236 research outputs found
Low-x hadronic final states at HERA
Measurements of the hadronic final state at HERA are reviewed, which aim at
the investigation of the parton dynamics of the proton at small Bjorken x.Comment: 7 pages, latex, 8 figures as ps/eps file
The Dawn of Galaxies
The development of primordial inhomogeneities into the non-linear regime and
the formation of the first astrophysical objects within dark matter halos mark
the transition from a simple, neutral, cooling universe -- described by just a
few parameters -- to a messy ionized one -- the realm of radiative,
hydrodynamic, and star formation processes. The recent measurement by the WMAP
satellite of a large optical depth to electron scattering implies that this
transition must have begun very early, and that the universe was reionized at
redshift z_ion=17\pm 5. It is an early generation of extremely metal-poor
massive stars and/or `seed' accreting black holes in subgalactic halos that may
have generated the ultraviolet radiation and mechanical energy that reheated
and reionized most of the hydrogen in the cosmos. The detailed thermal,
ionization, and chemical enrichment history of the universe during the crucial
formative stages around z=10-20 depends on the power-spectrum of density
fluctuations on small scales, the stellar initial mass function and star
formation efficiency, a complex network of poorly understood `feedback'
mechanisms, and remains one of the crucial missing links in galaxy formation
and evolution studies.Comment: 14 pages, 4 figures, to appear in the proceedings of the XXI Texas
Symposium on Relativistic Astrophysics held on December 9--13 2002, in
Florence, Ital
Numerical Simulations of the Dark Universe: State of the Art and the Next Decade
We present a review of the current state of the art of cosmological dark
matter simulations, with particular emphasis on the implications for dark
matter detection efforts and studies of dark energy. This review is intended
both for particle physicists, who may find the cosmological simulation
literature opaque or confusing, and for astro-physicists, who may not be
familiar with the role of simulations for observational and experimental probes
of dark matter and dark energy. Our work is complementary to the contribution
by M. Baldi in this issue, which focuses on the treatment of dark energy and
cosmic acceleration in dedicated N-body simulations. Truly massive dark
matter-only simulations are being conducted on national supercomputing centers,
employing from several billion to over half a trillion particles to simulate
the formation and evolution of cosmologically representative volumes (cosmic
scale) or to zoom in on individual halos (cluster and galactic scale). These
simulations cost millions of core-hours, require tens to hundreds of terabytes
of memory, and use up to petabytes of disk storage. The field is quite
internationally diverse, with top simulations having been run in China, France,
Germany, Korea, Spain, and the USA. Predictions from such simulations touch on
almost every aspect of dark matter and dark energy studies, and we give a
comprehensive overview of this connection. We also discuss the limitations of
the cold and collisionless DM-only approach, and describe in some detail
efforts to include different particle physics as well as baryonic physics in
cosmological galaxy formation simulations, including a discussion of recent
results highlighting how the distribution of dark matter in halos may be
altered. We end with an outlook for the next decade, presenting our view of how
the field can be expected to progress. (abridged)Comment: 54 pages, 4 figures, 3 tables; invited contribution to the special
issue "The next decade in Dark Matter and Dark Energy" of the new Open Access
journal "Physics of the Dark Universe". Replaced with accepted versio
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
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
Galactic Substructure and Dark Matter Annihilation in the Milky Way Halo
We study the effects of substructure on the rate of dark-matter annihilation
in the Galactic halo. We use an analytic model for substructure that can extend
numerical simulation results to scales too small to be resolved by the
simulations. We first calibrate the analytic model to numerical simulations,
and then determine the annihilation boost factor, for standard WIMP models as
well as those with Sommerfeld (or other) enhancements, as a function of
Galactocentric radius in the Milky Way. We provide an estimate of the
dependence of the gamma-ray intensity of WIMP annihilation as a function of
angular distance from the Galactic center. This methodology, coupled with
future numerical simulation results can be a powerful tool that can be used to
constrain WIMP properties using Fermi all-sky data.Comment: 10 pages, 7 figures, submitted to Phys. Rev. D; added a few
reference
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