6,253 research outputs found
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
Considerations for a design and operations knowledge support system for Space Station Freedom
Engineering and operations of modern engineered systems depend critically upon detailed design and operations knowledge that is accurate and authoritative. A design and operations knowledge support system (DOKSS) is a modern computer-based information system providing knowledge about the creation, evolution, and growth of an engineered system. The purpose of a DOKSS is to provide convenient and effective access to this multifaceted information. The complexity of Space Station Freedom's (SSF's) systems, elements, interfaces, and organizations makes convenient access to design knowledge especially important, when compared to simpler systems. The life cycle length, being 30 or more years, adds a new dimension to space operations, maintenance, and evolution. Provided here is a review and discussion of design knowledge support systems to be delivered and operated as a critical part of the engineered system. A concept of a DOKSS for Space Station Freedom (SSF) is presented. This is followed by a detailed discussion of a DOKSS for the Lyndon B. Johnson Space Center and Work Package-2 portions of SSF
Mapping Extragalactic Dark Matter Annihilation with Galaxy Surveys: A Systematic Study of Stacked Group Searches
Dark matter in the halos surrounding galaxy groups and clusters can
annihilate to high-energy photons. Recent advancements in the construction of
galaxy group catalogs provide many thousands of potential extragalactic targets
for dark matter. In this paper, we outline a procedure to infer the dark matter
signal associated with a given galaxy group. Applying this procedure to a
catalog of sources, one can create a full-sky map of the brightest
extragalactic dark matter targets in the nearby Universe (),
supplementing sources of dark matter annihilation from within the Local Group.
As with searches for dark matter in dwarf galaxies, these extragalactic targets
can be stacked together to enhance the signals associated with dark matter. We
validate this procedure on mock gamma-ray data sets using a
galaxy catalog constructed from the -body cosmological
simulation and demonstrate that the limits are robust, at
levels, to systematic uncertainties on halo mass and concentration. We also
quantify other sources of systematic uncertainty arising from the analysis and
modeling assumptions. Our results suggest that a stacking analysis using galaxy
group catalogs provides a powerful opportunity to discover extragalactic dark
matter and complements existing studies of Milky Way dwarf galaxies.Comment: 17+7 pages, 9+4 figures; v2, updated to PRD version with several
updates, results and conclusions unchange
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