81 research outputs found
A lower limit on the dark particle mass from dSphs
We use dwarf spheroidal galaxies as a tool to attempt to put precise lower
limits on the mass of the dark matter particle, assuming it is a sterile
neutrino. We begin by making cored dark halo fits to the line of sight velocity
dispersions as a function of projected radius (taken from Walker et al. 2007)
for six of the Milky Way's dwarf spheroidal galaxies. We test Osipkov-Merritt
velocity anisotropy profiles, but find that no benefit is gained over constant
velocity anisotropy. In contrast to previous attempts, we do not assume any
relation between the stellar velocity dispersions and the dark matter ones, but
instead we solve directly for the sterile neutrino velocity dispersion at all
radii by using the equation of state for a partially degenerate neutrino gas
(which ensures hydrostatic equilibrium of the sterile neutrino halo). This
yields a 1:1 relation between the sterile neutrino density and velocity
dispersion, and therefore gives us an accurate estimate of the Tremaine-Gunn
limit at all radii. By varying the sterile neutrino particle mass, we locate
the minimum mass for all six dwarf spheroidals such that the Tremaine-Gunn
limit is not exceeded at any radius (in particular at the centre). We find
sizeable differences between the ranges of feasible sterile neutrino particle
mass for each dwarf, but interestingly there exists a small range 270-280eV
which is consistent with all dSphs at the 1- level.Comment: 13 pages, 2 figures, 1 tabl
Conservative Constraints on Dark Matter from the Fermi-LAT Isotropic Diffuse Gamma-Ray Background Spectrum
We examine the constraints on final state radiation from Weakly Interacting
Massive Particle (WIMP) dark matter candidates annihilating into various
standard model final states, as imposed by the measurement of the isotropic
diffuse gamma-ray background by the Large Area Telescope aboard the Fermi
Gamma-Ray Space Telescope. The expected isotropic diffuse signal from dark
matter annihilation has contributions from the local Milky Way (MW) as well as
from extragalactic dark matter. The signal from the MW is very insensitive to
the adopted dark matter profile of the halos, and dominates the signal from
extragalactic halos, which is sensitive to the low mass cut-off of the halo
mass function. We adopt a conservative model for both the low halo mass
survival cut-off and the substructure boost factor of the Galactic and
extragalactic components, and only consider the primary final state radiation.
This provides robust constraints which reach the thermal production
cross-section for low mass WIMPs annihilating into hadronic modes. We also
reanalyze limits from HESS observations of the Galactic Ridge region using a
conservative model for the dark matter halo profile. When combined with the
HESS constraint, the isotropic diffuse spectrum rules out all interpretations
of the PAMELA positron excess based on dark matter annihilation into two lepton
final states. Annihilation into four leptons through new intermediate states,
although constrained by the data, is not excluded.Comment: 11 pages, 5 figures. v3: minor revisions, matches version to appear
in JCA
The multi-stream flows and the dynamics of the cosmic web
A new numerical technique to identify the cosmic web is proposed. It is based
on locating multi-stream flows, i.e. the places where the velocity field is
multi-valued. The method is local in Eulerian space, simple and computaionally
efficient. This technique uses the velocities of particles and thus takes into
account the dynamical information. This is in contrast with the majority of
standard methods that use the coordinates of particles only. Two quantities are
computed in every mesh cell: the mean and variance of the velocity field. In
the cells where the velocity is single-valued the variance must be equal to
zero exactly, therefore the cells with non-zero variance are identified as
multi-stream flows. The technique has been tested in a N-body simulation of the
\L CDM model. The preliminary analysis has shown that numerical noise does not
pose a significant problem. The web identified by the new method has been
compared with the web identified by the standard technique using only the
particle coordinates. The comparison has shown overall similarity of two webs
as expected, however they by no means are identical. For example, the
isocontours of the corresponding fields have significantly different shapes and
some density peaks of similar heights exhibit significant differences in the
velocity variance and vice versa. This suggest that the density and velocity
variance have a significant degree of independence. The shape of the
two-dimensional pdf of density and velocity variance confirms this proposition.
Thus, we conclude that the dynamical information probed by this technique
introduces an additional dimension into analysis of the web.Comment: 19 pages, 10 figure
On initial conditions for the Hot Big Bang
We analyse the process of reheating the Universe in the electroweak theory
where the Higgs field plays a role of the inflaton. We estimate the maximal
temperature of the Universe and fix the initial conditions for
radiation-dominated phase of the Universe expansion in the framework of the
Standard Model (SM) and of the nuMSM -- the minimal extension of the SM by
three right-handed singlet fermions. We show that the inflationary epoch is
followed by a matter dominated stage related to the Higgs field oscillations.
