Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi dSph) against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 1000 solar masses at a halo density in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses >3000 solar masses improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses $\sim 10^{5}$ solar masses can comprise no more than a halo mass fraction $\sim 0.01$. Better determinations of the velocity dispersion of old overdense regions in dSphs may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed, compact objects with masses above $\sim 100$ solar masses could be excluded from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter

On the Structure of Dark Matter Halos at the Damping Scale of the Power Spectrum with and without Relict Velocities

We report a series of high-resolution cosmological N-body simulations designed to explore the formation and properties of dark matter halos with masses close to the damping scale of the primordial power spectrum of density fluctuations. We further investigate the effect that the addition of a random component, v_rms, into the particle velocity field has on the structure of halos. We adopted as a fiducial model the Lambda Warm Dark Matter cosmology with a non-thermal sterile neutrino mass of 0.5 keV. The filtering mass corresponds then to M_f = 2.6x10^12 M_sun/h. Halos of masses close to M_f were simulated with several million of particles. The results show that, on one hand, the inner density slope of these halos (at radii <~0.02 the virial radius Rvir) is systematically steeper than the one corresponding to the NFW fit or to the CDM counterpart. On the other hand, the overall density profile (radii larger than 0.02Rvir) is less curved and less concentrated than the NFW fit, with an outer slope shallower than -3. For simulations with v_rms, the inner halo density profiles flatten significantly at radii smaller than 2-3 kpc/h (<~0.010-0.015Rvir). A constant density core is not detected in our simulations, with the exception of one halo for which the flat core radius is ~1 kpc/h. Nevertheless, if ``cored'' density profiles are used to fit the halo profiles, the inferred core radii are ~0.1-0.8 kpc/h, in rough agreement with theoretical predictions based on phase-space constrains, and on dynamical models of warm gravitational collapse. A reduction of v_rms by a factor of 3 produces a modest decrease in core radii, less than a factor of 1.5. We discuss the extension of our results into several contexts, for example, to the structure of the cold DM micro-halos at the damping scale of this model.Comment: 13 pages, 6 figures, accepted for publication in The Astrophysical Journa

On the origin of the HI holes in the interstellar medium of dwarf irregular galaxies

We suggest that large HI holes observed in the interstellar medium (ISM) of galaxies such as the Large Magellanic Cloud (LMC) and Holmberg II (Ho II, DDO 50, UGC 4305) can form as the combined result of turbulence coupled to thermal and gravitational instabilities. We investigate this problem with three dimensional hydrodynamical simulations, taking into account cooling and heating processes and the action of the self-gravity of the gas. We construct an algorithm for radiative transfer to post-process the simulated data and build emission maps in the 21 cm neutral hydrogen line. With this approach, we are able to reproduce the structure of the shells and holes as observed in regions of the ISM, where no stellar activity is detected. In order to quantify the comparison of our synthetic maps to the observations, we calculate the physical scale-autocorrelation length relation (L-L_{cr} relation) both on the synthetic HI maps and the HI map of Ho II. The L-L_{cr} relation shows a linear increase of the autocorrelation length with the physical scale up to the scale of energy injection and flattens for larger scales. The comparison of the L-L_{cr} relation between the observations and the synthetic maps suggests that turbulence is driven in the ISM of Ho II on large scales (~ 6 kpc). The slope of the L-L_{cr} relation in the linear regime in Ho II is better reproduced by models where turbulence is coupled with a weak efficiency cooling of the gas. These results demonstrate the importance of the interplay between turbulence and the thermodynamics of the gas for structure formation in the ISM. Our analysis can be used to determine the scale on which kinetic energy is injected in the ISM of dwarf irregular galaxies, and to derive, in a first approximation, the cooling rate of the gas.Comment: 36 pages, 13 figures, 1 table. Revised version, accepted to Ap

The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites

Available proper motion measurements of Milky Way (MW) satellite galaxies are used to calculate their orbital poles and projected uncertainties. These are compared to a set of recent cold dark-matter (CDM) simulations, tailored specifically to solve the MW satellite problem. We show that the CDM satellite orbital poles are fully consistent with being drawn from a random distribution, while the MW satellite orbital poles indicate that the disc-of-satellites of the Milky Way is rotationally supported. Furthermore, the bootstrapping analysis of the spatial distribution of theoretical CDM satellites also shows that they are consistent with being randomly drawn. The theoretical CDM satellite population thus shows a significantly different orbital and spatial distribution than the MW satellites, most probably indicating that the majority of the latter are of tidal origin rather than being DM dominated sub-structures. A statistic is presented that can be used to test a possible correlation of satellite galaxy orbits with their spatial distribution.Comment: Accepted for publication in Ap

Formation of the seed black holes: a role of quark nuggets?

Strange quark nuggets (SQNs) could be the relics of the cosmological QCD phase transition, and they could very likely be the candidate of cold quark matter if survived the cooling of the later Universe, although the formation and evolution of these SQNs depend on the physical state of the hot QGP (quark-gluon plasma) phase and the state of cold quark matter. We reconsider the possibility of SQNs as cold dark matter, and find that the formation of black holes in primordial halos could be significantly different from the standard scenario. In a primordial halo, the collision between gas and SQNs could be frequent enough, and thus the viscosity acting on each SQN would decrease its angular momentum and make it to sink into the center of the halo, as well as heat the gas. The SQNs with baryon numbers less than $10^{35}$ could assemble in the center of the halo before the formation of primordial stars. A black hole could form by merger of these SQNs, and then its mass could quickly become about $10^3\ M_\odot$ or higher, by accreting the surrounding SQNs or gas. The black holes formed in this way could be the seeds for the supermassive black holes at redshift as high as $z\sim 6$.Comment: 15 page

Milky Way potentials in CDM and MOND. Is the Large Magellanic Cloud on a bound orbit?

We compute the Milky Way potential in different cold dark matter (CDM) based models, and compare these with the modified Newtonian dynamics (MOND) framework. We calculate the axis ratio of the potential in various models, and find that isopotentials are less spherical in MOND than in CDM potentials. As an application of these models, we predict the escape velocity as a function of the position in the Galaxy. This could be useful in comparing with future data from planned or already-underway kinematic surveys (RAVE, SDSS, SEGUE, SIM, GAIA or the hypervelocity stars survey). In addition, the predicted escape velocity is compared with the recently measured high proper motion velocity of the Large Magellanic Cloud (LMC). To bind the LMC to the Galaxy in a MOND model, while still being compatible with the RAVE-measured local escape speed at the Sun's position, we show that an external field modulus of less than $0.03 a_0$ is needed.Comment: Accepted for publication in MNRAS, 13 pages, 7 figures, 3 table

Charged Lepton Production from Iron Induced by Atmospheric Neutrinos

The charged current lepton production induced by neutrinos in $^{56}Fe$ nuclei has been studied. The calculations have been done for the quasielastic as well as the inelastic reactions assuming $\Delta$ dominance and take into account the effect of Pauli blocking, Fermi motion and the renormalization of weak transition strengths in the nuclear medium. The quasielastic production cross section for lepton production are found to be strongly reduced due to nuclear effects while there is about 10% reduction in the inelastic cross sections in the absence of the final state interactions of the pions. The numerical results for the momentum and angular distributions of the leptons averaged over the various atmospheric neutrino spectra at the Soudan and Gransasso sites have been presented. The effect of nuclear model dependence and the atmospheric flux dependence on the relative yield of ${\mu}$ to e has been studied and discussed.Comment: 16pages, 18figure