1,013 research outputs found
The Dark Matter Annihilation Signal from Galactic Substructure: Predictions for GLAST
We present quantitative predictions for the detectability of individual
Galactic dark matter subhalos in gamma-rays from dark matter pair annihilations
in their centers. Our method is based on a hybrid approach, employing the
highest resolution numerical simulations available (including the recently
completed one billion particle Via Lactea II simulation) as well as analytical
models for the extrapolation beyond the simulations' resolution limit. We
include a self-consistent treatment of subhalo boost factors, motivated by our
numerical results, and a realistic treatment of the expected backgrounds that
individual subhalos must outshine. We show that for reasonable values of the
dark matter particle physics parameters (M_X ~ 50 - 500 GeV and ~
10^-26 - 10^-25 cm^3/s) GLAST may very well discover a few, even up to several
dozen, such subhalos, at 5 sigma significance, and some at more than 20 sigma.
We predict that the majority of luminous sources would be resolved with GLAST's
expected angular resolution. For most observer locations the angular
distribution of detectable subhalos is consistent with a uniform distribution
across the sky. The brightest subhalos tend to be massive (median Vmax of 24
km/s) and therefore likely hosts of dwarf galaxies, but many subhalos with Vmax
as low as 5 km/s are also visible. Typically detectable subhalos are 20 - 40
kpc from the observer, and only a small fraction are closer than 10 kpc. The
total number of observable subhalos has not yet converged in our simulations,
and we estimate that we may be missing up to 3/4 of all detectable subhalos.Comment: 19 pages, 12 figures, ApJ accepted, a version with higher resolution
figures can be downloaded from
http://www.sns.ias.edu/~mqk/transfer/VL2_GLAST_predictions.pd
Formation and evolution of galaxy dark matter halos and their substructure
We use the ``Via Lactea'' simulation to study the co-evolution of a Milky
Way-size LambdaCDM halo and its subhalo population. While most of the host halo
mass is accreted over the first 6 Gyr in a series of major mergers, the
physical mass distribution [not M_vir(z)] remains practically constant since
z=1. The same is true in a large sample of LambdaCDM galaxy halos. Subhalo mass
loss peaks between the turnaround and virialization epochs of a given mass
shell, and declines afterwards. 97% of the z=1 subhalos have a surviving bound
remnant at the present epoch. The retained mass fraction is larger for
initially lighter subhalos: satellites with maximum circular velocities Vmax=10
km/s at z=1 have today about 40% of their mass back then. At the first
pericenter passage a larger average mass fraction is lost than during each
following orbit. Tides remove mass in substructure from the outside in, leading
to higher concentrations compared to field halos of the same mass. This effect,
combined with the earlier formation epoch of the inner satellites, results in
strongly increasing subhalo concentrations towards the Galactic center. We
present individual evolutionary tracks and present-day properties of the likely
hosts of the dwarf satellites around the Milky Way. The formation histories of
``field halos'' that lie today beyond the Via Lactea host are found to strongly
depend on the density of their environment. This is caused by tidal mass loss
that affects many field halos on eccentric orbits.Comment: 20 pages, 18 figures. Figures 6,7 and 8 corrected in this version,
for details see the erratum in ApJ 679, 1680 and
http://www.ucolick.org/~diemand/vl/publ/vlevolerr.pdf. Data, movies and
images are available at http://www.ucolick.org/~diemand/vl
The structure and evolution of cold dark matter halos
In the standard cosmological model a mysterious cold dark matter (CDM)
component dominates the formation of structures. Numerical studies of the
formation of CDM halos have produced several robust results that allow unique
tests of the hierarchical clustering paradigm. Universal properties of halos,
including their mass profiles and substructure properties are roughly
consistent with observational data from the scales of dwarf galaxies to galaxy
clusters. Resolving the fine grained structure of halos has enabled us to make
predictions for ongoing and planned direct and indirect dark matter detection
experiments.
While simulations of pure CDM halos are now very accurate and in good
agreement (recently claimed discrepancies are addressed in detail in this
review), we are still unable to make robust, quantitative predictions about
galaxy formation and about how the dark matter distribution changes in the
process. Whilst discrepancies between observations and simulations have been
the subject of much debate in the literature, galaxy formation and evolution
needs to be understood in more detail in order to fully test the CDM paradigm.
