Tidal disruption of globular clusters in dwarf galaxies with triaxial dark matter haloes

We use N -body simulations to study the tidal evolution of globular clusters (GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically motivated scenario in which the dSph is approximated by a static Navarro, Frenk & White halo with a triaxial shape. We apply our models to five GCs spanning three orders of magnitude in stellar density and two in mass, chosen to represent the properties exhibited by the five GCs of the Fornax dSph. We show that only the object representing Fornax's least dense GC (F1) can be fully disrupted by Fornax's internal tidal field – the four denser clusters survive even if their orbits decay to the centre of Fornax. For a large set of orbits and projection angles, we examine the spatial and velocity distribution of stellar debris deposited during the complete disruption of an F1-like GC. Our simulations show that such debris appears as shells, isolated clumps and elongated overdensities at low surface brightness (≥26 mag arcsec⁻²), reminiscent of substructure observed in several Milky Way dSphs. Such features arise from the triaxiality of the galaxy potential and do not dissolve in time. The kinematics of the debris depends strongly on the progenitor's orbit. Debris associated with box and resonant orbits does not display stream motions and may appear 'colder'/'hotter' than the dSph's field population if the viewing angle is perpendicular/parallel to the progenitor's orbital plane. In contrast, debris associated with loop orbits shows a rotational velocity that may be detectable out to a few kpc from the galaxy centre. Chemical tagging that can distinguish GC debris from field stars may reveal whether the merger of GCs contributed to the formation of multiple stellar components observed in dSphs.     

Marriage à-la-MOND: Baryonic dark matter in galaxy clusters and the cooling flow puzzle

I start with a brief introduction to MOND phenomenology and its possible roots in cosmology—a notion that may turn out to be the most far reaching aspect of MOND. Next I discuss the implications of MOND for the dark matter (DM) doctrine: MOND’s successes imply that baryons determine everything. For DM this would mean that the puny tail of leftover baryons in galaxies wags the hefty DM dog. This has to occur in many intricate ways, and despite the haphazard construction history of galaxies—a very tall order. I then concentrate on galaxy clusters in light of MOND, which still requires some yet undetected cluster dark matter, presumably in some baryonic form (CBDM). This CBDM might contribute to the heating of the X-ray emitting gas and thus alleviate the cooling flow puzzle. MOND, qua theory of dynamics, does not directly enter the microphysics of the gas; however, it does force a new outlook on the role of DM in shaping the cluster gas dynamics: MOND tells us that the cluster DM is not cold dark matter, is not so abundant, and is not expected in galaxies; it is thus not subject to constraints on baryonic DM in galaxies. The mass in CBDM required in a whole cluster is, typically, similar to that in hot gas, but is rather more centrally concentrated, totally dominating the core. The CBDM contribution to the baryon budget in the universe is thus small. Its properties, deduced for isolated clusters, are consistent with the observations of the “bullet cluster”. Its kinetic energy reservoir is much larger than that of the hot gas in the core, and would suffice to keep the gas hot for many cooling times. Heating can be effected in various ways depending on the exact nature of the CBDM, from very massive black holes to cool, compact gas clouds.     

Dark matter influence on velocity dispersion profiles of clusters of galaxies

We have performed a series ofN-body experiments including the effects of a massive dominant background which follows Schuster's density law in order to simulate clusters of galaxies in which a smoothly distributed dark component is present. The existence of this background is inferred from the weak luminosity segregation observed in clusters which, however, show several characteristics of well-relaxed systems. The comparison of the velocity dispersion profiles of three clusters of galaxies (Coma, Perseus, and Virgo) with those obtained in the numerical experiments allows us to place some constraints on both the distribution and amount of distributed dark material in these clusters. The profiles are rather insensitive to variations in the ratio of the background mass to the mass attached to galaxies (M _b/M_g), but exhibit a strong dependence on their relative concentration. We conclude that highly concentrated background models are not consistent with observations. We find a maximum value for the ratio of the gravitational radius of the galaxies and the background (R _g/R_b) (approximately 0.6) and using previous results (Navarroet al., 1986) we conclude that virial theorem masses underestimate the total mass (M _b+M _g) of the clusters. As a final result, we derive a minimum value for theM _b/M_g ratio. All these conclusions could apply in general if Coma, Perseus, and Virgo constitute a fair sample of the rich clusters of galaxies in the Universe.     

