Dark Matter Dynamics Research Articles

Extended ΛCDM: generalized non-minimal coupling for dark matter fluids

In this paper we discuss a class of models that address the issue of explaining the gravitational dynamics at the galactic scale starting from a geometric point of view. Instead of claiming the existence of some hidden coupling between dark matter and baryons, or abandoning the existence of dark matter itself, we consider the possibility that dark matter and gravity have some non trivial interaction able to modify the dynamics at astrophysical scales. This interaction is implemented assuming that dark matter gets non-minimally coupled with gravity at suitably small scales and late times. After showing the predictions of the model in the Newtonian limit we also discuss the possible origin of it non-minimal coupling. This investigation seems to suggest that phenomenological mechanisms envisaged for the dark matter dynamics, such as the Bose-Einstein condensation of dark matter halos, could be connected to this class of models.

Dynamics and constraints of the massive graviton dark matter flat cosmologies

We discuss the dynamics of the Universe within the framework of the massive graviton cold dark matter scenario (MGCDM) in which gravitons are geometrically treated as massive particles. In this modified gravity theory, the main effect of the gravitons is to alter the density evolution of the cold dark matter component in such a way that the Universe evolves to an accelerating expanding regime, as presently observed. Tight constraints on the main cosmological parameters of the MGCDM model are derived by performing a joint likelihood analysis involving the recent supernovae type Ia data, the cosmic microwave background shift parameter, and the baryonic acoustic oscillations as traced by the Sloan Digital Sky Survey red luminous galaxies. The linear evolution of small density fluctuations is also analyzed in detail. It is found that the growth factor of the MGCDM model is slightly different ($\ensuremath{\sim}1--4%$) from the one provided by the conventional flat $\ensuremath{\Lambda}\mathrm{CDM}$ cosmology. The growth rate of clustering predicted by MGCDM and $\ensuremath{\Lambda}\mathrm{CDM}$ models are confronted to the observations and the corresponding best fit values of the growth index ($\ensuremath{\gamma}$) are also determined. By using the expectations of realistic future x-ray and Sunyaev-Zeldovich cluster surveys we derive the dark matter halo mass function and the corresponding redshift distribution of cluster-size halos for the MGCDM model. Finally, we also show that the Hubble flow differences between the MGCDM and the $\ensuremath{\Lambda}\mathrm{CDM}$ models provide a halo redshift distribution departing significantly from the those predicted by other dark energy models. These results suggest that the MGCDM model can observationally be distinguished from $\ensuremath{\Lambda}\mathrm{CDM}$ and also from a large number of dark energy models recently proposed in the literature.

QUANTIFYING THE COLLISIONLESS NATURE OF DARK MATTER AND GALAXIES IN A1689

We use extensive measurements of the cluster A1689 to assess the expected similarity in the dynamics of galaxies and dark matter (DM) in their motion as collisionless `particles' in the cluster gravitational potential. To do so we derive the radial profile of the specific kinetic energy of the cluster galaxies from the Jeans equation and observational data. Assuming that the specific kinetic energies of galaxies and DM are roughly equal, we obtain the mean value of the DM velocity anisotropy parameter, and the DM density profile. Since this deduced profile has a scale radius that is higher than inferred from lensing observations, we tested the validity of the assumption by repeating the analysis using results of simulations for the profile of the DM velocity anisotropy. Results of both analyses indicate a significant difference between the kinematics of galaxies and DM within $r \lesssim 0.3r_{\rm vir}$. This finding is reflected also in the shape of the galaxy number density profile, which flattens markedly with respect to the steadily rising DM profile at small radii. Thus, $r \sim 0.3r_{\rm vir}$ seems to be a transition region interior to which collisional effects significantly modify the dynamical properties of the galaxy population with respect to those of DM in A1689

