than 10 000 stars. We also demonstrate the need for additional measurements to make robust estimates of the expected dark matter annihilation signal strength. For the purpose of deriving robust indirect detection constraints, we identify Ursa Major II, Ursa Minor, and Draco as the systems that would most benefit from additional stars being observed.

Universal profiles for radio searches of Dark Matter in dwarf galaxies

The phenomenology of diffuse radio emission from Dark Matter annihilation or decay in dwarf spheroidal galaxies is examined. We introduce (in the context of cosmic-ray physics) a novel strategy for the computation of the relevant synchrotron signals. In particular, we identify various regimes where, in analogy to prompt gamma rays, the diffuse radio signal from dark matter annihilation/decay can be expressed as the multiplication of a halo times a spectral function. These functions are computed here for the first time for a number of benchmark cases. Furthermore, we find parameter regions in which the emissivity can be well approximated by a universal function ∼ sin(π r/r h)/r, where r is the galacto-centric distance and r h the diffusion-zone radius of the galaxy. Our theoretical setup differs from previous work in that, instead of employing a method-of-images strategy, we consider a Fourier-mode expansion of the relevant Green's functions. With this strategy, exact results can be obtained with very low computational cost and for generic dark matter models. In particular, \U0001d4aa(10–100) Fourier modes can be easily incorporated into the computations in order to probe the smallest scales of the problem. We also propose a new strategy to search for dark matter using radio observations of dwarf galaxies that is (1) easy to implement and (2) free of the otherwise large degeneracies in the description of synchrotron signals from dark matter. Finally, we correct a mistake in a widely used Green's function formula in this context. We show that the original expression leads to systematically incorrect — and in some cases divergent — results in the regime where the characteristic time-scale for diffusion is smaller than that for energy losses.

Cusp-to-core transition in low-mass dwarf galaxies induced by dynamical heating of cold dark matter by primordial black holes

ABSTRACT We performed a series of high-resolution N-body simulations to examine whether dark matter candidates in the form of primordial black holes (PBHs) can solve the cusp–core problem in low-mass dwarf galaxies. If some fraction of the dark matter in low-mass dwarf galaxies consists of PBHs and the rest is cold dark matter, dynamical heating of the cold dark matter by the PBHs induces a cusp-to-core transition in the total dark matter profile. The mechanism works for PBHs in the 25–100 M⊙ mass window, consistent with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detections, but requires a lower limit on the PBH mass fraction of 1 ${{\rm per\ cent}}$ of the total dwarf galaxy dark matter content. The cusp-to-core transition time-scale is between 1 and 8 Gyr. This time-scale is also a constant multiple of the relaxation time between cold dark matter particles and PBHs, which depends on the mass, the mass fraction, and the scale radius of the initial density profile of PBHs. We conclude that dark matter cores occur naturally in haloes composed of cold dark matter and PBHs, without the need to invoke baryonic processes.

On the Absence of Dark Matter in Dwarf Galaxies Surrounding the Milky Way

Abstract This paper presents an alternative scenario to explain the observed properties of the Milky Way dwarf Spheroidals (MW dSphs). We show that instead of resulting from large amounts of dark matter (DM), the large velocity dispersions observed along their lines of sight (σ los) can be entirely accounted for by dynamical heating of DM-free systems resulting from MW tidal shocks. Such a regime is expected if the progenitors of the MW dwarfs are infalling gas-dominated galaxies. In this case, gas lost through ram-pressure leads to a strong decrease of self-gravity, a phase during which stars can radially expand, while leaving a gas-free dSph in which tidal shocks can easily develop. The DM content of dSphs is widely derived from the measurement of the dSphs self-gravity acceleration projected along the line of sight. We show that the latter strongly anti-correlates with the dSph distance from the MW, and that it is matched in amplitude by the acceleration caused by MW tidal shocks on DM-free dSphs. If correct, this implies that the MW dSphs would have negligible DM content, putting in question, e.g., their use as targets for DM direct searches, or our understanding of the Local Group mass assembly history. Most of the progenitors of the MW dSphs are likely extremely tiny dIrrs, and deeper observations and more accurate modeling are necessary to infer their properties as well as to derive star formation histories of the faintest dSphs.

