1,135 research outputs found
Monte Carlo simulation of multiple scattered light in the atmosphere
We present a Monte Carlo simulation for the scattering of light in the case
of an isotropic light source. The scattering phase functions are studied
particularly in detail to understand how they can affect the multiple light
scattering in the atmosphere. We show that although aerosols are usually in
lower density than molecules in the atmosphere, they can have a non-negligible
effect on the atmospheric point spread function. This effect is especially
expected for ground-based detectors when large aerosols are present in the
atmosphere.Comment: 5 pages. Proceedings of the Atmospheric Monitoring for High-Energy
Astroparticle Detectors (AtmoHEAD) Conference, Saclay (France), June 10-12,
201
Atmospheric multiple scattering of fluorescence light from extensive air showers and effect of the aerosol size on the reconstruction of energy and depth of maximum
The reconstruction of the energy and the depth of maximum of an
extensive air shower depends on the multiple scattering of fluorescence photons
in the atmosphere. In this work, we explain how atmospheric aerosols, and
especially their size, scatter the fluorescence photons during their
propagation. Using a Monte Carlo simulation for the scattering of light, the
dependence on the aerosol conditions of the multiple scattered light
contribution to the recorded signal is fully parameterised. A clear dependence
on the aerosol size is proposed for the first time. Finally, using this new
parameterisation, the effect of atmospheric aerosols on the energy and on the
reconstructions is presented for a vertical extensive air shower
observed by a ground-based detector at km: for typical aerosol conditions,
multiple scattering leads to a systematic over-estimation of for
the energy and g/cm for the , where the
uncertainties refer to a variation of the aerosol size.Comment: 12 pages, 9 figures, journal paper, accepted in Astroparticle
Physics. arXiv admin note: text overlap with arXiv:1310.170
Void Statistics and Hierarchical Scaling in the Halo Model
We study scaling behaviour of statistics of voids in the context of the halo
model of nonlinear large-scale structure. The halo model allows us to
understand why the observed galaxy void probability obeys hierarchical scaling,
even though the premise from which the scaling is derived is not satisfied. We
argue that the commonly observed negative binomial scaling is not fundamental,
but merely the result of the specific values of bias and number density for
typical galaxies. The model implies quantitative relations between void
statistics measured for two populations of galaxies, such as SDSS red and blue
galaxies, and their number density and bias.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Building Merger Trees from Cosmological N-body Simulations
Although a fair amount of work has been devoted to growing Monte-Carlo merger
trees which resemble those built from an N-body simulation, comparatively
little effort has been invested in quantifying the caveats one necessarily
encounters when one extracts trees directly from such a simulation. To somewhat
revert the tide, this paper seeks to provide its reader with a comprehensive
study of the problems one faces when following this route. The first step to
building merger histories of dark matter haloes and their subhaloes is to
identify these structures in each of the time outputs (snapshots) produced by
the simulation. Even though we discuss a particular implementation of such an
algorithm (called AdaptaHOP) in this paper, we believe that our results do not
depend on the exact details of the implementation but extend to most if not all
(sub)structure finders. We then highlight different ways to build merger
histories from AdaptaHOP haloes and subhaloes, contrasting their various
advantages and drawbacks. We find that the best approach to (sub)halo merging
histories is through an analysis that goes back and forth between
identification and tree building rather than one which conducts a
straightforward sequential treatment of these two steps. This is rooted in the
complexity of the merging trees which have to depict an inherently dynamical
process from the partial temporal information contained in the collection of
instantaneous snapshots available from the N-body simulation.Comment: 19 pages, 28 figure
Reconstruction of primordial density fields
The Monge-Ampere-Kantorovich (MAK) reconstruction is tested against
cosmological N-body simulations. Using only the present mass distribution
sampled with particles, and the assumption of homogeneity of the primordial
distribution, MAK recovers for each particle the non-linear displacement field
between its present position and its Lagrangian position on a primordial
uniform grid. To test the method, we examine a standard LCDM N-body simulation
with Gaussian initial conditions and 6 models with non-Gaussian initial
conditions: a chi-squared model, a model with primordial voids and four weakly
non-Gaussian models. Our extensive analyses of the Gaussian simulation show
that the level of accuracy of the reconstruction of the nonlinear displacement