REIONIZATION SIMULATIONS POWERED BY GRAPHICS PROCESSING UNITS. I. ON THE STRUCTURE OF THE ULTRAVIOLET RADIATION FIELD

We present a set of cosmological simulations with radiative transfer in order to model the reionization history of the universe from z = 18 down to z = 6. Galaxy formation and the associated star formation are followed self-consistently with gas and dark matter dynamics using the RAMSES code, while radiative transfer is performed as a post-processing step using a moment-based method with the M1 closure relation in the ATON code. The latter has been ported to a multiple Graphics Processing Unit (GPU) architecture using the CUDA language together with the MPI library, resulting in an overall acceleration that allows us to tackle radiative transfer problems at a significantly higher resolution than previously reported: 10243 + 2 levels of refinement for the hydrodynamic adaptive grid and 10243 for the radiative transfer Cartesian grid. We reach a typical acceleration factor close to 100× when compared to the CPU version, allowing us to perform 1/4 million time steps in less than 3000 GPU hr. We observe good convergence properties between our different resolution runs for various volume- and mass-averaged quantities such as neutral fraction, UV background, and Thomson optical depth, as long as the effects of finite resolution on the star formation history are properly taken into account. We also show that the neutral fraction depends on the total mass density, in a way close to the predictions of photoionization equilibrium, as long as the effect of self-shielding are included in the background radiation model. Although our simulation suite has reached unprecedented mass and spatial resolution, we still fail in reproducing the z ∼ 6 constraints on the neutral fraction of hydrogen and the intensity of the UV background. In order to account for unresolved density fluctuations, we have modified our chemistry solver with a simple clumping factor model. Using our most spatially resolved simulation (12.5 Mpc h−1 with 10243 particles) to calibrate our subgrid model, we have resimulated our largest box (100 Mpc h−1 with 10243 particles) with the modified chemistry, successfully reproducing the observed level of neutral hydrogen in the spectra of high-redshift quasars. We however did not reproduce the average photoionization rate inferred from the same observations. We argue that this discrepancy could be partly explained by the fact that the average radiation intensity and the average neutral fraction depend on different regions of the gas density distribution, so that one quantity cannot be simply deduced from the other.

Semi-annihilation of dark matter

We show that the thermal relic abundance of dark matter can be affected by a new type of reaction: semi-annihilation. Semi-annihilation takes the schematic form X_i X_j -> X_k phi, where X_i are stable dark matter particles and phi is an unstable state. Such reactions are generically present when dark matter is composed of more than one species with "flavor" and/or "baryon" symmetries. We give a complete set of coupled Boltzmann equations in the presence of semi-annihilations, and study two toy models featuring this process. Semi-annihilation leads to non-trivial dark matter dynamics in the early universe, often dominating over ordinary annihilation in determining the relic abundance. This process also has important implications for indirect detection experiments, by enriching the final state spectrum from dark matter (semi-)annihilation in the Milky Way.

Disentangling dark matter dynamics with directional detection

Inelastic dark matter reconciles the DAMA anomaly with other null direct detection experiments and points to a non-minimal structure in the dark matter sector. In addition to the dominant inelastic interaction, dark matter scattering may have a subdominant elastic component. If these elastic interactions are suppressed at low momentum transfer, they will have similar nuclear recoil spectra to inelastic scattering events. While upcoming direct detection experiments will see strong signals from such models, they may not be able to unambiguously determine the presence of the subdominant elastic scattering from the recoil spectra alone. We show that directional detection experiments can separate elastic and inelastic scattering events and discover the underlying dynamics of dark matter models.

Lyα versus X-ray heating in the high-zintergalactic medium

In this paper we examine the effect of X-ray and Lyalpha photons on the intergalactic medium temperature. We calculate the photon production from a population of stars and micro-quasars in a set of cosmological hydrodynamic simulations which self-consistently follow the dark matter dynamics, radiative processes as well as star formation, black hole growth and associated feedback processes. We find that, (i) IGM heating is always dominated by X-rays unless the Lyalpha photon contribution from stars in objects with mass M<10^8 Msun becomes significantly enhanced with respect to the X-ray contribution from BHs in the same halo (which we do not directly model). (ii) Without overproducing the unresolved X-ray background, the gas temperature becomes larger than the CMB temperature, and thus an associated 21 cm signal should be expected in emission, at z<11.5. We discuss how in such a scenario the transition redshift between a 21 cm signal in absorption and in emission could be used to constraint BHs accretion and associated feedback processes.