Formation of LISA Black Hole Binaries in Merging Dwarf Galaxies: The Imprint of Dark Matter

Abstract Theoretical models for the expected merger rates of intermediate-mass black holes (IMBHs) are vital for planned gravitational-wave detection experiments such as the Laser Interferometer Space Antenna (LISA). Using collisionless N-body simulations of dwarf galaxy (DG) mergers, we examine how the orbital decay of IMBHs and the efficiency of IMBH binary formation depend on the central dark matter (DM) density profile of the merging DGs. Specifically, we explore various asymptotic inner slopes γ of the DG’s DM density distribution, ranging from steep cusps (γ = 1) to shallower density profiles (γ < 1), motivated by well-known baryonic-feedback effects as well as by DM models that differ from cold DM at the scales of DGs. We find that the inner DM slope is crucial for the formation (or lack thereof) of an IMBH binary; only mergers between DGs with cuspy DM profiles (γ = 1) are favorable to forming a hard IMBH binary, whereas when γ < 1 the IMBHs stall at a separation of 50–100 pc. Consequently, the rate of LISA signals from IMBH coalescence will be determined by the fraction of DGs with a cuspy DM profile. Conversely, the LISA event rates at IMBH mass scales offer in principle a novel way to place constraints on the inner structure of DM halos in DGs and address the core–cusp controversy. We also show that, with spatial resolutions of ∼0.1 kpc, as often adopted in cosmological simulations, all IMBHs stall, independent of γ. This suggests caution should be taken when employing cosmological simulations of galaxy formation to study BH dynamics in DGs.

The Structural and Kinematic Evolution of Central Star Clusters in Dwarf Galaxies and Their Dependence on Dark Matter Halo Profiles

Through a suite of direct N-body simulations, we explore how the structural and kinematic evolution of a star cluster located at the center of a dwarf galaxy is affected by the shape of its host's dark matter density profile. The stronger central tidal fields of cuspier halos minimize the cluster's ability to expand in response to mass loss due to stellar evolution during its early evolutionary stages and during its subsequent long-term evolution driven by two-body relaxation. Hence clusters evolving in cuspier dark matter halos are characterized by more compact sizes, higher velocity dispersions and remain approximately isotropic at all clustercentric distances. Conversely, clusters in cored halos can expand more and develop a velocity distribution profile that becomes increasingly radially anisotropic at larger clustercentric distances. Finally, the larger velocity dispersion of clusters evolving in cuspier dark matter profiles results in them having longer relaxation times. Hence clusters in cuspy galaxies relax at a slower rate and, consequently, they are both less mass segregated and farther from complete energy equipartition than cluster's in cored galaxies. Application of this work to observations allows for star clusters to be used as tools to measure the distribution of dark matter in dwarf galaxies and to distinguish isolated star clusters from ultra-faint dwarf galaxies.

Mapping extragalactic dark matter annihilation with galaxy surveys: A systematic study of stacked group searches

Dark matter in the halos surrounding galaxy groups and clusters can annihilate to high-energy photons. Recent advancements in the construction of galaxy group catalogs provide many thousands of potential extragalactic targets for dark matter. In this paper, we outline a procedure to infer the dark matter signal associated with a given galaxy group. Applying this procedure to a catalog of sources, one can create a full-sky map of the brightest extragalactic dark matter targets in the nearby Universe ($z\lesssim 0.03$), supplementing sources of dark matter annihilation from within the Local Group. As with searches for dark matter in dwarf galaxies, these extragalactic targets can be stacked together to enhance the signals associated with dark matter. We validate this procedure on mock $\textit{Fermi}$ gamma-ray data sets using a galaxy catalog constructed from the $\texttt{DarkSky}$ $N$-body cosmological simulation and demonstrate that the limits are robust, at $\mathcal{O}(1)$ levels, to systematic uncertainties on halo mass and concentration. We also quantify other sources of systematic uncertainty arising from the analysis and modeling assumptions. Our results suggest that a stacking analysis using galaxy group catalogs provides a powerful opportunity to discover extragalactic dark matter and complements existing studies of Milky Way dwarf galaxies.

