71 research outputs found
The baryons in the Milky Way satellites
We investigate the formation and evolution of satellite galaxies using smoothed particle hydrodynamics (SPH) simulations of a Milky Way (MW) like system, focusing on the best resolved examples, analogous to the classical MW satellites. Comparing with a pure dark matter simulation, we find that the condensation of baryons has had a relatively minor effect on the structure of the satellites’ dark matter haloes. The stellar mass that forms in each satellite agrees relatively well over three levels of resolution (a factor of ∼64 in particle mass) and scales with (sub)halo mass in a similar way in an independent semi-analytical model. Our model provides a relatively good match to the average luminosity function of the MW and M31. To establish whether the potential wells of our satellites are realistic, we measure their masses within observationally determined half-light radii, finding that they have somewhat higher mass-to-light ratios than those derived for the MW dSphs from stellar kinematic data; the most massive examples are most discrepant. A statistical test yields an ∼6 per cent probability that the simulated and observationally derived distributions of masses are consistent. If the satellite population of the MW is typical, our results could imply that feedback processes not properly captured by our simulations have reduced the central densities of subhaloes, or that they initially formed with lower concentrations, as would be the case, for example, if the dark matter were made of warm, rather than cold particles
Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion
We perform a suite of smoothed particle hydrodynamics simulations to investigate in detail the results of a giant impact on the young Uranus. We study the internal structure, rotation rate, and atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit. Most of the material from the impactor's rocky core falls in to the core of the target. However, for higher angular momentum impacts, significant amounts become embedded anisotropically as lumps in the ice layer. Furthermore, most of the impactor's ice and energy is deposited in a hot, high-entropy shell at a radius of ~3 R ⊕. This could explain Uranus' observed lack of heat flow from the interior and be relevant for understanding its asymmetric magnetic field. We verify the results from the single previous study of lower resolution simulations that an impactor with a mass of at least 2 M ⊕ can produce sufficiently rapid rotation in the post-impact Uranus for a range of angular momenta. At least 90% of the atmosphere remains bound to the final planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 M ⊕ impactor. This atmospheric erosion peaks for intermediate impactor angular momenta (~3 × 1036 kg m2 s−1). Rock is more efficiently placed into orbit and made available for satellite formation by 2 M ⊕ impactors than 3 M ⊕ ones, because it requires tidal disruption that is suppressed by the more massive impactors
Evidence for explosive silicic volcanism on the Moon from the extended distribution of thorium near the Compton-Belkovich Volcanic Complex
We reconstruct the abundance of thorium near the Compton-Belkovich Volcanic Complex on the Moon, using data from the Lunar Prospector Gamma Ray Spectrometer. We enhance the resolution via a pixon image reconstruction technique and find that the thorium is distributed over a larger (40km × 75 km) area than the (25km × 35 km) high-albedo region normally associated with Compton-Belkovich. Our reconstructions show that inside this region, the thorium concentration is 14–26ppm. We also find additional thorium, spread up to 300km eastward of the complex at ∼2 ppm. The thorium must have been deposited during the formation of the volcanic complex, because subsequent lateral transport mechanisms, such as small impacts, are unable to move sufficient material. The morphology of the feature is consistent with pyroclastic dispersal, and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma
An Isocurvature Mechanism for Structure Formation
We examine a novel mechanism for structure formation involving initial number
density fluctuations between relativistic species, one of which then undergoes
a temporary downward variation in its equation of state and generates
superhorizon-scale density fluctuations. Isocurvature decaying dark matter
models (iDDM) provide concrete examples. This mechanism solves the
phenomenological problems of traditional isocurvature models, allowing iDDM
models to fit the current CMB and large-scale structure data, while still
providing novel behavior. We characterize the decaying dark matter and its
decay products as a single component of ``generalized dark matter''. This
simplifies calculations in decaying dark matter models and others that utilize
this mechanism for structure formation.Comment: 4 pages, 3 figures, submitted to PRD (rapid communications
Cluster Masses Accounting for Structure along the Line of Sight
Weak gravitational lensing of background galaxies by foreground clusters
offers an excellent opportunity to measure cluster masses directly without
using gas as a probe. One source of noise which seems difficult to avoid is
large scale structure along the line of sight. Here I show that, by using
standard map-making techniques, one can minimize the deleterious effects of
this noise. The resulting uncertainties on cluster masses are significantly
smaller than when large scale structure is not properly accounted for, although
still larger than if it was absent altogether.Comment: 5 pages, 5 figure
Effect of halo modelling on WIMP exclusion limits
WIMP direct detection experiments are just reaching the sensitivity required
to detect galactic dark matter in the form of neutralinos. Data from these
experiments are usually analysed under the simplifying assumption that the
Milky Way halo is an isothermal sphere with maxwellian velocity distribution.