Dynamics of scalar field dark matter with a cosh-like potential

The dynamics of a cosmological model of dark matter and dark energy represented by a scalar field endowed with a cosh-like potential plus a cosmological constant is investigated in detail. By studying the appropriate phase space of the equations of motion, it is shown that a standard evolution of the Universe is recovered for appropriate values of the free parameters, and that the only late-time attractor is always the de Sitter solution. We also discuss the appearance of scalar field oscillations corresponding to dark matter behavior.

Dark Matter and Elliptical Galaxy Dynamics

The halos of elliptical galaxies, through their orbit and angular momentum distribution, contain important information about the formation and evolution of these systems.

Measurement of the dark matter velocity anisotropy profile in galaxy clusters

Dark matter halos contribute the major part of the mass of galaxy clusters and the formation of these cosmological structures have been investigated in numerical simulations. Observations have been found to be in good agreement with the numerical predictions regarding the spatial distribution of dark matter, i.e. the mass profile. However, the dynamics of dark matter in halos has so far proved a greater challenge to probe observationally. We have used observations of 16 relaxed galaxy clusters to show that the dark matter velocity dispersion is larger along the radial direction than along the tangential, and that the magnitude of this velocity anisotropy β varies with radius. This measurement implies that the collective behaviour of dark matter particles is fundamentally different from that of baryonic particles and constrains the self-interaction per unit mass. The radial variation of the anisotropy velocity agrees with the predictions so that, on cluster scales, there is now excellent agreement between numerical predictions and observations regarding the phase space of dark matter.

Formation of globular clusters induced by external ultraviolet radiation

We present a novel scenario for globular cluster (GC) formation, where the ultraviolet (UV) background radiation effectively works so as to produce compact star clusters. Here, we explore the formation of GCs in UV radiation fields. For this purpose, we calculate baryon and dark matter (DM) dynamics in spherical symmetry, incorporating the self-shielding effects by solving the radiative transfer of UV radiation. In addition, we prescribe the star formation in cooled gas components and pursue the dynamics of formed stars. As a result, we find that the evolution of subgalactic objects in UV background radiation are separated into three types, that is, (1) prompt star formation, where less massive clouds ~10^{5-8} M_sun are promptly self-shielded and undergo star formation, (2) delayed star formation, where photoionized massive clouds >10^8 M_sun collapse despite high thermal pressure and are eventually self-shielded to form stars in a delayed fashion, and (3) supersonic infall, where photoionized less massive clouds ~10^{5-8} M_sun contract with supersonic infall velocity and are self-shielded when a compact core forms. In particular, the type (3) is a novel type found in the present simulations, and eventually produces a very compact star cluster. The resultant mass-to-light ratios, half-mass radii, and velocity dispersions for the three types are compared to the observations of GCs, dwarf spheroidals (dSphs), and ultra-compact dwarfs (UCDs). It turns out that the properties of star clusters resulting from supersonic infall match well with those of observed GCs, whereas the other two types are distinct from GCs. Hence, we conclude that supersonic infall in a UV background is a promising mechanism to form GCs.

Probing dark matter dynamics via earthborn neutrinos at IceCube

Recent results from PAMELA and ATIC hint that O(TeV) dark matter (DM) is annihilating, in our galactic neighborhood, predominantly to leptons. The annihilation rate is much larger now than during freeze-out, one possible explanation of this is a low-velocity enhancement of the annihilation cross section. In a model independent fashion, we show that in this case the rate of neutrino emission from the Earth, due to DM annihilation, may be greatly enhanced while the rate from the Sun is unaltered. There is potential for IceCube to see these earthborn neutrinos while the same parameter space will be soon covered by direct detection experiments. Combining these near-future data will allow extraction of valuable information about the DM sector dynamics.

