Collisionless Dark Matter Research Articles

Supermassive black holes from collapsing dark matter Bose–Einstein condensates

The discovery of active galactic nuclei at redshifts 6 suggests that supermassive black holes (SMBHs) formed early on. Growth of the remnants of population III stars by accretion of matter, both baryonic as well as collisionless dark matter (DM), leading up to formation of SMBHs is a very slow process. Therefore, such models encounter difficulties in explaining quasars detected at . Furthermore, massive particles making up collisionless DM have not only so far eluded experimental detection but they also do not satisfactorily explain gravitational structures on small scales.In recent years, there has been a surge in research activities concerning cosmological structure formation that involve coherent, ultra-light bosons in a dark fluid-like or fuzzy cold DM state. In this paper, we study collapse of such ultra-light bosonic halo DM that is in a Bose–Einstein condensate (BEC) phase to give rise to SMBHs on dynamical time scales. Time evolution of such self-gravitating BECs is examined using the Gross–Pitaevskii equation in the framework of time-dependent variational method. Comprised of identical dark bosons of mass m, BECs can collapse to form black holes of mass Meff on time scales ∼108 yrs provided . In particular, ultra-light dark bosons of mass can lead to SMBHs with mass at . Recently observed radio-galaxies in the ELAIS-N1 deep field with aligned jets can also possibly be explained if vortices of a rotating cluster size BEC collapse to form spinning SMBHs with angular momentum , where nW and M are the winding number and mass of a vortex, respectively.

Weak annihilation cusp inside the dark matter spike about a black hole.

We reinvestigate the effect of annihilations on the distribution of collisionless dark matter (DM) in a spherical density spike around a massive black hole. We first construct a very simple, pedagogic, analytic model for an isotropic phase space distribution function that accounts for annihilation and reproduces the "weak cusp" found by Vasiliev for DM deep within the spike and away from its boundaries. The DM density in the cusp varies as r-1/2 for s-wave annihilation, where r is the distance from the central black hole, and is not a flat "plateau" profile. We then extend this model by incorporating a loss cone that accounts for the capture of DM particles by the hole. The loss cone is implemented by a boundary condition that removes capture orbits, resulting in an anisotropic distribution function. Finally, we evolve an initial spike distribution function by integrating the Boltzmann equation to show how the weak cusp grows and its density decreases with time. We treat two cases, one for s-wave and the other for p-wave DM annihilation, adopting parameters characteristic of the Milky Way nuclear core and typical WIMP models for DM. The cusp density profile for p-wave annihilation is weaker, varying like ~r-0.34, but is still not a flat plateau.

Boson stars from self-interacting dark matter

We study the possibility that self-interacting bosonic dark matter forms star-like objects. We study both the case of attractive and repulsive self-interactions, and we focus particularly in the parameter phase space where self-interactions can solve well standing problems of the collisionless dark matter paradigm. We find the mass radius relations for these dark matter bosonic stars, their density profile as well as the maximum mass they can support.

Dark Matter Halos as Particle Colliders: Unified Solution to Small-Scale Structure Puzzles from Dwarfs to Clusters.

Astrophysical observations spanning dwarf galaxies to galaxy clusters indicate that dark matter (DM) halos are less dense in their central regions compared to expectations from collisionless DM N-body simulations. Using detailed fits to DM halos of galaxies and clusters, we show that self-interacting DM (SIDM) may provide a consistent solution to the DM deficit problem across all scales, even though individual systems exhibit a wide diversity in halo properties. Since the characteristic velocity of DM particles varies across these systems, we are able to measure the self-interaction cross section as a function of kinetic energy and thereby deduce the SIDM particle physics model parameters. Our results prefer a mildly velocity-dependent cross section, from σ/m≈2 cm^{2}/g on galaxy scales to σ/m≈0.1 cm^{2}/g on cluster scales, consistent with the upper limits from merging clusters. Our results dramatically improve the constraints on SIDM models and may allow the masses of both DM and dark mediator particles to be measured even if the dark sector is completely hidden from the standard model, which we illustrate for the dark photon model.