Bose-Einstein Condensate Dark Matter Halos Confronted with Galactic Rotation Curves

We present a comparative confrontation of both the Bose-Einstein Condensate (BEC) and the Navarro-Frenk-White (NFW) dark halo models with galactic rotation curves. We employ 6 High Surface Brightness (HSB), 6 Low Surface Brightness (LSB), and 7 dwarf galaxies with rotation curves falling into two classes. In the first class rotational velocities increase with radius over the observed range. The BEC and NFW models give comparable fits for HSB and LSB galaxies of this type, while for dwarf galaxies the fit is significantly better with the BEC model. In the second class the rotational velocity of HSB and LSB galaxies exhibits long flat plateaus, resulting in better fit with the NFW model for HSB galaxies and comparable fits for LSB galaxies. We conclude that due to its central density cusp avoidance the BEC model fits better dwarf galaxy dark matter distribution. Nevertheless it suffers from sharp cutoff in larger galaxies, where the NFW model performs better. The investigated galaxy sample obeys the Tully-Fisher relation, including the particular characteristics exhibited by dwarf galaxies. In both models the fitting enforces a relation between dark matter parameters: the characteristic density and the corresponding characteristic distance scale with an inverse power.

Missing dark matter in dwarf galaxies?

We use cosmological hydrodynamical simulations of the APOSTLE project along with high-quality rotation curve observations to examine the fraction of baryons in ΛCDM haloes that collect into galaxies. This ‘galaxy formation efficiency’ correlates strongly and with little scatter with halo mass, dropping steadily towards dwarf galaxies. The baryonic mass of a galaxy may thus be used to place a lower limit on total halo mass and, consequently, on its asymptotic maximum circular velocity. A number of observed dwarfs seem to violate this constraint, having baryonic masses up to 10 times higher than expected from their rotation speeds, or, alternatively, rotating at only half the speed expected for their mass. Taking the data at face value, either these systems have formed galaxies with extraordinary efficiency – highly unlikely given their shallow potential wells – or their dark matter content is much lower than expected from ΛCDM haloes. This ‘missing dark matter’ is reminiscent of the inner mass deficit of galaxies with slowly rising rotation curves, but cannot be explained away by star formation-induced ‘cores’ in the dark mass profile, since the anomalous deficit applies to regions larger than the luminous galaxies themselves. We argue that explaining the structure of these galaxies would require either substantial modification of the standard ΛCDM paradigm or else significant revision to the uncertainties in their inferred mass profiles, which should be much larger than reported. Systematic errors in inclination may provide a simple resolution to what would otherwise be a rather intractable problem for the current paradigm.

Monochromatic neutrino lines from sneutrino dark matter

We investigate the possibility of observing monochromatic neutrino lines originating from annihilation of dark matter. We analyse several astrophysical sources with overdensities of dark matter that can amplify the signal. As a case study, we consider mixed left and right handed sneutrino dark matter. We demonstrate that in the physically viable region of the model, one can obtain a prominent monochromatic neutrino line. We propose a search strategy to observe these neutrino lines in future generations of neutrino telescopes that is especially sensitive to dwarf spheroidal galaxies. We demonstrate that the presence of massive black holes in the cores of dwarfs as well as of more massive galaxies substantially boosts any putative signal. In particular, dark matter in dwarf galaxies spiked by IMBH provides a powerful means of probing low annihilation cross-sections well below $10^{-26} \rm cm^3 s^{-1}$ that are otherwise inaccessible by any future direct detection or collider experiment.

Stellar kinematics and dark matter in dwarf galaxies

AbstractIn this review I will discuss the current status on determinations of the dark matter content and distribution in Milky Way dwarf spheroidals, for which the available data-sets allow the application of sophisticated mass modeling techniques.

Prospects for detecting gamma rays from annihilating dark matter in dwarf galaxies in the era of the Dark Energy Survey and Large Synoptic Survey Telescope

Among the most stringent constraints on the dark matter annihilation cross section are those derived from observations of dwarf galaxies by the Fermi Gamma-Ray Space Telescope. As current (e.g., Dark Energy Survey, DES) and future (Large Synoptic Survey Telescope, LSST) optical imaging surveys discover more of the Milky Way's ultra-faint satellite galaxies, they may increase Fermi's sensitivity to dark matter annihilations. In this study, we use a semi-analytic model of the Milky Way's satellite population to predict the characteristics of the dwarfs likely to be discovered by DES and LSST, and project how these discoveries will impact Fermi's sensitivity to dark matter. While we find that modest improvements are likely, the dwarf galaxies discovered by DES and LSST are unlikely to increase Fermi's sensitivity by more than a factor of ~2-4. However, this outlook may be conservative, given that our model underpredicts the number of ultra-faint galaxies with large potential annihilation signals actually discovered in the Sloan Digital Sky Survey. Our simulation-based approach focusing on the Milky Way satellite population demographics complements existing empirically-based estimates.