Whatever the true nature of the dark matter particle is, its clustering
properties must not be too different from a cold neutralino like particle to
maintain all the successes of the model in matching large scale structure data
and the global properties of halos which are mostly in good agreement with
observations.Comment: Invited Review to appear on Advanced Science Letters (ASL), Special
Issue on Computational Astrophysics, edited by Lucio Mayer. Higher quality
version available at
http://www.ucolick.org/~diemand/vl/publ/dm_dm_minirev.pdf ; movies, images
and data at http://www.ucolick.org/~diemand/v
Dark matter subhalos and the dwarf satellites of the Milky Way
The Via Lactea simulation of the dark matter halo of the Milky Way predicts
the existence of many thousands of bound subhalos distributed approximately
with equal mass per decade of mass. Here we show that: a) a similar steeply
rising subhalo mass function is also present at redshift 0.5 in an
elliptical-sized halo simulated with comparable resolution in a different
cosmology. Compared to Via Lactea, this run produces nearly a factor of two
more subhalos with large circular velocities; b) the fraction of Via Lactea
mass brought in by subhalos that have a surviving bound remnant today with
present-day peak circular velocity Vmax>2 km/s (>10 km/s) is 45% (30%); c)
because of tidal mass loss, the number of subhalos surviving today that reached
a peak circular velocity of >10 km/s throughout their lifetime exceeds half a
thousand, five times larger than their present-day abundance and more than
twenty times larger than the number of known satellites of the Milky Way; e)
unless the circular velocity profiles of Galactic satellites peak at values
significantly higher that expected from the stellar line-of-sight velocity
dispersion, only about one in five subhalos with Vmax>20 km/s today must be
housing a luminous dwarf; f) small dark matter clumps appear to be relatively
inefficient at forming stars even well beyond the virial radius; g) the
observed Milky Way satellites appear to follow the overall dark matter
distribution of Via Lactea, while the largest simulated subhalos today are
found preferentially at larger radii; h) subhalos have central densities that
increase with Vmax and reach 0.1-0.3 Msun/pc3 comparable to the central
densities inferred in dwarf spheroidals with core radii >250 pc.Comment: 14 pages, 8 figures, ApJ in press. A few typos correcte
Bound and unbound substructures in Galaxy-scale Dark Matter haloes
We analyse the coarse-grained phase-space structure of the six Galaxy-scale
dark matter haloes of the Aquarius Project using a state-of-the-art 6D
substructure finder. Within r_50, we find that about 35% of the mass is in
identifiable substructures, predominantly tidal streams, but including about
14% in self-bound subhaloes. The slope of the differential substructure mass
function is close to -2, which should be compared to around -1.9 for the
population of self-bound subhaloes. Near r_50 about 60% of the mass is in
substructures, with about 30% in self-bound subhaloes. The inner 35 kpc of the
highest resolution simulation has only 0.5% of its mass in self-bound
subhaloes, but 3.3% in detected substructure, again primarily tidal streams.
The densest tidal streams near the solar position have a 3-D mass density about
1% of the local mean, and populate the high velocity tail of the velocity
distribution.Comment: Submitted to MNRAS on 12/10/2010, 11 pages, 10 figure
Molecular dynamics simulations of bubble nucleation in dark matter detectors
Bubble chambers and droplet detectors used in dosimetry and dark matter
particle search experiments use a superheated metastable liquid in which
nuclear recoils trigger bubble nucleation. This process is described by the
classical heat spike model of F. Seitz [Phys. Fluids (1958-1988) 1, 2 (1958)],
which uses classical nucleation theory to estimate the amount and the
localization of the deposited energy required for bubble formation. Here we
report on direct molecular dynamics simulations of heat-spike-induced bubble
formation. They allow us to test the nanoscale process described in the
classical heat spike model. 40 simulations were performed, each containing
about 20 million atoms, which interact by a truncated force-shifted
Lennard-Jones potential. We find that the energy per length unit needed for
bubble nucleation agrees quite well with theoretical predictions, but the
allowed spike length and the required total energy are about twice as large as
predicted. This could be explained by the rapid energy diffusion measured in
the simulation: contrary to the assumption in the classical model, we observe
significantly faster heat diffusion than the bubble formation time scale.
Finally we examine {\alpha}-particle tracks, which are much longer than those
of neutrons and potential dark matter particles. Empirically, {\alpha} events
were recently found to result in louder acoustic signals than neutron events.
This distinction is crucial for the background rejection in dark matter
searches. We show that a large number of individual bubbles can form along an
{\alpha} track, which explains the observed larger acoustic amplitudes.Comment: 7 pages, 5 figures, accepted for publication in Phys. Rev. E, matches
published versio
The Graininess of Dark Matter Haloes
We use the recently completed one billion particle Via Lactea II LambdaCDM
simulation to investigate local properties like density, mean velocity,
velocity dispersion, anisotropy, orientation and shape of the velocity
dispersion ellipsoid, as well as structure in velocity space of dark matter
haloes. We show that at the same radial distance from the halo centre, these
properties can deviate by orders of magnitude from the canonical, spherically
averaged values, a variation that can only be partly explained by triaxiality
and the presence of subhaloes. The mass density appears smooth in the central
relaxed regions but spans four orders of magnitude in the outskirts, both
because of the presence of subhaloes as well as of underdense regions and holes
in the matter distribution. In the inner regions the local velocity dispersion
ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in
the outer parts where the orientation becomes more isotropic. The clumpy
structure in local velocity space of the outer halo can not be well described
by a smooth multivariate normal distribution. Via Lactea II also shows the
presence of cold streams made visible by their high 6D phase space density.
Generally, the structure of dark matter haloes shows a high degree of
graininess in phase space that cannot be described by a smooth distribution
function.Comment: 22 pages, 14 figures, 1 table, accepted for publication by MNRA
On the age-radius relation and orbital history of cluster galaxies
We explore the region of influence of a galaxy cluster using numerical
simulations of cold dark matter halos. Many of the observed galaxies in a
cluster are expected to be infalling for the first time. Half of the halos at
distances of one to two virial radii today have previously orbited through the
cluster, most of them have even passed through the dense inner regions of the
cluster. Some halos at distances of up to three times the virial radius have
also passed through the cluster core. We do not find a significant correlation
of ``infall age'' versus present day position for substructures and the scatter
at a given position is very large. This relation may be much more significant
if we could resolve the physically overmerged galaxies in the central region.Comment: To appear in the proceedings of IAU Colloquium 195: "Outskirts of
galaxy clusters: intense life in the suburbs", Torino, Italy, March 12-16,
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