14,455 research outputs found
Dark Matter Halos from the Inside Out
The balance of evidence indicates that individual galaxies and groups or
clusters of galaxies are embedded in enormous distributions of cold, weakly
interacting dark matter. These dark matter 'halos' provide the scaffolding for
all luminous structure in the universe, and their properties comprise an
essential part of the current cosmological model. I review the internal
properties of dark matter halos, focussing on the simple, universal trends
predicted by numerical simulations of structure formation. Simulations indicate
that halos should all have roughly the same spherically-averaged density
profile and kinematic structure, and predict simple distributions of shape,
formation history and substructure in density and kinematics, over an enormous
range of halo mass and for all common variants of the concordance cosmology. I
describe observational progress towards testing these predictions by measuring
masses, shapes, profiles and substructure in real halos, using baryonic tracers
or gravitational lensing. An important property of simulated halos (possibly
the most important property) is their dynamical 'age', or degree of internal
relaxation. The age of a halo may have almost as much effect as its mass in
determining the state of its baryonic contents, so halo ages are also worth
trying to measure observationally. I review recent gravitational lensing
studies of galaxy clusters which should measure substructure and relaxation in
a large sample of individual cluster halos, producing quantitative measures of
age that are well-matched to theoretical predictions. The age distributions
inferred from these studies will lead to second-generation tests of the
cosmological model, as well as an improved understanding of cluster assembly
and the evolution of galaxies within clusters.Comment: v2: additional references and minor corrections to match the
published versio
Multivariable proportional-integral-plus (PIP) control of the ALSTOM nonlinear gasifier simulation
Multivariable proportional-integral-plus (PIP) control methods are applied to the nonlinear ALSTOM Benchmark Challenge II. The approach utilises a data-based combined model reduction and linearisation step, which plays an essential role in satisfying the design specifications. The discrete-time transfer function models obtained in this manner are represented in a non-minimum state space form suitable for PIP control system design. Here, full state variable feedback control can be implemented directly from the measured input and output signals of the controlled process, without resorting to the design and implementation of a deterministic state reconstructor or a stochastic Kalman filter. Furthermore, the non-minimal formulation provides more design freedom than the equivalent minimal case, a characteristic that proves particularly useful in tuning the algorithm to meet the Benchmark specifications. The latter requirements are comfortably met for all three operating conditions by using a straightforward to implement, fixed gain, linear PIP algorithm
Can Supersymmetry Naturally Explain the Positron Excess?
It has often been suggested that the cosmic positron excess observed by the
HEAT experiment could be the consequence of supersymmetric dark matter
annihilating in the galactic halo. Although it is well known that evenly
distributed dark matter cannot account for the observed excess, if substantial
amounts of local dark matter substructure are present, the positron flux would
be enhanced, perhaps to the observed magnitude. In this paper, we attempt to
identify the nature of the substructure required to match the HEAT data,
including the location, size and density of any local dark matter clump(s).
Additionally, we attempt to assess the probability of such substructure being
present. We find that if the current density of neutralino dark matter is the
result of thermal production, very unlikely ( or less) conditions
must be present in local substructure to account for the observed excess.Comment: Version accepted by Physical Review
The Phase-Space Density Profiles of Cold Dark Matter Halos
We examine the coarse-grained phase-space density profiles of a set of
recent, high-resolution simulations of galaxy-sized Cold Dark Matter (CDM)
halos. Over two and a half decades in radius the phase-space density closely
follows a power-law, , with . This behaviour matches the self-similar solution obtained by
Bertschinger for secondary infall in a uniformly expanding universe. On the
other hand, the density profile corresponding to Bertschinger's solution (a
power-law of slope ) differs significantly from the density
profiles of CDM halos. We show that isotropic mass distributions with power-law
phase-space density profiles form a one-parameter family of structures
controlled by , the ratio of the velocity dispersion to the peak
circular velocity. For one recovers the power-law
solution . For larger than some critical
value, , solutions become non-physical, leading to negative
densities near the center. The critical solution, , has
the narrowest phase-space density distribution compatible with the power-law
phase-space density stratification constraint. Over three decades in radius the
critical solution is indistinguishable from an NFW profile. Our results thus
suggest that the NFW profile is the result of a hierarchical assembly process
that preserves the phase-space stratification of Bertschinger's infall model
but which ``mixes'' the system maximally, perhaps as a result of repeated
merging.Comment: 16 pages, 4 figures; submitted to The Astrophysical Journa
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