1,505 research outputs found
RX J0911+05: A Massive Cluster Lens at z=0.769
We report the detection of a massive high-redshift cluster of galaxies near
the quadruple quasar RX J0911+05, using the LRIS instrument on the Keck-II
telescope. The cluster is found to have a mean redshift of =0.7689+/-0.002
and a velocity dispersion of sigma=836{+180-200} km/s, based on redshift
measurements for 24 member galaxies. This massive high-redshift cluster is the
origin of the unusually large external shear required by lensing models of the
quadruple quasar system. We predict the expected time delay depending on the
exact contribution of the cluster. A measurement of the time delay and further
deep lensing and X-ray observations will unravel useful properties of this
serendipitously discovered high-redshift cluster, and may put interesting
cosmological constraints on H0.Comment: Submitted to ApJL, 7 pages, 5 figure
A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing
Spergel & Steinhardt proposed the possibility that the dark matter particles
are self-interacting, as a solution to two discrepancies between the
predictions of cold dark matter models and the observations: first, the
observed dark matter distribution in some dwarf galaxies has large,
constant-density cores, as opposed to the predicted central cusps; and second,
small satellites of normal galaxies are much less abundant than predicted. The
dark matter self-interaction would produce isothermal cores in halos, and would
also expel the dark matter particles from dwarfs orbiting within large halos.
However, another inevitable consequence of the model is that halos should
become spherical once most particles have interacted. Here, I rule out this
model by the fact that the innermost regions of dark matter halos in massive
clusters of galaxies are elliptical, as shown by gravitational lensing and
other observations. The absence of collisions in the lensing cores of massive
clusters implies that any dark matter self-interaction is too weak to have
affected the observed density profiles in the dark-matter dominated dwarf
galaxies, or to have eased the destruction of dwarf satellites in galactic
halos. If is the cross section and the mass of the dark matter
particle, then s_x/m_x < 10^{-25.5} \cm^2/\gev.Comment: to appear in ApJ, January 1 200
Gravitational Lensing in Clusters of Galaxies
Gravitational lensing in clusters of galaxies is an efficient tool to probe
the mass distribution of galaxies and clusters, high redshift objects thanks to
the gravitational amplification, and the geometry of the universe. We review
some important aspects of cluster lensing and related issues in observational
cosmology.Comment: invited review of the journal: Progress of Theoretical Physics (in
press) 51 pages - 33 figure
Galaxy Galaxy Lensing as a Probe of Galaxy Dark Matter Halos
Gravitational lensing has now become a popular tool to measure the mass
distribution of structures in the Universe on various scales. Here we focus on
the study of galaxy's scale dark matter halos with galaxy-galaxy lensing
techniques: observing the shapes of distant background galaxies which have been
lensed by foreground galaxies allows us to map the mass distribution of the
foreground galaxies. The lensing effect is small compared to the intrinsic
ellipticity distribution of galaxies, thus a statistical approach is needed to
derive some constraints on an average lens population. An advantage of this
method is that it provides a probe of the gravitational potential of the halos
of galaxies out to very large radii, where few classical methods are viable,
since dynamical and hydrodynamical tracers of the potential cannot be found at
this radii. We will begin by reviewing the detections of galaxy-galaxy lensing
obtained so far. Next we will present a maximum likelihood analysis of
simulated data we performed to evaluate the accuracy and robustness of
constraints that can be obtained on galaxy halo properties. Then we will apply
this method to study the properties of galaxies which stand in massive cluster
lenses at z~0.2. The main result of this work is to find dark matter halos of
cluster galaxies to be significantly more compact compared to dark matter halos
around field galaxies of equivalent luminosity, in agreement with early
galaxy-galaxy lensing studies and with theoretical expectations, in particular
with the tidal stripping scenario. We thus provide a strong confirmation of
tidal truncation from a homogeneous sample of galaxy clusters. Moreover, it is
the first time that cluster galaxies are probed successfully using
galaxy-galaxy lensing techniques from ground based data.Comment: 8 pages, 5 figures, to appear in Moriond Proceedings, From Dark Halos
to Ligh
Multiple-Images in the Cluster Lens Abell 2218: Constraining the Geometry of the Universe ?
In this Letter we present a detailed study of the lensing configuration in
the cluster Abell 2218. Four multiple-images systems with measured
spectroscopic redshifts have been identified in this cluster. These multiple
images are very useful to constrain accurately the mass distribution in the
cluster core, but they are also sensitive to the value of the geometrical
cosmological parameters of the Universe. Using a simplified maximum likelihood
analysis we find 0 < Omega_M < 0.30 assuming a flat Universe, and 0 < Omega_M <
0.33 and w < -0.85 for a flat Universe with dark energy. Interestingly, an
Einstein-de Sitter model is excluded at more than 4sigma. These constraints are
consistent with the current constraints derived with CMB anisotropies or
supernovae studies. The proposed method constitutes an independent test of the
geometrical cosmological parameters of the Universe and we discuss the limits
of this method and this particular application to Abell 2218. Application of
this method with more sophisticated tools and to a larger number of clusters or
with more multiple images constraints, will put stringent constraints on the
geometrical cosmological parameters.Comment: 5 pages, 3 figures. Accepted for publication by Astronomy &
Astrophysic
Semiparametric Multinomial Logit Models for Analysing Consumer Choice Behaviour
The multinomial logit model (MNL) is one of the most frequently used statistical models in marketing applications. It allows to relate an unordered categorical response variable, for example representing the choice of a brand, to a vector of covariates such as the price of the brand or variables characterising the consumer. In its classical form, all covariates enter in strictly parametric, linear form into the utility function of the MNL model. In this paper, we introduce semiparametric extensions, where smooth effects of continuous covariates are modelled by penalised splines. A mixed model representation of these penalised splines is employed to obtain estimates of the corresponding smoothing parameters, leading to a fully automated estimation procedure. To validate semiparametric models against parametric models, we utilise proper scoring rules and compare parametric and semiparametric approaches for a number of brand choice data sets
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