Ionization yield from nuclear recoils in liquid-xenon dark matter detection

The ionization yield in a two-phase liquid xenon dark-matter detector has been studied in keV nuclear recoil energy region. The newly obtained nuclear quenching as well as the average energy required to produce an electron–ion pair from the measurement in Seguinot (1992) are used to calculate the total electric charges produced. To estimate the fraction of the electron charges collected, the Thomas-Imel model is generalized to describe the field dependence for nuclear recoils in liquid xenon. With free parameters fitted to experimentally measured 56.5 keV nuclear recoils, the energy dependence of ionization yield for nuclear recoils is predicted, which increases as recoil energy decreases and reaches the maximum value at 2∼3 keV. This prediction agrees well with existing data and may help to lower the energy detection threshold for nuclear recoils to ∼1 keV.     

Sterile neutrinos as dark matter in clusters and galaxies

The recent analysis of MiniBooNE experiment suggests that a better fit of the data arises if there are 2 types of sterile neutrinos. If the sterile neutrinos were produced during the early epoch of the Big Bang, they would be slightly degenerate. I show that the existence of 2 types slightly degenerate sterile neutrinos can fully explain the dark matter problem, the cusp problem, the hot gas density profile in clusters and the rotation curves of galaxies.     

The dark matter problem in disc galaxies

In the generic CDM cosmogony, dark-matter haloes emerge too lumpy and centrally concentrated to host observed galactic discs. Moreover, discs are predicted to be smaller than those observed. We argue that the resolution of these problems may lie with a combination of the effects of protogalactic discs, which would have had a mass comparable to that of the inner dark halo and be plausibly non-axisymmetric, and of massive galactic winds, which at early times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intracluster gas through the G-dwarf problem, point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disc will be the relic of a massive outflow. The inner halo expanded after absorbing energy and angular momentum from the ejected material. Observed discs formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budget and strong radial streaming of the gas had died down.     

Constraining the dark matter annihilation cross-section with Cherenkov telescope observations of dwarf galaxies

The presence of dark matter in the halo of our Galaxy could be revealed through indirect detection of its annihilation products. Dark matter annihilation is one possible interpretation of the recently measured excesses in positron and electron fluxes, provided that boost factors of the order of 10³ or more are taken into account. Such boost factors are actually achievable through the velocity-dependent Sommerfeld enhancement of the annihilation cross-section. Here, we study the expected γ-ray flux from two local dwarf galaxies for which Cherenkov telescope measurements are available, namely Draco and Sagittarius. We use recent stellar kinematical measurements to model the dark matter haloes of the dwarfs and the results of numerical simulations to model the presence of an associated population of subhaloes. We incorporate the Sommerfeld enhancement of the annihilation cross-section. We compare our predictions with the observations of Draco and Sagittarius performed by MAGIC and HESS, respectively, and derive exclusion limits on the effective annihilation cross-section. We also study the sensitivities of Fermi and of the future Cherenkov telescope array to cross-section enhancements. We find that the boost factor due to the Sommerfeld enhancement is already constrained by the MAGIC and HESS data, with enhancements greater than ∼10⁴ being excluded.     

TeV dark matter in the disk

DAMA and CoGeNT annual modulation data and, CDMS-II, EDELWEISS-II, CRESST excesses of events over the expected background are reanalyzed in terms of a dark matter particle signal considering the case of a rotating halo. It is found that the configurations of very high mass dark matter particles in a corotating cold flux are favored by data. A similar high-mass/low-velocity solution could be of interest in the light of the positron/electron excess measured by PAMELA and Fermi LAT in cosmic rays.     

