19 research outputs found
Exploring Dark Matter with Milky Way substructure
The unambiguous detection of Galactic dark matter annihilation would unravel
one of the most outstanding puzzles in particle physics and cosmology. Recent
observations have motivated models in which the annihilation rate is boosted by
the Sommerfeld effect, a non-perturbative enhancement arising from a long range
attractive force. Here we apply the Sommerfeld correction to Via Lactea II, a
high resolution N-body simulation of a Milky-Way-size galaxy, to investigate
the phase-space structure of the Galactic halo. We show that the annihilation
luminosity from kinematically cold substructure can be enhanced by orders of
magnitude relative to previous calculations, leading to the prediction of
gamma-ray fluxes from up to hundreds of dark clumps that should be detectable
by the Fermi satellite.Comment: 23 pages, 9 figures (includes Supporting Online Material), accepted
for publication in Science, v2: added reference, fixed typo
Simulations of recoiling massive black holes
The coalescence of black hole binaries is a significant source of gravitational wave radiation. The typically asymmetric nature of this emission, which carries linear momentum, can result in the recoil of the black hole remnant with velocities in the range 100 < Vrecoil < 3750 km s‑1. The detectability of recoiling massive black holes (MBH) as off-nuclear QSOs is tightly connected with the properties of the host galaxy, which determine the MBH's orbit and fuel reservoir. We present the results of N-body simulations of recoiling MBHs in high-resolution, non-axisymmetric potentials. We find that if the recoil velocities are high enough to reach regions of the galaxy dominated by the generally triaxial dark matter distribution, the return time is significantly extended when compared to a spherical distribution. We also perform simulations of recoiling MBHs traveling in gas merger remnants, where large amounts of gas have been funneled to the central regions, In this case, the MBHs remain within R<1 kpc from the center of the host even for high recoil velocities (Vrecoil = 1200 km s‑1) due to the compactness of the remnant galaxy's nuclear disk. We discuss the implications of both scenarios for detectability
The Via Lactea INCITE Simulation: Galactic Dark Matter Substructure at High Resolution
It is a clear unique prediction of the cold dark matter paradigm of
cosmological structure formation that galaxies form hierarchically and are
embedded in massive, extended dark halos teeming with self-bound substructure
or "subhalos". The amount and spatial distribution of subhalos around their
host provide unique information and clues on the galaxy assembly process and
the nature of the dark matter. Here we present results from the Via Lactea
INCITE simulation, a one billion particle, one million cpu-hour simulation of
the formation and evolution of a Galactic dark matter halo and its substructure
population.Comment: 10 pages, Proceedings of the SciDAC 2008 conference, (Seattle, July
13-17, 2008
Fossil remnants of reionization in the halo of the Milky Way
Our recently completed one billion particle Via Lactea II simulation of a
Milky Way-sized dark matter halo resolves over 50,000 gravitationally bound
clumps orbiting today within the virialized region of the main host. About
2,300 of these subhalos have one or more "progenitors" above 1e6 Msun at
redshift 11, i.e. massive enough for their gas to have cooled via excitation of
H2 and fragmented prior to the epoch of cosmic reionization. We count 4,500
such progenitors: if these were able to convert a fraction of their gas content
into very metal-poor stars with a Salpeter initial mass function (IMF), they
would be shining today with a visual magnitude M_V=6.7 per solar mass in stars.
Assuming a universal baryon fraction, we show that mean star formation
efficiencies as low as 0.1% in progenitors below 1e8 Msun would overproduce the
abundance of the faint Galatic dwarf spheroidals observed by the Sloan Digital
Sky Survey. Star formation at first light must have occurred either with an IMF
lacking stars below 0.9 Msun, or was intrinsically very inefficient in small
dark matter halos. If the latter, our results may be viewed as another hint of
a minimum scale in galaxy formation.Comment: 5 pages, 4 figures, ApJL, in pres
Quantifying the heart of darkness with GHALO - a multi-billion particle simulation of our galactic halo
We perform a series of simulations of a Galactic mass dark matter halo at
different resolutions, our largest uses over three billion particles and has a
mass resolution of 1000 M_sun. We quantify the structural properties of the
inner dark matter distribution and study how they depend on numerical
resolution. We can measure the density profile to a distance of 120 pc (0.05%
of R_vir) where the logarithmic slope is -0.8 and -1.4 at (0.5% of R_vir). We
propose a new two parameter fitting function that has a linearly varying
logarithmic density gradient which fits the GHALO and VL2 density profiles
extremely well. Convergence in the density profile and the halo shape scales as
N^(-1/3), but the shape converges at a radius three times larger at which point
the halo becomes more spherical due to numerical resolution. The six
dimensional phase-space profile is dominated by the presence of the
substructures and does not follow a power law, except in the smooth
under-resolved inner few kpc.Comment: 6 pages, 4 figures, submitted to MNRAS Letters, for full sized