We investigate the energy transfer from Higgs-inflaton to the SM particles and
show that the radiation dominated phase of the Universe expansion starts at
temperature T_r~(3-15)*10^{13} GeV, where the upper bound depends on the Higgs
boson mass. We estimate the production rate of singlet fermions at preheating
and find that their concentrations at T_r are negligibly small. This suggests
that the sterile neutrino Dark Matter (DM) production and baryogenesis in the
nuMSM with Higgs-driven inflation are low energy phenomena, having nothing to
do with inflation. We study then a modification of the nuMSM, adding to its
Lagrangian higher dimensional operators suppressed by the Planck scale. The
role of these operators in Higgs-driven inflation is clarified. We find that
these operators do not contribute to the production of Warm Dark Matter (WDM)
and to baryogenesis. We also demonstrate that the sterile neutrino with mass
exceeding 100 keV (a Cold Dark Matter (CDM) candidate) can be created during
the reheating stage of the Universe in necessary amounts. We argue that the
mass of DM sterile neutrino should not exceed few MeV in order not to overclose
the Universe.Comment: 41 pages, 5 figures. Journal version accepted in JCA
Catalog of Nearby Isolated Galaxies in the Volume z<0.01
We present a catalog of 520 most isolated nearby galaxies with radial
velocities V_LG<3500 km/s covering the entire sky. This population of "space
orphans" makes up 4.8% among 10900 galaxies with measured radial velocities. We
describe the isolation criterion used to select our sample, called the "Local
Orphan Galaxies" (LOG), and discuss their basic optical and HI properties. A
half of the LOG catalog is occupied by the Sdm, Im and Ir morphological type
galaxies without a bulge. The median ratio M_gas/M_star in the LOG galaxies
exceeds 1. The distribution of the catalog galaxies on the sky looks uniform
with some signatures of a weak clustering on the scale of about 0.5 Mpc. The
LOG galaxies are located in the regions where the mean local density of matter
is approximately 50 times lower than the mean global density. We indicate a
number of LOG galaxies with distorted structures, which may be the consequence
of interaction of isolated galaxies with massive dark objects
Reopening the window on charged dark matter
We reexamine the limits on charged dark matter particles. We show that if
their mass and charge fall in the range 100(q_X/e)^2< m_X < 10^8(q_X/e) TeV,
then magnetic fields prevent particles in the halo from entering the galactic
disk, while those initially trapped inside are accelerated through the Fermi
mechanism and ejected within about 0.1-1 Gyrs. Consequently, previous
constraints on charged dark matter based on terrestrial non-observation are
invalid within that range. Further, we find that charged massive particles may
simultaneously solve several long-standing astrophysical problems, including
the underabundance of dwarf galaxies, the shallow density profiles in the cores
of the LSB galaxies, the absence of cooling flows in the cores of galaxy
clusters, and several others.Comment: 9 pages, 1 figure, accepted for publication in JCA
Constraining scalar fields with stellar kinematics and collisional dark matter
The existence and detection of scalar fields could provide solutions to
long-standing puzzles about the nature of dark matter, the dark compact objects
at the centre of most galaxies, and other phenomena. Yet, self-interacting
scalar fields are very poorly constrained by astronomical observations, leading
to great uncertainties in estimates of the mass and the
self-interacting coupling constant of these fields. To counter this,
we have systematically employed available astronomical observations to develop
new constraints, considerably restricting this parameter space. In particular,
by exploiting precise observations of stellar dynamics at the centre of our
Galaxy and assuming that these dynamics can be explained by a single boson
star, we determine an upper limit for the boson star compactness and impose
significant limits on the values of the properties of possible scalar fields.
Requiring the scalar field particle to follow a collisional dark matter model
further narrows these constraints. Most importantly, we find that if a scalar
dark matter particle does exist, then it cannot account for both the
dark-matter halos and the existence of dark compact objects in galactic nucleiComment: 23 pages, 8 figures; accepted for publication by JCAP after minor
change
Radiative Seesaw Mechanism at Weak Scale
We investigate an alternative seesaw mechanism for neutrino mass generation.
Neutrino mass is generated at loop level but the basic concept of usual seesaw
mechanism is kept. One simple model is constructed to show how this mechanism
is realized. The applications of this seesaw mechanism at weak scale to
cosmology and neutrino physics are discussed.Comment: 12 Pages, latex, no figure
Atomic Dark Matter
We propose that dark matter is dominantly comprised of atomic bound states.
We build a simple model and map the parameter space that results in the early
universe formation of hydrogen-like dark atoms. We find that atomic dark matter
has interesting implications for cosmology as well as direct detection:
Protohalo formation can be suppressed below for weak scale dark matter due to Ion-Radiation interactions in the
dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine
splittings of order 100 \kev, consistent with the inelastic dark matter
interpretation of the DAMA data while naturally evading direct detection
bounds.Comment: 17 pages, 3 figure
Dark Matter Direct Detection with Non-Maxwellian Velocity Structure
The velocity distribution function of dark matter particles is expected to
show significant departures from a Maxwell-Boltzmann distribution. This can
have profound effects on the predicted dark matter - nucleon scattering rates
in direct detection experiments, especially for dark matter models in which the
scattering is sensitive to the high velocity tail of the distribution, such as
inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for
experiments that require high energy recoil events, such as many directionally
sensitive experiments. Here we determine the velocity distribution functions
from two of the highest resolution numerical simulations of Galactic dark
matter structure (Via Lactea II and GHALO), and study the effects for these
scenarios. For directional detection, we find that the observed departures from
Maxwell-Boltzmann increase the contrast of the signal and change the typical
direction of incoming DM particles. For iDM, the expected signals at direct
detection experiments are changed dramatically: the annual modulation can be
enhanced by more than a factor two, and the relative rates of DAMA compared to
CDMS can change by an order of magnitude, while those compared to CRESST can
change by a factor of two. The spectrum of the signal can also change
dramatically, with many features arising due to substructure. For LDM the
spectral effects are smaller, but changes do arise that improve the
compatibility with existing experiments. We find that the phase of the
modulation can depend upon energy, which would help discriminate against
background should it be found.Comment: 34 pages, 16 figures, submitted to JCAP. Tables of g(v_min), the
integral of f(v)/v from v_min to infinity, derived from our simulations, are
available for download at http://astro.berkeley.edu/~mqk/dmdd
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