Reproducing properties of MW dSphs as descendants of DM-free TDGs

The Milky Way (MW) dwarf spheroidal (dSph) satellites are known to be the most dark-matter (DM) dominated galaxies with estimates of dark to baryonic matter reaching even above one hundred. It comes from the assumption that dwarfs are dynamically supported by their observed velocity dispersions. However their spatial distributions around the MW is not at random and this could challenge their origin, previously assumed to be residues of primordial galaxies accreted by the MW potential. Here we show that alternatively, dSphs could be the residue of tidal dwarf galaxies (TDGs), which would have interacted with the Galactic hot gaseous halo and disk. TDGs are gas-rich and have been formed in a tidal tail produced during an ancient merger event at the M31 location, and expelled towards the MW. Our simulations show that low-mass TDGs are fragile to an interaction with the MW disk and halo hot gas. During the interaction, their stellar content is progressively driven out of equilibrium and strongly expands, leading to low surface brightness feature and mimicking high dynamical M/L ratios. Our modeling can reproduce the properties, including the kinematics, of classical MW dwarfs within the mass range of the Magellanic Clouds to Draco. An ancient gas-rich merger at the M31 location could then challenge the currently assumed high content of dark matter in dwarf galaxies. We propose a simple observational test with the coming GAIA mission, to follow their expected stellar expansion, which should not be observed within the current theoretical framework.

Dark Matter in Dwarf Galaxies: High Resolution Observations

We present observations and analysis of rotation curves and dark matter halo density profiles in the central regions of four nearby dwarf galaxies. This observing program has been designed to overcome some of the limitations of other rotation curve studies that rely mostly on longslit spectra. We find that these objects exhibit the full range of central density profiles between ρ ∝ r0 (constant density) and ρ ∝ r–1 (NFW halo). This result suggests that there is a distribution of central density slopes rather than a unique halo density profile.

Interacting hot dark matter

We discuss the viability of a light particle ($\ensuremath{\sim}30\mathrm{eV}$ neutrino) with strong self-interactions as a dark matter candidate. The interaction prevents the neutrinos from free-streaming during the radiation-dominated regime so galaxy-sized density perturbations can survive. Smaller scale perturbations are damped due to neutrino diffusion. We calculate the power spectrum in the imperfect fluid approximation, and show that it is damped at the length scale one would estimate due to neutrino diffusion. The strength of the neutrino-neutrino coupling is only weakly constrained by observations, and could be chosen by fitting the power spectrum to the observed amplitude of matter density perturbations. The main shortcoming of our model is that interacting neutrinos can- not provide the dark matter in dwarf galaxies.

The Structure of Dark Matter Halos in Dwarf Galaxies

Recent observations indicate that dark matter haloes have flat central density profiles. Cosmological simulations with non-baryonic dark matter predict however self similar haloes with central density cusps. This contradiction has lead to the conclusion that dark matter must be baryonic. Here it is shown that the dark matter haloes of dwarf spiral galaxies represent a one parameter family with self similar density profiles. The observed global halo parameters are coupled with each other through simple scaling relations which can be explained by the standard cold dark matter model if one assumes that all the haloes formed from density fluctuations with the same primordial amplitude. We find that the finite central halo densities correlate with the other global parameters. This result rules out scenarios where the flat halo cores formed subsequently through violent dynamical processes in the baryonic component. These cores instead provide important information on the origin and nature of dark matter in dwarf galaxies.

Dark Matter in Dwarf Galaxies

This paper reviews the observational evidence for dark matter (DM) in dwarf spiral (dS) and dwarf spheroidal (dE) galaxies. The most secure detection of DM in dwarf galaxies is given by HI rotation curves. They provide estimates of DM halo parameters, i. e., isothermal core radii rc, central densities ρ0 and one-dimensional velocity dispersions σ. The smallest DM halo measured so far is in DDO 127 (MB = −14.5 for H0 = 75 km s−1 Mpc−1, rc ≃ 2.3 kpc, σ ≃ 27 km s−1). If this halo is made of neutrinos of mass mv, then phase-space constraints imply that mv > 110 eV. This is difficult to reconcile with cosmological upper limits giving Ω ≤ 1. Ultimately, dE galaxies will provide the strongest constraints on DM in dwarf galaxies; a detailed look at present results shows that they are not yet conclusive.