On the interaction of jet plasma with dark matter subhaloes in active galaxies

The interaction of fragmented plasma of active galactic nuclei jets with galactic haloes via gravitational scattering and lensing by dark matter subhaloes is studied using analytical calculations and numerical Monte-Carlo method. The lensing of jet radiation by halo masses is found to be negligible and unobservable. Moving through a galactic halo jet plasma fragments are sequentially deflected on hyperbolic orbits by gravitational field of subhaloes and deviates at some angles when leaving halo, causing widening of the jet. Based on this model jet opening angles are calculated numerically for various values of jet and halo characteristics. Though these angles are very small, gravitational scattering by halo masses results in specific radial profile of jet radiation intensity, that does not depend on halo mass distribution and jet properties. The intensity of jet radiation, obeying the derived profile, decreases by reasonable observable factors giving possibility to probe the presence of dark matter subhaloes.     

Sommerfeld enhancement of invisible dark matter annihilation in galaxies and galaxy clusters

Recent observations indicate that core-like dark matter structures exist in many galaxies, while numerical simulations reveal a singular dark matter density profile at the center. In this article, I show that if the annihilation of dark matter particles gives invisible sterile neutrinos, the Sommerfeld enhancement of the annihilation cross-section can give a sufficiently large annihilation rate to solve the core-cusp problem. The resultant core density, core radius, and their scaling relation generally agree with recent empirical fits from observations. Also, this model predicts that the resultant core-like structures in dwarf galaxies can be easily observed, but not for large normal galaxies and galaxy clusters.     

Rotation velocity and neutral hydrogen distribution dependency on magnetic field strength in spiral galaxies

The rotation velocity of a simulated plasma galaxy is compared to the rotation curves of Sc type spiral galaxies. Both show flat rotation curves with velocities of the order of several hundred kilometers per second, modified by E × B instabilities. Maps of the strength and distribution of galactic magnetic fields and neutral hydrogen regions, as-well-as as predictions by particle-in-cell simulations run in the late 1970s, are compared to Effelsberg observations.Agreement between simulation and observation is best when the simulation galaxy masses are identical to the observational masses of spiral galaxies. No dark matter is needed.     

The evolution of disk galaxies in clusters

We are carrying out a programme to measure the evolution of the stellar and dynamical masses and M/L ratios for a sizeable sample of morphologically-classified disk galaxies in rich galaxy clusters at 0.2 < z < 0.9. Using FORS2 at the VLT we are obtaining rotation curves for the cluster spirals so that their Tully-Fisher relation can be studied as a function of redshift and compared with that of field spirals. We already have rotation curves for ∼ 10 cluster spirals at z = 0.83, and 25 field spirals at lower redshifts and we plan to increase this sample by one order of magnitude. We present here the first results of our study, and discuss the implications of our data in the context of current ideas and models of galaxy formation and evolution. This revised version was published online in September 2006 with corrections to the Cover Date.     

Gravitational stability of dark energy in galaxies and clusters of galaxies

We analyze the behavior of the scalar field as dark energy of the Universe in a static world of galaxies and clusters of galaxies. We find the analytical solutions of evolution equations of the density and velocity perturbations of dark matter and dark energy, which interact only gravitationally, along with the perturbations of metric in a static world with background Minkowski metric. It was shown that quintessential and phantom dark energy in the static world of galaxies and clusters of galaxies is gravitationally stable and can only oscillate by the influence of self-gravity. In the gravitational field of dark matter perturbations, it is able to condense monotonically, but the amplitude of density and velocity perturbations on all scales remains small. It was also illustrated that the “accretion” of phantom dark energy in the region of dark matter overdensities causes formation of dark energy underdensities-the regions with negative amplitude of density perturbations of dark energy.