92 research outputs found
Transforming the Einstein static Universe into physically acceptable static fluid spheres II: A two - fold infinity of exact solutions
Following a solution generating technique introduced recently by one of us,
we transform the Einstein static Universe into a two - fold infinity class of
physically acceptable exact perfect fluid solutions of Einstein's equations.
Whereas the entire class of solutions can be considered as generalizations of
the familiar Tolman IV solution, no member of the class can be written
explicitly in isotropic coordinates. Further, except for a set of measure zero,
no member of the class can be written explicitly in curvature coordinates
either.Comment: 6 pages, 6 figures, revtex4,improved figures. To appear in Phys. Rev.
Warm Dark Haloes Accretion Histories and their Gravitational Signatures
We study clusters in Warm Dark Matter (WDM) models of a thermally produced
dark matter particle keV in mass. We show that, despite clusters in WDM
cosmologies having similar density profiles as their Cold Dark Matter (CDM)
counterparts, the internal properties, such as the amount of substructure,
shows marked differences. This result is surprising as clusters are at mass
scales that are {\em a thousand times greater} than that at which structure
formation is suppressed. WDM clusters gain significantly more mass via smooth
accretion and contain fewer substructures than their CDM brethren. The higher
smooth mass accretion results in subhaloes which are physically more extended
and less dense. These fine-scale differences can be probed by strong
gravitational lensing. We find, unexpectedly, that WDM clusters have {\em
higher} lensing efficiencies than those in CDM cosmologies, contrary to the
naive expectation that WDM clusters should be less efficient due to the fewer
substructures they contain. Despite being less dense, the larger WDM subhaloes
are more likely to have larger lensing cross-sections than CDM ones.
Additionally, WDM subhaloes typically reside at larger distances, which
radially stretches the critical lines associated with strong gravitational
lensing, resulting in excess in the number of clusters with large radial
cross-sections at the level. Though lensing profile for an
individual cluster vary significantly with the line-of-sight, the radial arc
distribution based on a sample of clusters may prove to be the
crucial test for the presence of WDM.Comment: 13 pages, 14 figures, submitted to MNRA
Hidden from view: Coupled Dark Sector Physics and Small Scales
We study cluster mass dark matter haloes, their progenitors and surroundings
in an coupled Dark Matter-Dark Energy model and compare it to quintessence and
CDM models with adiabatic zoom simulations. When comparing cosmologies
with different expansions histories, growth functions & power spectra, care
must be taken to identify unambiguous signatures of alternative cosmologies.
Shared cosmological parameters, such as , need not be the same for
optimal fits to observational data. We choose to set our parameters to
CDM values. We find that in coupled models, where DM decays into
DE, haloes appear remarkably similar to CDM haloes despite DM
experiencing an additional frictional force. Density profiles are not
systematically different and the subhalo populations have similar mass, spin,
and spatial distributions, although (sub)haloes are less concentrated on
average in coupled cosmologies. However, given the scatter in related
observables (), this difference is unlikely to
distinguish between coupled and uncoupled DM. Observations of satellites of MW
and M31 indicate a significant subpopulation reside in a plane. Coupled models
do produce planar arrangements of satellites of higher statistical significance
than CDM models, however, in all models these planes are dynamically
unstable. In general, the nonlinear dynamics within and near large haloes masks
the effects of a coupled dark sector. The sole environmental signature we find
is that small haloes residing in the outskirts are more deficient in baryons
than their CDM counterparts. The lack of a pronounced signal for a
coupled dark sector strongly suggests that such a phenomena would be
effectively hidden from view.Comment: 13 pages, 14 figures, 2 tables, accepted for publication in MNRA
Heating of galactic gas by dark matter annihilation in ultracompact minihalos
The existence of substructure in halos of annihilating dark matter would be
expected to substantially boost the rate at which annihilation occurs.
Ultracompact minihalos of dark matter (UCMHs) are one of the more extreme
examples of this. The boosted annihilation can inject significant amounts of
energy into the gas of a galaxy over its lifetime. Here we determine the impact
of the boost factor from UCMH substructure on the heating of galactic gas in a
Milky Way-type galaxy, by means of N-body simulation. If of the dark
matter exists as UCMHs, the corresponding boost factor can be of order .
For reasonable values of the relevant parameters (annihilation cross section
, dark matter mass 100 GeV,
10% heating efficiency), we show that the presence of UCMHs at the 0.1% level
would inject enough energy to eject significant amounts of gas from the halo,
potentially preventing star formation within 1 kpc of the halo centre.Comment: 14 pages, 3 figure
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