Constraints on small-scale cosmological fluctuations from SNe lensing dispersion

We provide predictions on small-scale cosmological density power spectrum from supernova lensing dispersion. Parameterizing the primordial power spectrum with running $\alpha$ and running of running $\beta$ of the spectral index, we exclude large positive $\alpha$ and $\beta$ parameters which induce too large lensing dispersions over current observational upper bound. We ran cosmological N-body simulations of collisionless dark matter particles to investigate non-linear evolution of the primordial power spectrum with positive running parameters. The initial small-scale enhancement of the power spectrum is largely erased when entering into the non-linear regime. For example, even if the linear power spectrum at $k>10h {\rm Mpc}^{-1}$ is enhanced by $1-2$ orders of magnitude, the enhancement much decreases to a factor of $2-3$ at late time ($z \leq 1.5$). Therefore, the lensing dispersion induced by the dark matter fluctuations weakly constrains the running parameters. When including baryon-cooling effects (which strongly enhance the small-scale clustering), the constraint is comparable or tighter than the PLANCK constraint, depending on the UV cut-off. Further investigations of the non-linear matter spectrum with baryonic processes is needed to reach a firm constraint.

Simplex-in-cell technique for collisionless plasma simulations

We extend the simplex-in-cell (SIC) technique recently introduced in the context of collisionless dark matter fluids [1,2] to the case of collisionless plasmas. The six-dimensional phase space distribution function f(x,v) is represented by an ensemble of three-dimensional manifolds, which we refer to as sheets. The electric potential field is obtained by solving the Poisson equation on a uniform mesh, where the charge density is evaluated by a spatial projection of the phase space sheets. The SIC representation of phase space density facilitates robust, high accuracy numerical evolution of the Vlasov–Poisson system using significantly fewer tracer particles than comparable particle-in-cell (PIC) approaches by reducing the numerical shot-noise associated with the latter. We introduce the SIC formulation and describe its implementation in a new code, which we validate using standard test problems including plasma oscillations, Landau damping, and two stream instabilities in one dimension. Merits of the new scheme are shown to include higher accuracy and faster convergence rates in the number of particles. We finally motivate and outline the efficient application of SIC to higher dimensional problems.

Asymmetric dark matter stars

We study the possibility of asymmetric dark matter with self-interactions forming compact stable objects. We solve the Tolman-Oppenheimer-Volkoff equation and find the mass-radius relation of such "dark stars", their density profile and their Chandrasekhar mass limit. We consider fermionic asymmetric dark matter with Yukawa-type self-interactions appropriate for solving the well known problems of the collisionless dark matter paradigm. We find that in several cases the relativistic effects are significant.

TESTING THE DARK MATTER CAUSTIC THEORY AGAINST OBSERVATIONS IN THE MILKY WAY

We test a particular theory of dark matter in which dark matter axions form ring "caustics" in the plane of the Milky Way against actual observations of Milky Way stars. According to this theory, cold, collisionless dark matter particles with angular momentum flow in and out of the Milky Way on sheets. These flows form caustic rings (at the positions of the rings, the density of the flow is formally infinite) at the locations of closest approach to the Galactic center. We show that the caustic ring dark matter theory reproduces a roughly logarithmic halo, with large perturbations near the rings. We show that the theory can reasonably match the observed rotation curve of the Milky Way. We explore the effects of the caustic rings on dwarf galaxy tidal disruption using N-body simulations. In particular, simulations of the Sagittarius dwarf galaxy tidal disruption in a caustic ring halo potential match observations of the trailing tidal tail as far as 90 kpc from the Galactic center; they do not, however, match the leading tidal tail. None of the caustic ring, NFW, or triaxial logarithmic halos fit all of the data. The source code for calculating the acceleration due to a caustic ring halo has been made publicly available in the NEMO Stellar Dynamics Toolbox and the Milkyway@home client repository.

Dark matter self-interactions via collisionless shocks in cluster mergers

While dark matter self-interactions may solve several problems with structure formation, so far only the effects of two-body scatterings of dark matter particles have been considered. We show that, if a subdominant component of dark matter is charged under an unbroken U(1) gauge group, collective dark plasma effects need to be taken into account to understand its dynamics. Plasma instabilities can lead to collisionless dark matter shocks in galaxy cluster mergers which might have been already observed in the Abell 3827 and 520 clusters. As a concrete model we propose a thermally produced dark pair plasma of vector-like fermions. In this scenario the interacting dark matter component is expected to be separated from the stars and the non-interacting dark matter halos in cluster collisions. In addition, the missing satellite problem is softened, while constraints from all other astrophysical and cosmological observations are avoided.