field achieved by MAK is unprecedented, at scales as small as about 3 Mpc. In
particular, it captures in a nontrivial way the nonlinear contribution from
gravitational instability, well beyond the Zel'dovich approximation. This is
also confirmed by our analyses of the non-Gaussian samples. Applying the
spherical collapse model to the probability distribution function of the
divergence of the displacement field, we also show that from a
well-reconstructed displacement field, such as that given by MAK, it is
possible to accurately disentangle dynamical contributions induced by
gravitational clustering from possible initial non-Gaussianities, allowing one
to efficiently test the non-Gaussian nature of the primordial fluctuations. In
addition, a simple application of MAK using the Zel'dovich approximation allows
one to also recover accurately the present-day peculiar velocity field on
scales of about 8 Mpc.Comment: Version to appear in MNRAS, 24 pages, 21 figures appearing (uses 35
figure files), 1 tabl
Fluid-loaded metasurfaces
We consider wave propagation along fluid-loaded structures which take the
form of an elastic plate augmented by an array of resonators forming a
metasurface, that is, a surface structured with sub-wavelength resonators. Such
surfaces have had considerable recent success for the control of wave
propagation in electromagnetism and acoustics, by combining the vision of
sub-wavelength wave manipulation, with the design, fabrication and size
advantages associated with surface excitation. We explore one aspect of recent
interest in this field: graded metasurfaces, but within the context of
fluid-loaded structures.
Graded metasurfaces allow for selective spatial frequency separation and are
often referred to as exhibiting rainbow trapping. Experiments, and theory, have
been developed for acoustic, electromagnetic, and even elastic, rainbow devices
but this has not been approached for fluid-loaded structures that support
surface waves coupled with the acoustic field in a bulk fluid. This surface
wave, coupled with the fluid, can be used to create an additional effect by
designing a metasurface to mode convert from surface to bulk waves. We
demonstrate that sub-wavelength control is possible and that one can create
both rainbow trapping and mode conversion phenomena for a fluid-loaded elastic
plate model.Comment: 13 pages, 10 figure
The redshift evolution of bias and baryonic matter distribution
We study the distribution of baryonic and luminous matter within the
framework of a hierarchical scenario. Using an analytical model for structure
formation which has already been checked against observations for galaxies,
Lyman- clouds, clusters and reionization processes, we present its
predictions for the bias of these objects. We describe its dependence on the
luminosity (for galaxies or quasars) or the column density (for Lyman-
absorbers) of the considered objects. We also study its redshift evolution,
which can exhibit an intricate behaviour. These astrophysical objects do not
trace the dark matter density field, the Lyman- forest clouds being
undercorrelated and the bright galaxies overcorrelated, while the intermediate
class of Lyman-limit systems is seen to sample the matter field quite well.
We also present the distribution of baryonic matter over these various
objects. We show that light does not trace baryonic mass, since bright galaxies
which contain most of the stars only form a small fraction of the mass
associated with virialized and cooled halos. We consider two cosmologies: a
critical density universe and an open universe. In both cases, our results
agree with observations and show that hierarchical scenarios provide a good
model for structure formation and can describe a wide range of objects which
spans at least the seven orders of magnitude in mass for which data exist. More
detailed observations, in particular of the clustering evolution of galaxies,
will constrain the astrophysical models involved.Comment: 13 pages, final version published in A&
Evolution of hierarchical clustering in the CFHTLS-Wide since z~1
We present measurements of higher order clustering of galaxies from the
latest release of the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS)
Wide. We construct a volume-limited sample of galaxies that contains more than
one million galaxies in the redshift range 0.2<z<1 distributed over the four
independent fields of the CFHTLS. We use a counts in cells technique to measure
the variance and the hierarchical moments S_n = /^(n-1)
(3<n<5) as a function of redshift and angular scale.The robustness of our
measurements if thoroughly tested, and the field-to-field scatter is in very
good agreement with analytical predictions. At small scales, corresponding to
the highly non-linear regime, we find a suggestion that the hierarchical
moments increase with redshift. At large scales, corresponding to the weakly
non-linear regime, measurements are fully consistent with perturbation theory
predictions for standard LambdaCDM cosmology with a simple linear bias.Comment: 17 pages, 11 figures, submitted to MNRA
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