Dark matter and dark radiation

We explore the feasibility and astrophysical consequences of a new long-range $U(1)$ gauge field (``dark electromagnetism'') that couples only to dark matter, not to the standard model. The dark matter consists of an equal number of positive and negative charges under the new force, but annihilations are suppressed if the dark-matter mass is sufficiently high and the dark fine-structure constant $\stackrel{^}{\ensuremath{\alpha}}$ is sufficiently small. The correct relic abundance can be obtained if the dark matter also couples to the conventional weak interactions, and we verify that this is consistent with particle-physics constraints. The primary limit on $\stackrel{^}{\ensuremath{\alpha}}$ comes from the demand that the dark matter be effectively collisionless in galactic dynamics, which implies $\stackrel{^}{\ensuremath{\alpha}}\ensuremath{\lesssim}{10}^{\ensuremath{-}3}$ for TeV-scale dark matter. These values are easily compatible with constraints from structure formation and primordial nucleosynthesis. We raise the prospect of interesting new plasma effects in dark-matter dynamics, which remain to be explored.

EQUAL- AND UNEQUAL-MASS MERGERS OF DISK AND ELLIPTICAL GALAXIES WITH BLACK HOLES

We present binary galaxy merger simulations with varying mass ratios and different progenitor morphologies. The simulations include mergers of gas-rich disks (Sp-Sp), of early-type galaxies and disks (E-Sp, mixed mergers), and mergers of early-type galaxies (E-E, dry mergers). We follow the dynamics of gas, stars, and dark matter, and include radiative cooling, star formation and black hole (BH) accretion, and the associated feedback processes as in Springel et al. We study the mass assembly of the BHs and discuss technical issues of the implemented model in detail. For Sp-Sp mergers, the peak star formation rate and BH accretion rate decrease and the growth timescales of the central BHs and newly formed stars increase with higher progenitor mass ratios. The peak BH accretion rate typically occurs shortly after the time of BH merging for low progenitor mass ratios (e.g., 3:1 and lower), whereas for higher progenitor mass ratios there is no clear correlation between the peak BH accretion rate and the BH merging time. The termination of star formation by BH feedback in disk mergers is significantly less important for higher progenitor mass ratios (e.g., 3:1 and higher). In addition, the inclusion of BH feedback efficiently suppresses star formation in dry E-E mergers and mixed E-Sp mergers. All merger remnants, independent of their progenitors, follow the observed relations between the central BH mass and the stellar velocity dispersion (MBH − σ), the bulge mass (MBH − M*), and the bulge binding energy (MBH − M*σ2), with the dominant source of scatter arising from variations in the initial gas mass fraction. The normalizations for all relations and the simulated slope of the MBH − σ and MBH − M*σ2 relations are in good agreement with the observations, whereas the simulated slope of the MBH − M* relation is slightly steeper compared to the observations. This indicates that the simple BH feedback model self-regulates the mass growth of the BHs in accordance with the observed relations for a very wide range of merger progenitors.

Dark matter dynamics in the galactic center

The evolution of dark matter in central areas of galaxies is considered (the Milky Way is taken as an example). It is driven by the scattering off of dark matter particles by bulge stars, their absorption by the supermassive black hole, and self-annihilation. This process is described by the diffusion equation in the phase space of energy and angular momentum. The equation was integrated for several different models of initial dark matter distribution and using various assumptions about the dynamical factors. It turns out that because the Milky Way center is rather dynamically old ({approx}4 relaxation times t{sub r}), the difference in initial conditions almost vanishes. The density attains a nearly universal profile, and the {gamma}-ray flux from dark matter annihilation lies in a rather narrow range, which enables a more robust determination of the dark matter parameters. By presenting the mass of dark matter inside the black hole, the sphere of influence (r or approx. t{sub r} is determined mainly by stars outside the sphere of influence.