The rise and fall of a challenger: the Bullet Cluster in Λ cold dark matter simulations

The Bullet Cluster has provided some of the best evidence for the $\Lambda$ cold dark matter ($\Lambda\rm{CDM}$) model via direct empirical proof of the existence of collisionless dark matter, while posing a serious challenge owing to the unusually high inferred pairwise velocities of its progenitor clusters. Here we investigate the probability of finding such a high-velocity pair in large-volume N-body simulations, particularly focusing on differences between halo finding algorithms. We find that algorithms that do not account for the kinematics of infalling groups yield vastly different statistics and probabilities. When employing the ROCKSTAR halo finder that considers particle velocities, we find numerous Bullet-like pair candidates that closely match not only the high pairwise velocity, but also the mass, mass ratio, separation distance, and collision angle of the initial conditions that have been shown to produce the Bullet Cluster in non-cosmological hydrodynamic simulations. The probability of finding a high pairwise velocity pair among haloes with $M_{\rm halo}\geq10^{14} M_{\odot}$ is $4.6\times 10^{-4}$ using ROCKSTAR, while it is $\approx 34\times$ lower using a friends-of-friends (FOF) based approach as in previous studies. This is because the typical spatial extent of Bullet progenitors is such that FOF tends to group them into a single halo despite clearly distinct kinematics. Further requiring an appropriately high average mass among the two progenitors, we find the comoving number density of potential Bullet-like candidates to be on the order of $\approx10^{-10} \,{\rm Mpc}^{-3}$. Our findings suggest that $\Lambda\rm{CDM}$ straightforwardly produces massive, high relative velocity halo pairs analogous to Bullet Cluster progenitors, and hence the Bullet Cluster does not present a challenge to the $\Lambda\rm{CDM}$ model.

MASS ACCRETION AND ITS EFFECTS ON THE SELF-SIMILARITY OF GAS PROFILES IN THE OUTSKIRTS OF GALAXY CLUSTERS

Galaxy clusters exhibit remarkable self-similar behavior which allows us to establish simple scaling relationships between observable quantities and cluster masses, making galaxy clusters useful cosmological probes. Recent X-ray observations suggest that self-similarity may be broken in the outskirts of galaxy clusters. In this work, we analyze a mass-limited sample of massive galaxy clusters from the Omega500 cosmological hydrodynamic simulation to investigate the self-similarity of the diffuse X-ray emitting intracluster medium (ICM) in the outskirts of galaxy clusters. We find that the self-similarity of the outer ICM profiles is better preserved if they are normalized with respect to the mean density of the universe, while the inner profiles are more self-similar when normalized using the critical density. However, the outer ICM profiles as well as the location of accretion shock around clusters are sensitive to their mass accretion rate, which causes the apparent breaking of self-similarity in cluster outskirts. We also find that the collisional gas does not follow the distribution of collisionless dark matter perfectly in the infall regions of galaxy clusters, leading to 10% departures in the gas-to-dark matter density ratio from the cosmic mean value. Our results have a number implications for interpreting observations of galaxy clusters in X-ray and through the Sunyaev-Zel'dovich effect and their application to cluster cosmology.

Dark matter–radiation interactions: the impact on dark matter haloes

Interactions between dark matter (DM) and radiation (photons or neutrinos) in the early Universe suppress density fluctuations on small mass scales. Here we perform a thorough analysis of structure formation in the fully non-linear regime using N-body simulations for models with DM-radiation interactions and compare the results to a traditional calculation in which DM only interacts gravitationally. Significant differences arise due to the presence of interactions, in terms of the number of low-mass DM haloes and their properties, such as their spin and density profile. These differences are clearly seen even for haloes more massive than the scale on which density fluctuations are suppressed. We also show that semi-analytical descriptions of the matter distribution in the non-linear regime fail to reproduce our numerical results, emphasizing the challenge of predicting structure formation in models with physics beyond collisionless DM.

Ram pressure stripping of hot coronal gas from group and cluster galaxies and the detectability of surviving X-ray coronae

