202 research outputs found
Contending cultures of counterterrorism: transatlantic divergence or convergence?
Terrorist attacks on the United States, Spain and the United Kingdom have underlined the differing responses of Europe and the United States to the 'new terrorism'. This article analyses these responses through the prism of historically determined strategic cultures. For the last four years the United States has directed the full resources of a 'national security' approach towards this threat and has emphasized unilateralism. Europe, based on its own past experience of terrorism, has adopted a regulatory approach pursued through multilateralism. These divergences in transatlantic approaches, with potentially major implications for the future of the relationship, have appeared to be mitigated by a revised American strategy of counterterrorism that has emerged during 2005. However, this article contends that while strategic doctrines may change, the more immutable nature of strategic culture will make convergence difficult. This problem will be compounded by the fact that neither Europe nor America have yet addressed the deeper connections between terrorism and the process of globalization
Direct measurement of dark matter halo ellipticity from two-dimensional lensing shear maps of 25 massive clusters
We present new measurements of dark matter distributions in 25 X-ray luminous
clusters by making a full use of the two-dimensional (2D) weak lensing signals
obtained from high-quality Subaru/Suprime-Cam imaging data. Our approach to
directly compare the measured lensing shear pattern with elliptical model
predictions allows us to extract new information on the mass distributions of
individual clusters, such as the halo ellipticity and mass centroid. We find
that these parameters on the cluster shape are little degenerate with cluster
mass and concentration parameters. By combining the 2D fitting results for a
subsample of 18 clusters, the elliptical shape of dark matter haloes is
detected at 7\sigma significance level. The mean ellipticity is found to be e =
0.46 \pm 0.04 (1\sigma), which is in excellent agreement with the standard
collisionless CDM model prediction. The mass centroid can be constrained with a
typical accuracy of ~20" (~50 kpc/h) in radius for each cluster with some
significant outliers, enabling to assess one of the most important systematic
errors inherent in the stacked cluster weak lensing technique, the mass
centroid uncertainty. In addition, the shape of the dark mass distribution is
found to be only weakly correlated with that of the member galaxy distribution.
We carefully examine possible sources of systematic errors in our measurements,
finding none of them to be significant. Our results demonstrate the power of
high-quality imaging data for exploring the detailed spatial distribution of
dark matter (Abridged).Comment: 17 pages, 10 figures, MNRAS in pres
The Halos of Satellite Galaxies: the Companion of the Massive Elliptical Lens SL2S J08544-0121
Strong gravitational lensing by groups or clusters of galaxies provides a
powerful technique to measure the dark matter properties of individual lens
galaxies. We study in detail the mass distribution of the satellite lens galaxy
in the group-scale lens SL2S J08544-0121 by modelling simultaneously the
spatially extended surface brightness distribution of the source galaxy and the
lens mass distribution using Markov chain Monte Carlo methods. In particular,
we measure the dark matter halo size of the satellite lens galaxy to be
6.0^{+2.9}_{-2.0} kpc with a fiducial velocity dispersion of 127^{+21}_{-12}
km/s. This is the first time the size of an individual galaxy halo in a galaxy
group has been measured using strong gravitational lensing without assumptions
of mass following light. We verify the robustness of our halo size measurement
using mock data resembling our lens system. Our measurement of the halo size is
compatible with the estimated tidal radius of the satellite galaxy, suggesting
that halos of galaxies in groups experience significant tidal stripping, a
process that has been previously observed on galaxies in clusters. Our mass
model of the satellite galaxy is elliptical with its major axis misaligned with
that of the light by ~50 deg. The major axis of the total matter distribution
is oriented more towards the centre of the host halo, exhibiting the radial
alignment found in N-body simulations and observational studies of satellite
galaxies. This misalignment between mass and light poses a significant
challenge to modified Newtonian dynamics.Comment: 13 pages, 10 figures, minor revisions based on referee's comments,
accepted for publication in A&
Miscentring in Galaxy Clusters: Dark Matter to Brightest Cluster Galaxy Offsets in 10,000 SDSS Clusters
We characterise the typical offset between the Dark Matter (DM) projected
centre and the Brightest Cluster Galaxy (BCG) in 10,000 SDSS clusters. To place
constraints on the centre of DM, we use an automated strong-lensing analysis,
mass-modelling technique which is based on the well-tested assumption that
light traces mass. The cluster galaxies are modelled with a steep power-law,
and the DM component is obtained by smoothing the galaxy distribution fitting a
low-order 2D polynomial (via spline interpolation), while probing a whole range