images, see http://www.itp.uzh.ch/news.htm
Ultra-cold WIMPs: relics of non-standard pre-BBN cosmologies
Weakly interacting massive particles (WIMPs) are one of very few probes of
cosmology before Big Bang nucleosynthesis (BBN). We point out that in scenarios
in which the Universe evolves in a non-standard manner during and after WIMP
kinetic decoupling, the horizon mass scale at decoupling can be smaller and the
dark matter WIMPs can be colder than in standard cosmology. This would lead to
much smaller first objects in hierarchical structure formation. In low
reheating temperature scenarios the effect may be large enough as to noticeably
enhance indirect detection signals in GLAST and other detectors, by up to two
orders of magnitude.Comment: Six pages, one figure- Extensive additions and rewriting with respect
to v1. Figure change
The detection of sub-solar mass dark matter halos
Dark matter halos of sub-solar mass are the first bound objects to form in
cold dark matter theories. In this article, I discuss the present understanding
of "microhalos'', their role in structure formation, and the implications of
their potential presence, in the interpretation of dark matter experiments.Comment: 18 pages, 7 figures. Invited contribution to NJP Focus Issue on "Dark
Matter and Particle Physics
Clumps and streams in the local dark matter distribution
In cold dark matter cosmological models, structures form and grow by merging
of smaller units. Numerical simulations have shown that such merging is
incomplete; the inner cores of halos survive and orbit as "subhalos" within
their hosts. Here we report a simulation that resolves such substructure even
in the very inner regions of the Galactic halo. We find hundreds of very
concentrated dark matter clumps surviving near the solar circle, as well as
numerous cold streams. The simulation reveals the fractal nature of dark matter
clustering: Isolated halos and subhalos contain the same relative amount of
substructure and both have cuspy inner density profiles. The inner mass and
phase-space densities of subhalos match those of recently discovered faint,
dark matter-dominated dwarf satellite galaxies and the overall amount of
substructure can explain the anomalous flux ratios seen in strong gravitational
lenses. Subhalos boost gamma-ray production from dark matter annihilation, by
factors of 4-15, relative to smooth galactic models. Local cosmic ray
production is also enhanced, typically by a factor 1.4, but by more than a
factor of ten in one percent of locations lying sufficiently close to a large
subhalo. These estimates assume that gravitational effects of baryons on dark
matter substructure are small.Comment: 14 pages, 5 figures, to appear in Nature, includes supplementary
information. Full version of Figure 1 available at
http://www.ucolick.org/~diemand/vl2/fig1.pn
Indirect Dark Matter Detection from Dwarf Satellites: Joint Expectations from Astrophysics and Supersymmetry
We present a general methodology for determining the gamma-ray flux from
annihilation of dark matter particles in Milky Way satellite galaxies, focusing
on two promising satellites as examples: Segue 1 and Draco. We use the
SuperBayeS code to explore the best-fitting regions of the Constrained Minimal
Supersymmetric Standard Model (CMSSM) parameter space, and an independent MCMC
analysis of the dark matter halo properties of the satellites using published
radial velocities. We present a formalism for determining the boost from halo
substructure in these galaxies and show that its value depends strongly on the
extrapolation of the concentration-mass (c(M)) relation for CDM subhalos down
to the minimum possible mass. We show that the preferred region for this
minimum halo mass within the CMSSM with neutralino dark matter is ~10^-9-10^-6
solar masses. For the boost model where the observed power-law c(M) relation is
extrapolated down to the minimum halo mass we find average boosts of about 20,
while the Bullock et al (2001) c(M) model results in boosts of order unity. We
estimate that for the power-law c(M) boost model and photon energies greater
than a GeV, the Fermi space-telescope has about 20% chance of detecting a dark
matter annihilation signal from Draco with signal-to-noise greater than 3 after
about 5 years of observation
Fitting the Gamma-Ray Spectrum from Dark Matter with DMFIT: GLAST and the Galactic Center Region
We study the potential of GLAST to unveil particle dark matter properties
with gamma-ray observations of the Galactic center region. We present full
GLAST simulations including all gamma-ray sources known to date in a region of
4 degrees around the Galactic center, in addition to the diffuse gamma-ray
background and to the dark matter signal. We introduce DMFIT, a tool that
allows one to fit gamma-ray emission from pair-annihilation of generic particle
dark matter models and to extract information on the mass, normalization and
annihilation branching ratios into Standard Model final states. We assess the
impact and systematic effects of background modeling and theoretical priors on
the reconstruction of dark matter particle properties. Our detailed simulations
demonstrate that for some well motivated supersymmetric dark matter setups with
one year of GLAST data it will be possible not only to significantly detect a
dark matter signal over background, but also to estimate the dark matter mass
and its dominant pair-annihilation mode.Comment: 37 pages, 16 figures, submitted to JCA