Ram pressure stripping can remove hot and cold gas from galaxies in the intracluster medium (ICM), as shown by observations of X-ray and HI galaxy wakes in nearby clusters of galaxies. However, ram pressure stripping, including pre-processing in group environments, does not remove all the hot coronal gas from cluster galaxies. Recent high-resolution Chandra observations have shown that $\sim 1 - 4$ kpc extended, hot galactic coronae are ubiquitous in group and cluster galaxies. To better understand this result, we simulate ram pressure stripping of a cosmologically motivated population of galaxies in isolated group and cluster environments. The galaxies and the host group and cluster are composed of collisionless dark matter and hot gas initially in hydrostatic equilibrium with the galaxy and host potentials. We show that the rate at which gas is lost depends on the galactic and host halo mass. Using synthetic X-ray observations, we evaluate the detectability of stripped galactic coronae in real observations by stacking images on the known galaxy centers. We find that coronal emission should be detected within $\sim 10$ arcsec, or $\sim 5$ kpc up to $\sim 2.3$ Gyr in the lowest (0.1 - 1.2 keV) energy band. Thus the presence of observed coronae in cluster galaxies significantly smaller than the hot X-ray halos of field galaxies indicates that at least some gas removal occurs within cluster environments for recently accreted galaxies. Finally, we evaluate the possibility that existing and future X-ray cluster catalogs can be used in combination with optical galaxy positions to detect galactic coronal emission via stacking analysis. We briefly discuss the effects of additional physical processes on coronal survival, and will address them in detail in future papers in this series.

Dynamics of One-dimensional Self-gravitating Systems Using Hermite-Legendre Polynomials

The current paradigm for understanding galaxy formation in the universe de- pends on the existence of self-gravitating collisionless dark matter. Modeling such dark matter systems has been a major focus of astrophysicists, with much of that effort directed at computational techniques. Not surprisingly, a comprehensive understanding of the evolution of these self-gravitating systems still eludes us, since it involves the collective nonlinear dynamics of many-particle systems inter- acting via long-range forces described by the Vlasov equation. As a step towards developing a clearer picture of collisionless self-gravitating relaxation, we analyze the linearized dynamics of isolated one-dimensional systems near thermal equi- librium by expanding their phase space distribution functions f(x,v) in terms of Hermite functions in the velocity variable, and Legendre functions involving the position variable. This approach produces a picture of phase-space evolution in terms of expansion coefficients, rather than spatial and velocity variables. We obtain equations of motion for the expansion coefficients for both test-particle dis- tributions and self-gravitating linear perturbations of thermal equilibrium. This development presents the opportunity to avoid time-consuming N-body simula- tions that are limited by statistical uncertainty and provides a powerful analysis tool for understanding the relaxation to equilibrium.Â

Schrödinger method asN-body double and UV completion of dust

We investigate large-scale structure formation of collisionless dark matter in the phase space description based on the Vlasov equation whose nonlinearity is induced solely by gravitational interaction according to the Poisson equation. Determining the time-evolution of density and peculiar velocity demands solving the full Vlasov hierarchy for the moments of the phase space distribution function. In the presence of long-range interaction no consistent truncation of the hierarchy is known apart from the pressure- less fluid (dust) model which is incapable of describing virialization due to the occurrence of shell-crossing singularities and the inability to generate vorticity and higher cumulants like velocity dispersion. Our goal is to find a simple ansatz for the phase space distribution function that approximates the full Vlasov distribution function without pathologies in a controlled way and therefore can serve as theoretical N-body double and as a replacement for the dust model. We argue that the coarse-grained Wigner probability distribution obtained from a wave function fulfilling the Schr\"odinger-Poisson equation (SPE) is the sought-after function. We show that its evolution equation approximates the Vlasov equation and therefore also the dust equations before shell-crossing, but cures the shell-crossing singularities and is able to describe regions of multi-streaming and virialization. This feature was already employed in simulations by Widrow & Kaiser (1993). The coarse-grained Wigner ansatz allows to calculate all higher moments from density and velocity analytically, thereby incorporating nonzero higher cumulants in a self-consistent manner. On this basis we are able to show that the Schr\"odinger method (ScM) automatically closes the corresponding hierarchy such that it suffices to solve the SPE in order to directly determine density and velocity and all higher cumulants.

Dark matter–baryons separation at the lowest mass scale: the Bullet Group★

Abstract We report on the X-ray observation of a strong lensing selected group, SL2S J08544-0121, with a total mass of 2.4 ± 0.6 × 1014 M⊙ which revealed a separation of 124 ± 20 kpc between the X-ray emitting collisional gas and the collisionless galaxies and dark matter (DM), traced by strong lensing. This source allows to put an order of magnitude estimate to the upper limit to the interaction cross-section of DM of 10 cm2 g−1. It is the lowest mass object found to date showing a DM–baryons separation, and it reveals that the detection of bullet-like objects is not rare and confined to mergers of massive objects opening the possibility of a statistical detection of DM–baryons separation with future surveys.