of polynomial degrees and galaxy power laws. We find that the offsets between
the BCG and the peak of the smoothed light map representing the DM, \Delta, are
distributed equally around zero with no preferred direction, and are well
described by a log-normal distribution with <log_{10}(\Delta [h^{-1}
Mpc])>=-1.895^{+0.003}_{-0.004}, and \sigma=0.501\pm0.004 (95% confidence
levels), or =0.564\pm0.005, and
\sigma=0.475\pm0.007. Some of the offsets originate in prior misidentifications
of the BCG or other bright cluster members by the cluster finding algorithm,
whose level we make an additional effort to assess, finding that ~10% of the
clusters in the probed catalogue are likely to be misidentified, contributing
to higher-end offsets in general agreement with previous studies. Our results
constitute the first statistically-significant high-resolution distributions of
DM-to-BCG offsets obtained in an observational analysis, and importantly show
that there exists such a typical non-zero offset in the probed catalogue. The
offsets show a weak positive correlation with redshift, so that higher
separations are generally found for higher-z clusters in agreement with the
hierarchical growth of structure, which in turn could help characterise the
merger, relaxation and evolution history of clusters, in future studies.
[ABRIDGED]Comment: 15 pages, 11 figures; MNRAS in press; V3 includes minor text update
Cooperative Material Handling by Human and Robotic Agents:Module Development and System Synthesis
In this paper we present the results of a collaborative effort to design and implement a system for cooperative material handling by a small team of human and robotic agents in an unstructured indoor environment. Our approach makes fundamental use of human agents\u27 expertise for aspects of task planning, task monitoring, and error recovery. Our system is neither fully autonomous nor fully teleoperated. It is designed to make effective use of human abilities within the present state of the art of autonomous systems. It is designed to allow for and promote cooperative interaction between distributed agents with various capabilities and resources. Our robotic agents refer to systems which are each equipped with at least one sensing modality and which possess some capability for self-orientation and/or mobility. Our robotic agents are not required to be homogeneous with respect to either capabilities or function. Our research stresses both paradigms and testbed experimentation. Theory issues include the requisite coordination principles and techniques which are fundamental to the basic functioning of such a cooperative multi-agent system. We have constructed a testbed facility for experimenting with distributed multi-agent architectures. The required modular components of this testbed are currently operational and have been tested individually. Our current research focuses on the integration of agents in a scenario for cooperative material handling
Combined strong and weak lensing analysis of 28 clusters from the Sloan Giant Arcs Survey
We study the mass distribution of a sample of 28 galaxy clusters using strong
and weak lensing observations. The clusters are selected via their strong
lensing properties as part of the Sloan Giant Arcs Survey (SGAS) from the Sloan
Digital Sky Survey (SDSS). Mass modelling of the strong lensing information
from the giant arcs is combined with weak lensing measurements from deep
Subaru/Suprime-cam images to primarily obtain robust constraints on the
concentration parameter and the shape of the mass distribution. We find that
the concentration c_vir is a steep function of the mass, c_vir \propto
M_vir^-0.59\pm0.12, with the value roughly consistent with the
lensing-bias-corrected theoretical expectation for high mass (10^15 h^-1 M_sun)
clusters. However, the observationally inferred concentration parameters appear
to be much higher at lower masses (10^14 h^-1 M_sun), possibly a consequence of
the modification to the inner density profiles provided by baryon cooling. The
steep mass-concentration relation is also supported from direct stacking
analysis of the tangential shear profiles. In addition, we explore the
two-dimensional shape of the projected mass distribution by stacking weak
lensing shear maps of individual clusters with prior information on the
position angle from strong lens modelling, and find significant evidence for a
large mean ellipticity with the best-fit value of e = 0.47 \pm 0.06 for the
mass distribution of the stacked sample. We find that the luminous cluster
member galaxy distribution traces the overall mass distribution very well,
although the distribution of fainter cluster galaxies appears to be more
extended than the total mass.Comment: 29 pages, 15+9 figures, 7 tables, accepted for publication in MNRA
On the over-concentration problem of strong lensing clusters
Lambda cold dark matter paradigm predicts that galaxy clusters follow an
universal mass density profile and fit a well defined mass-concentration
relation, with lensing clusters being preferentially triaxial haloes elongated
along the line of sight. Oddly, recent strong and weak lensing analyses of
clusters with a large Einstein radius suggested those haloes to be highly
over-concentrated. Here, we investigate what intrinsic shape and orientation an
halo should have to account for both theoretical predictions and observations.