Baryon Dynamics, Dark Matter Substructure, and Galaxies

By comparing a collisionless cosmological N-body simulation (DM) to an SPH simulation with the same initial conditions, we investigate the correspondence between the dark matter subhalos produced by collisionless dynamics and the galaxies produced by dissipative gas dynamics in a dark matter background. When galaxies in the SPH simulation become satellites in larger groups, they retain local dark matter concentrations (SPH subhalos) whose mass is typically five times their baryonic mass. The more massive subhalos of the SPH simulation have corresponding subhalos of similar mass and position in the DM simulation; at lower masses, there is fairly good correspondence, but some DM subhalos are in different spatial positions and some suffer tidal stripping or disruption. The halo occupation statistics of DM subhalos -- the mean number of subhalos, pairs, and triples as a function of host halo mass -- are very similar to those of SPH subhalos and SPH galaxies. Gravity of the dissipative baryon component amplifies the density contrast of subhalos in the SPH simulation, making them more resistant to tidal disruption. Relative to SPH galaxies and SPH subhalos, the DM subhalo population is depleted in the densest regions of the most massive halos. The good agreement of halo occupation statistics between the DM subhalo and SPH galaxy populations leads to good agreement of their two-point correlation functions and higher order moments on large scales. The depletion of DM subhalos in dense regions depresses their clustering at R<1 Mpc/h. In these simulations, the "conversation" between dark matter and baryons is mostly one-way, with dark matter dynamics telling galaxies where to form and how to cluster, but the "back talk" of the baryons influences small scale clustering by enhancing the survival of substructure in the densest environments.

Direct Cosmological Simulations of the Growth of Black Holes and Galaxies

We investigate the coupled formation and evolution of galaxies and their embedded supermassive black holes using state-of-the-art hydrodynamic simulations of cosmological structure formation. For the first time, we self-consistently follow the dark matter dynamics, radiative gas cooling, and star formation, as well as BH growth and associated feedback processes, starting directly from initial conditions appropriate for the ΛCDM cosmology. Our modeling of the black hole physics is based on an approach that we have developed in simulations of isolated galaxy mergers. Here we examine (1) the predicted global history of BH mass assembly, (2) the evolution of the local black hole-host mass correlations, and (3) the conditions that allow rapid growth of the first quasars, and the properties of their hosts and descendants today. We find a total BH mass density in good agreement with observational estimates. The BH accretion rate density peaks at lower redshift and evolves more strongly at high redshift than the star formation rate density, but the ratio of black hole to stellar mass density shows only a moderate evolution at low redshifts. We find strong correlations between BH masses and properties of the stellar systems, agreeing well with the measured local MBH-σ and MBH-M* relationships, but also suggesting (dependent on the mass range) a weak evolution with redshift in the normalization and the slope. Our simulations also produce massive black holes at high redshift, due to extended periods of exponential growth in regions that collapse early and exhibit strong gas inflows. These first supermassive BH systems, however, are not necessarily the most massive ones today, since they are often overtaken in growth by quasars that form later.

Cosmological MHD simulation of a cooling flow cluster

Context. Various observations of magnetic fields in the intra-cluster medium (ICM), most of the time restricted to cluster cores, point towards a field strength of a few µG (synchrotron radiation from radio relics and radio halos, inverse Compton radiation in X-rays and Faraday rotation measure of polarised background sources). Both the origin and the spatial structure of galaxy cluster magnetic fields are still under debate. In particular, the radial profile of the magnetic field, from the core of clusters to their outskirts, is important for cosmic ray propagation within the cosmic web. Aims. In this letter, we highlight the importance of cooling processes in amplifying the magnetic field in the core of galaxy clusters up to one order of magnitude above the typical amplification obtained for a purely adiabatic evolution. Methods. We performed a “zoom” cosmological simulation of a 3 keV cluster, including dark matter and gas dynamics, atomic cooling, UV heating, and star formation using the newly developed MHD solver in the AMR code RAMSES. Results. Magnetic field amplification proceeds mainly through gravitational contraction. Shearing motions due to turbulence provide additional amplification in the outskirts of the cluster, while magnetic reconnection during mergers causes magnetic field dissipation in the core. Conclusions. Cooling processes have a strong impact on the magnetic field structure in the cluster. First, due to the sharp rise of the gas density in the centre, gravitational amplification is significantly higher, when compared to the non-radiative run. Second, cooling processes cause shearing motions to be much stronger in the core than in the adiabatic case, leading to additional field amplification and no significant magnetic reconnection. Cooling processes are therefore essential for determining the magnetic field profile in galaxy clusters.