Beyond single-stream with the Schrödinger method

AbstractWe investigate large scale structure formation of collisionless dark matter in the phase space description based on the Vlasov-Poisson equation. We present the Schrödinger method, originally proposed by \cite{WK93} as numerical technique based on the Schrödinger Poisson equation, as an analytical tool which is superior to the common standard pressureless fluid model. Whereas the dust model fails and develops singularities at shell crossing the Schrödinger method encompasses multi-streaming and even virialization.

Concepts of Dark Energy and Dark Matter: The Understanding and Calculation of 'Dark Energy and Dark Matter'

Mounting observational data collected during the last decades have provided growing evidence that the vast majority of the universe comprises two “dark” components, a mysterious, collisionless dark matter that holds galaxies together and even more mysterious, almost uniform dark energy component with negative isotropic pressure that produces cosmic acceleration. Together, these components capture around 96% of the present day cosmic energy budget. Dark energy is responsible for around 74% while dark matter captures almost a quarter, making it six times more abundant than visible matter. The visible matter is a very minor component in the universe since it captures around 4% of the universe. So far, dark matter and dark energy are inferred from their cosmic dynamic effects. Efforts to explain their makeup and interaction physics, however, have not been fruitful. This research also explorers the opinions and views of different professionals concerning the topic under investigation in order to identify their thoughts concerning dark energy and dark matter. The data gathered from secondary sources plus the thoughts of the various professionals will help in the design of the core fundamentals that should govern the design of a theory that would satisfactory explain the concepts of dark energy. The cosmological principle explains that the universe is highly isotropic and homogenous at super-galactic scales.

The large-scale distribution of cool gas around luminous red galaxies

We present a measurement of the correlation function between luminous red galaxies and cool gas traced by Mg II \lambda \lambda 2796, 2803 absorption, on scales ranging from about 30 kpc to 20 Mpc. The measurement is based on cross-correlating the positions of about one million red galaxies at z~0.5 and the flux decrements induced in the spectra of about 10^5 background quasars from the Sloan Digital Sky Survey. We find that: (i) This galaxy-gas correlation reveals a change of slope on scales of about 1 Mpc, consistent with the expected transition from a dark matter halo dominated environment to a regime where clustering is dominated by halo-halo correlations. Assuming that, on average, the distribution of Mg II gas follows that of dark matter up to a gas-to-mass ratio, we find the standard halo model to provide an accurate description of the gas distribution over three orders of magnitude in scale. Within this framework we estimate the average host halo mass of luminous red galaxies to be about 10^{13.5} M_solar, in agreement with other methods. We also find the Mg II gas-to-mass ratio around LRGs to be consistent with the cosmic value estimated on Mpc scales. Combining our galaxy-gas correlation and the galaxy-mass correlation function from galaxy-galaxy lensing analyses we can directly measure the Mg II gas-to-mass ratio as a function of scale and reach the same conclusion. (ii) From line-width estimates, we show that the velocity dispersion of the gas clouds also shows the expected 1- and 2-halo behaviors. On large scales the gas distribution follows the Hubble flow, whereas on small scales we observe the velocity dispersion of the Mg II gas clouds to be lower than that of collisionless dark matter particles within their host halo. This is in line with the fact that cool clouds are subject to the pressure of the virialized hot gas.

Dynamics of one-dimensional self-gravitating systems using Hermite–Legendre polynomials

The current paradigm for understanding galaxy formation in the universe depends on the existence of self-gravitating collisionless dark matter. Modeling such dark matter systems has been a major focus of astrophysicists, with much of that effort directed at computational techniques. Not surprisingly, a comprehensive understanding of the evolution of these self-gravitating systems still eludes us, since it involves the collective nonlinear dynamics of many-particle systems interacting via long-range forces described by the Vlasov equation. As a step towards developing a clearer picture of collisionless self-gravitating relaxation, we analyze the linearized dynamics of isolated one-dimensional systems near thermal equilibrium by expanding their phase space distribution functions f(x,v) in terms of Hermite functions in the velocity variable, and Legendre functions involving the position variable. This approach produces a picture of phase-space evolution in terms of expansion coefficients, rather than spatial and velocity variables. We obtain equations of motion for the expansion coefficients for both test-particle distributions and self-gravitating linear perturbations of thermal equilibrium. N-body simulations of perturbed equilibria are performed and found to be in excellent agreement with the expansion coefficient approach over a time duration that depends on the size of the expansion series used.