67 research outputs found
Cosmology through arc statistics I: sensitivity to and
The next generation of large sky photometric surveys will finally be able to
use arc statistics as a cosmological probe. Here we present the first of a
series of papers on this topic. In particular, we study how arc counts are
sensitive to the variation of two cosmological parameters: the (total) matter
density parameter, , and the normalisation of the primordial power
spectrum, expressed in terms of . Both these parameters influence the
abundances of collapsed structures and their internal structure. We compute the
expected number of gravitational arcs with various length-to-width ratios in
mock light cones, by varying these cosmological parameters in the ranges
and . We find that the arc
counts dependence on and is similar, but not identical,
to that of the halo counts. We investigate how the precision of the constraints
on the cosmological parameters based on arc counts depends on the survey area.
We find that the constraining power of arc statistics degrades critically only
for surveys covering an area smaller than of the whole sky. Finally, we
consider the case in which the search for arcs is done only in frames where
galaxy clusters have been previously identified. Adopting the selection
function for galaxy clusters expected to be detected from photometric data in
future wide surveys, we find that less than of the arcs will be missed,
with only a small degradation of the corresponding cosmological constraints.Comment: 12 pages, 12 figures, accepted by MNRA
Trapping light with micro lenses in thin film organic photovoltaic cells.
We demonstrate a novel light trapping configuration based on an array of micro lenses in conjunction with a self aligned array of micro apertures located in a highly reflecting mirror. When locating the light trapping element, that displays strong directional asymmetric transmission, in front of thin film organic photovoltaic cells, an increase in cell absorption is obtained. By recycling reflected photons that otherwise would be lost, thinner films with more beneficial electrical properties can effectively be deployed. The light trapping element enhances the absorption rate of the solar cell and increases the photocurrent by as much as 25%. (C) 2008 Optical Society of Americ
Mass Distributions of HST Galaxy Clusters from Gravitational Arcs
Although N-body simulations of cosmic structure formation suggest that dark
matter halos have density profiles shallower than isothermal at small radii and
steeper at large radii, whether observed galaxy clusters follow this profile is
still ambiguous. We use one such density profile, the asymmetric NFW profile,
to model the mass distributions of 11 galaxy clusters with gravitational arcs
observed by HST. We characterize the galaxy lenses in each cluster as NFW
ellipsoids, each defined by an unknown scale convergence, scale radius,
ellipticity, and position angle. For a given set of values of these parameters,
we compute the arcs that would be produced by such a lens system. To define the
goodness of fit to the observed arc system, we define a chi^2 function
encompassing the overlap between the observed and reproduced arcs as well as
the agreement between the predicted arc sources and the observational
constraints on the source system. We minimize this chi^2 to find the values of
the lens parameters that best reproduce the observed arc system in a given
cluster. Here we report our best-fit lens parameters and corresponding mass
estimates for each of the 11 lensing clusters. We find that cluster mass models
based on lensing galaxies defined as NFW ellipsoids can accurately reproduce
the observed arcs, and that the best-fit parameters to such a model fall within
the reasonable ranges defined by simulations. These results assert NFW profiles
as an effective model for the mass distributions of observed clusters.Comment: Submitted to ApJ, 14 figures include
Strong lensing by cluster-sized haloes in dark-energy cosmologies
We study the efficiency of numerically simulated galaxy clusters for
producing strong gravitational lensing events in different dark-energy
cosmologies with constant and time-variable equation of state and we compare it
with ``standard'' CDM and OCDM models. Our main results are that: (1)
the expected abundance of gravitational arcs with large length-to-width ratio
depends on the equation of state of dark energy at the epoch of formation of
the halo; (2) as found in previous studies, the strong lensing cross section of
galaxy clusters is very sensitive to dynamical processes like mergers, whose
impact however is different for different cosmologies, being stronger for
models in which halos are less concentrated. As expected, the largest
differences in the lensing optical depth occur at moderate and high redshift.Comment: 10 pages, 6 figures, submitted to A&
Evolution of dark-matter haloes in a variety of dark-energy cosmologies
High-resolution, numerical simulations of 17 cluster-sized dark-matter haloes
in eight different cosmologies with and without dynamical dark energy confirm
the picture that core halo densities are imprinted early during their formation
by the mean cosmological density. Quite independent of cosmology, halo
concentrations have a log-normal distribution with a scatter of ~0.2 about the
mean. We propose a simple scaling relation for halo concentrations in
dark-energy cosmologies.Comment: proceedings contribution to "Dark Matter/Dark Energy 2004
The impact of cluster mergers on arc statistics
We study the impact of merger events on the strong lensing properties of
galaxy clusters. Previous lensing simulations were not able to resolve
dynamical time scales of cluster lenses, which arise on time scales which are
of order a Gyr. In this case study, we first describe qualitatively with an
analytic model how some of the lensing properties of clusters are expected to
change during merging events. We then analyse a numerically simulated lens
model for the variation in its efficiency for producing both tangential and
radial arcs while a massive substructure falls onto the main cluster body. We
find that: (1) during the merger, the shape of the critical lines and caustics
changes substantially; (2) the lensing cross sections for long and thin arcs
can grow by one order of magnitude and reach their maxima when the extent of
the critical curves is largest; (3) the cross section for radial arcs also
grows, but the cluster can efficiently produce this kind of arcs only while the
merging substructure crosses the main cluster centre; (4) while the arc cross
sections pass through their maxima as the merger proceeds, the cluster's X-ray
emission increases by a factor of . Thus, we conclude that accounting
for these dynamical processes is very important for arc statistics studies. In
particular, they may provide a possible explanation for the arc statistics
problem.Comment: 16 pages, submitted to MNRAS, revised version after referee'
Comments. Gzipped file including full resolution images can be downloaded at
http://dipastro.pd.astro.it/~cosmo/massimo/high-res-images.tar.g
Growth Rate, Slaughter Traits and Meat Quality of Lambs of Three Alpine Sheep Breeds
A fattening and slaughter trial was carried out on 36 lambs of Alpagota, Brogna and Foza sheep breeds native of Veneto Region Alps (six male and six female lambs per breed) divided in three groups depending on the type of feed used: pasture, hay and concentrate, hay and concentrate supplemented with conjugated linoleic acid. Lambs were slaughtered at 225 days (mean weight: 30 kg). Infra-vitam and post-mortem data were analyzed by using a linear model that included the cross-classified effect of breed, sex, feeding system and age of lambs as linear covariate. The three breeds showed some specificity: Foza lambs, of both sexes, were larger-sized and faster growing, with a lower incidence of gastro-intestinal tract and lower cooking losses of the hind-leg samples compared to the other two middle-sized breeds. Alpagota breed tended to be leaner, with heavier shin and greater cooking losses than Brogna breed.
In conclusion the three Alpine breeds of the Veneto Region confirmed to be able to produce lamb carcasses and meat with valuable characteristics that can be exploited through typical products and food preparation in local markets and gastronomy, according to the tradition. The valorisation of these productions can be an important instrument for in situ conservation of these breeds
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