White noise from dark matter: 21 cm observations of early baryon collapse

In concordance cosmology, dark matter density perturbations generated by inflation lead to nonlinear, virialized minihalos, into which baryons collapse at redshift $z\ensuremath{\sim}20$. We survey here novel baryon evolution produced by a modification of the power spectrum from white noise density perturbations at scales below $k\ensuremath{\sim}10h\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$ (the smallest scales currently measured with the Lyman-$\ensuremath{\alpha}$ forest). Exotic dark matter dynamics, such as would arise from scalar dark matter with a late phase transition (similar to an axion, but with lower mass), or primordial black hole dark matter, create such an amplification of small scale power. The dark matter produced in such a phase transition collapses into minihalos, with a size given by the dark matter mass within the horizon at the phase transition. If the mass of the initial minihalos is larger than $\ensuremath{\sim}{10}^{\ensuremath{-}3}{M}_{\ensuremath{\bigodot}}$, the modified power spectrum is found to cause widespread baryon collapse earlier than standard $\ensuremath{\Lambda}\mathrm{CDM}$, leading to earlier gas heating. It also results in higher spin temperature of the baryons in the 21 cm line relative to $\ensuremath{\Lambda}\mathrm{CDM}$ at redshifts $zg20$ if the mass of the minihalo is larger than $1{M}_{\ensuremath{\bigodot}}$. It is estimated that experiments probing 21 cm radiation at high redshift will contribute a significant constraint on dark matter models of this type for initial minihalos larger than $\ensuremath{\sim}10{M}_{\ensuremath{\bigodot}}$. These experiments may also detect (or rule out) primordial black holes as the dark matter in the window $30{M}_{\ensuremath{\bigodot}}\ensuremath{\lesssim}{M}_{H}\ensuremath{\lesssim}4\ifmmode\times\else\texttimes\fi{}{10}^{3}{M}_{\ensuremath{\bigodot}}$ still left open by strong microlensing experiments and other astrophysical constraints. Early experiments reaching to $z\ensuremath{\approx}15$ will constrain minihalos down to $\ensuremath{\sim}{10}^{3}{M}_{\ensuremath{\bigodot}}$.

Systematic Uncertainties in Stellar Mass Estimation for Distinct Galaxy Populations

We show that different stellar-mass estimation methods yield overall mass scales that disagree by factors up to ~2 for the z=0 galaxy population, and more importantly, relative mass scales that sometimes disagree by factors >~3 between distinct classes of galaxies (spiral/irregular types, classical E/S0s, and E/S0s whose colors reflect recent star formation). This comparison considers stellar mass estimates based on (a) two different calibrations of the correlation between K-band mass-to-light ratio and B-R color (Bell et al., Portinari et al.) and (b) detailed fitting of UBRJHK photometry and optical spectrophotometry using two different population synthesis models (Bruzual-Charlot, Maraston), with the same initial mass function in all cases. We also compare stellar+gas masses with dynamical masses. This analysis offers only weak arguments for preferring a particular stellar-mass estimation method, given the plausibility of real variations in dynamical properties and dark matter content. These results help to calibrate the systematic uncertainties inherent in mass-based evolutionary studies of galaxies, including comparisons of low and high redshift galaxies.