2,037 research outputs found
Surface Curvature Effects on Reflectance from Translucent Materials
Most of the physically based techniques for rendering translucent objects use
the diffusion theory of light scattering in turbid media. The widely used
dipole diffusion model (Jensen et al. 2001) applies the diffusion-theory
formula derived for a planar interface to objects of arbitrary shapes. This
paper presents first results of our investigation of how surface curvature
affects the diffuse reflectance from translucent materials.Comment: 10 pages, 2 figures. The first version of this paper was published in
the Communication Papers Proceedings of 18th International Conference on
Computer Graphics, Visualization and Computer Vision 2010 - WSCG201
Extensions of differential representations of SL(2) and tori
Linear differential algebraic groups (LDAGs) measure differential algebraic
dependencies among solutions of linear differential and difference equations
with parameters, for which LDAGs are Galois groups. The differential
representation theory is a key to developing algorithms computing these groups.
In the rational representation theory of algebraic groups, one starts with
SL(2) and tori to develop the rest of the theory. In this paper, we give an
explicit description of differential representations of tori and differential
extensions of irreducible representation of SL(2). In these extensions, the two
irreducible representations can be non-isomorphic. This is in contrast to
differential representations of tori, which turn out to be direct sums of
isotypic representations.Comment: 21 pages; few misprints corrected; Lemma 4.6 adde
Mapping extragalactic dark matter structures through gamma-rays
If dark matter is composed of neutralinos, the gamma-ray radiation produced
in their annihilation offers an attractive possibility for dark matter
detection. This process may contribute significantly to the extragalactic
gamma-ray background (EGB) radiation, which is being measured by the FERMI
satellite with unprecedented sensitivity. Using the high-resolution
Millennium-II simulation of cosmic structure formation we have produced the
first full-sky maps of the expected contribution of dark matter annihilation to
the EGB radiation. Our maps include a proper normalization of the signal
according to a specific supersymmetric model based on minimal supergravity. The
new simulated maps allow a study of the angular power spectrum of the gamma-ray
background from dark matter annihilation, which has distinctive features
associated with the nature of the annihilation process. Our results are in
broad agreement with analytic models for the gamma-ray background, but they
also include higher-order correlations not readily accessible in analytic
calculations and, in addition, provide detailed spectral information for each
pixel. In particular, we find that color maps combining different energies can
reveal the cosmic large-scale structure at low and intermediate redshifts.Comment: 7 pages, 5 figures, 2009 Fermi Symposium, eConf Proceedings C09112
Dynamical Friction and Galaxy Merging Timescales
The timescale for galaxies within merging dark matter halos to merge with
each other is an important ingredient in galaxy formation models. Accurate
estimates of merging timescales are required for predictions of astrophysical
quantities such as black hole binary merger rates, the build-up of stellar mass
in central galaxies, and the statistical properties of satellite galaxies
within dark matter halos. In this paper, we study the merging timescales of
extended dark matter halos using N-body simulations. We compare these results
to standard estimates based on the Chandrasekhar theory of dynamical friction.
We find that these standard predictions for merging timescales, which are often
used in semi-analytic galaxy formation models, are systematically shorter than
those found in simulations. The discrepancy is approximately a factor of 1.7
for and becomes larger for more disparate
satellite-to-host mass ratios, reaching a factor of for
. Based on our simulations, we propose a new, easily
implementable fitting formula that accurately predicts the timescale for an
extended satellite to sink from the virial radius of a host halo down to the
halo's center for a wide range of and orbits. Including a
central bulge in each galaxy changes the merging timescale by \la 10%. To
highlight one concrete application of our results, we show that merging
timescales often used in the literature overestimate the growth of stellar mass
by satellite accretion by , with the extra mass gained in low
mass ratio mergers.Comment: 10 pages, 7 figures; MNRAS, in press. Minor revisions, including
results from additional simulations with baryonic components; conclusions
unchange
Dynamics of the Magellanic Clouds in a LCDM Universe
We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II
Simulation in order to place the orbits of the Magellanic Clouds in a
cosmological context. Our analysis shows that satellites massive enough to be
LMC analogs are typically accreted at late times. Moreover, those that are
accreted at early times and survive to the present have orbital properties that
are discrepant with those observed for the LMC. The high velocity of the LMC,
coupled with the dearth of unbound orbits seen in the simulation, argues that
the mass of the MW's halo is unlikely to be less than 2 x 10^12 Msun. This
conclusion is further supported by statistics of halos hosting satellites with
masses, velocities, and separations comparable to those of the LMC. We further
show that: (1) LMC and SMC-mass objects are not particularly uncommon in
MW-mass halos; (2) the apparently high angular momentum of the LMC is not
cosmologically unusual; and (3) it is rare for a MW halo to host a LMC-SMC
binary system at z=0, but high speed binary pairs accreted at late times are
possible. Based on these results, we conclude that the LMC was accreted within
the past four Gyr and is currently making its first pericentric passage about
the MW.Comment: 14 pages, 13 figures; MNRAS, in press. Minor revisions, conclusions
unchange
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