Observations and numerical simulations indicate that galaxy halos are in fact
triaxial and anisotropic. Furthermore, in the cold dark matter paradigm
galactic halos form via the merger of smaller subhalos, and at least some
residual substructure survives. We examine the effect of halo modelling on WIMP
exclusion limits, taking into account the detector response. Triaxial and
anisotropic halo models, with parameters motivated by observations and
numerical simulations, lead to significant changes which are different for
different experiments, while if the local WIMP distribution is dominated by
small scale clumps then the exclusion limits are changed dramatically.Comment: 9 pages, 9 figures, version to appear in Phys. Rev. D, minor change
Measuring in the Early Universe: CMB Temperature, Large-Scale Structure and Fisher Matrix Analysis
We extend our recent work on the effects of a time-varying fine-structure
constant in the cosmic microwave background, by providing a thorough
analysis of the degeneracies between and the other cosmological
parameters, and discussing ways to break these with both existing and/or
forthcoming data. In particular, we present the state-of-the-art CMB
constraints on , through a combined analysis of the BOOMERanG, MAXIMA
and DASI datasets. We also present a novel discussion of the constraints on
coming from large-scale structure observations, focusing in particular
on the power spectrum from the 2dF survey. Our results are consistent with no
variation in from the epoch of recombination to the present day, and
restrict any such (relative) variation to be less than about 4%. We show that
the forthcoming MAP and (particularly) Planck experiments will be able to break
most of the currently existing degeneracies between and other
parameters, and measure to better than percent accuracy.Comment: 11 pages in RevTex4 format. Low-quality figures to comply with arXiv
restrictions (better ones available from the authors). v2: Updated Oklo
discussion, plus other cosmetic changes. Version to appear in Phys Rev
Self-Calibration of Cluster Dark Energy Studies: Counts in Cells
Cluster number counts can constrain the properties of dark energy if and only
if the evolution in the relationship between observable quantities and the
cluster mass can be calibrated. Next generation surveys with ~10000 clusters
will have sufficient statistics to enable some degree of self-calibration. The
excess variance of counts due to the clustering of clusters provides such an
opportunity and can be measured from the survey without additional
observational cost. It can minimize the degradation in dark energy constraints
due to an unknown power law evolution in the mass-observable relation improving
constraints on the dark energy equation of state by a factor of 2 or more to
sigma(w)=0.06 for a deep 4000 deg2 survey.Comment: 4 pages 2 figures submitted to PR
An Extended Zel'dovich Model for the Halo Mass Function
A new way to construct a fitting formula for the halo mass function is
presented. Our formula is expressed as a solution to the modified Jedamzik
matrix equation that automatically satisfies the normalization constraint. The
characteristic parameters expressed in terms of the linear shear eigenvalues
are empirically determined by fitting the analytic formula to the numerical
results from the high-resolution N-body simulation and found to be independent
of scale, redshift and background cosmology. Our fitting formula with the
best-fit parameters is shown to work excellently in the wide mass-range at
various redshifts: The ratio of the analytic formula to the N-body results
departs from unity by up to 10% and 5% over 10^{11}<= M/(M_sun/h)<= 5x10^{15}
at z=0,\ 0.5 and 1 for the FoF-halo and SO-halo cases, respectively.Comment: Accepted for publication in JCAP; 19pages, 9 figures, significantly
revised, discussion on the limitation of our model adde
On Dark Matter Annihilation in the Local Group
Under the hypothesis of a Dark Matter composed by supersymmetric particles
like neutralinos, we investigate the possibility that their annihilation in the
haloes of nearby galaxies could produce detectable fluxes of -photons.
Expected fluxes depend on several, poorly known quantities such as the density
profiles of Dark Matter haloes, the existence and prominence of central density
cusps and the presence of a population of sub-haloes. We find that, for all
reasonable choices of Dark Matter halo models, the intensity of the
-ray flux from some of the nearest extragalactic objects, like M31, is
comparable or higher than the diffuse Galactic foreground. We show that next
generation ground-based experiments could have the sensitivity to reveal such
fluxes which could help us unveiling the nature of Dark Matter particles.Comment: 11 pages, 10 figures. Accepted for publication in Phys. Rev. D.;
added a new paragraph on the detectability of Galactic sub-halos in our
Galaxy; added a discussion on their model dependence. The relation of our
results with the "CDM crisis" issue has also been adde
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