We considered a sample of 10 strong lensing clusters. We first measured their
elongation assuming a given mass-concentration relation. Then, for each cluster
we found the intrinsic shape and orientation which are compatible with the
inferred elongation and the measured projected ellipticity. We distinguished
two groups. The first one (nearly one half) seems to be composed of outliers of
the mass-concentration relation, which they would fit only if they were
characterised by a filamentary structure extremely elongated along the line of
sight, that is not plausible considering standard scenarios of structure
formations. The second sample supports expectations of N-body simulations which
prefer mildly triaxial lensing clusters with a strong orientation bias.Comment: 11 pages, 8 figures, in press on MNRA
Polarization as an indicator of intrinsic alignment in radio weak lensing
We propose a new technique for weak gravitational lensing in the radio band
making use of polarization information. Since the orientation of a galaxy's
polarized emission is both unaffected by lensing and is related to the galaxy's
intrinsic orientation, it effectively provides information on the unlensed
galaxy position angle. We derive a new weak lensing estimator which exploits
this effect and makes full use of both the observed galaxy shapes and the
estimates of the intrinsic position angles as provided by polarization. Our
method has the potential to both reduce the effects of shot noise, and to
reduce to negligible levels, in a model-independent way, all effects of
intrinsic galaxy alignments. We test our technique on simulated weak lensing
skies, including an intrinsic alignment contaminant consistent with recent
observations, in three overlapping redshift bins. Adopting a standard weak
lensing analysis and ignoring intrinsic alignments results in biases of 5-10%
in the recovered power spectra and cosmological parameters. Applying our new
estimator to one tenth the number of galaxies used for the standard case, we
recover both power spectra and the input cosmology with similar precision as
compared to the standard case and with negligible residual bias, even in the
presence of a substantial (astrophysical) scatter in the relationship between
the observed orientation of the polarized emission and the intrinsic
orientation. Assuming a reasonable polarization fraction for star-forming
galaxies, and no cosmological conspiracy in the relationship between
polarization direction and intrinsic morphology, our estimator should prove a
valuable tool for weak lensing analyses of forthcoming radio surveys, in
particular, deep wide field surveys with e-MERLIN, MeerKAT and ASKAP and
ultimately, definitive radio lensing surveys with the SKA.Comment: 18 pages, 10 figures, submitted to MNRA
The Third Gravitational Lensing Accuracy Testing (GREAT3) Challenge Handbook
The GRavitational lEnsing Accuracy Testing 3 (GREAT3) challenge is the third
in a series of image analysis challenges, with a goal of testing and
facilitating the development of methods for analyzing astronomical images that
will be used to measure weak gravitational lensing. This measurement requires
extremely precise estimation of very small galaxy shape distortions, in the
presence of far larger intrinsic galaxy shapes and distortions due to the
blurring kernel caused by the atmosphere, telescope optics, and instrumental
effects. The GREAT3 challenge is posed to the astronomy, machine learning, and
statistics communities, and includes tests of three specific effects that are
of immediate relevance to upcoming weak lensing surveys, two of which have
never been tested in a community challenge before. These effects include
realistically complex galaxy models based on high-resolution imaging from
space; spatially varying, physically-motivated blurring kernel; and combination
of multiple different exposures. To facilitate entry by people new to the
field, and for use as a diagnostic tool, the simulation software for the
challenge is publicly available, though the exact parameters used for the
challenge are blinded. Sample scripts to analyze the challenge data using
existing methods will also be provided. See http://great3challenge.info and
http://great3.projects.phys.ucl.ac.uk/leaderboard/ for more information.Comment: 30 pages, 13 figures, submitted for publication, with minor edits
(v2) to address comments from the anonymous referee. Simulated data are
available for download and participants can find more information at
http://great3.projects.phys.ucl.ac.uk/leaderboard
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