59,190 research outputs found
Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies
We use a new non-parametric Bayesian approach to obtain the most probable
mass distributions and circular velocity curves along with their confidence
ranges, given deprojected density and temperature profiles of the hot gas
surrounding X-ray bright elliptical galaxies. For a sample of six X-ray bright
ellipticals, we find that all circular velocity curves are rising in the outer
parts due to a combination of a rising temperature profile and a logarithmic
pressure gradient that increases in magnitude. Comparing the circular velocity
curves we obtain from X-rays to those obtained from dynamical models, we find
that the former are often lower in the central ~10 kpc. This is probably due to
a combination of: i) Non-thermal contributions of up to ~35% in the pressure
(with stronger effects in NGC 4486), ii) multiple-temperature components in the
hot gas, iii) incomplete kinematic spatial coverage in the dynamical models,
and iv) mass profiles that are insufficiently general in the dynamical
modelling. Complementing the total mass information from the X-rays with
photometry and stellar population models to infer the dark matter content, we
find evidence for massive dark matter haloes with dark matter mass fractions of
~35-80% at 2Re, rising to a maximum of 80-90% at the outermost radii. We also
find that the six galaxies follow a Tully-Fisher relation with slope ~4 and
that their circular velocities at 1Re correlate strongly with the velocity
dispersion of the local environment. As a result, the galaxy luminosity at 1Re
also correlates with the velocity dispersion of the environment. These
relations suggest a close link between the properties of central X-ray bright
elliptical galaxies and their environments (abridged).Comment: 20 pages, 11 figures, accepted for publication in MNRA
Connectivity-Enforcing Hough Transform for the Robust Extraction of Line Segments
Global voting schemes based on the Hough transform (HT) have been widely used
to robustly detect lines in images. However, since the votes do not take line
connectivity into account, these methods do not deal well with cluttered
images. In opposition, the so-called local methods enforce connectivity but
lack robustness to deal with challenging situations that occur in many
realistic scenarios, e.g., when line segments cross or when long segments are
corrupted. In this paper, we address the critical limitations of the HT as a
line segment extractor by incorporating connectivity in the voting process.
This is done by only accounting for the contributions of edge points lying in
increasingly larger neighborhoods and whose position and directional content
agree with potential line segments. As a result, our method, which we call
STRAIGHT (Segment exTRAction by connectivity-enforcInG HT), extracts the
longest connected segments in each location of the image, thus also integrating
into the HT voting process the usually separate step of individual segment
extraction. The usage of the Hough space mapping and a corresponding
hierarchical implementation make our approach computationally feasible. We
present experiments that illustrate, with synthetic and real images, how
STRAIGHT succeeds in extracting complete segments in several situations where
current methods fail.Comment: Submitted for publicatio
Universality of the Ising and the S=1 model on Archimedean lattices: A Monte Carlo determination
The Ising model S=1/2 and the S=1 model are studied by efficient Monte Carlo
schemes on the (3,4,6,4) and the (3,3,3,3,6) Archimedean lattices. The
algorithms used, a hybrid Metropolis-Wolff algorithm and a parallel tempering
protocol, are briefly described and compared with the simple Metropolis
algorithm. Accurate Monte Carlo data are produced at the exact critical
temperatures of the Ising model for these lattices. Their finite-size analysis
provide, with high accuracy, all critical exponents which, as expected, are the
same with the well known 2d Ising model exact values. A detailed finite-size
scaling analysis of our Monte Carlo data for the S=1 model on the same lattices
provides very clear evidence that this model obeys, also very well, the 2d
Ising model critical exponents. As a result, we find that recent Monte Carlo
simulations and attempts to define effective dimensionality for the S=1 model
on these lattices are misleading. Accurate estimates are obtained for the
critical amplitudes of the logarithmic expansions of the specific heat for both
models on the two Archimedean lattices.Comment: 9 pages, 11 figure
Universality aspects of the d=3 random-bond Blume-Capel model
The effects of bond randomness on the universality aspects of the simple
cubic lattice ferromagnetic Blume-Capel model are discussed. The system is
studied numerically in both its first- and second-order phase transition
regimes by a comprehensive finite-size scaling analysis. We find that our data
for the second-order phase transition, emerging under random bonds from the
second-order regime of the pure model, are compatible with the universality
class of the 3d random Ising model. Furthermore, we find evidence that, the
second-order transition emerging under bond randomness from the first-order
regime of the pure model, belongs to a new and distinctive universality class.
The first finding reinforces the scenario of a single universality class for
the 3d Ising model with the three well-known types of quenched uncorrelated
disorder (bond randomness, site- and bond-dilution). The second, amounts to a
strong violation of universality principle of critical phenomena. For this case
of the ex-first-order 3d Blume-Capel model, we find sharp differences from the
critical behaviors, emerging under randomness, in the cases of the
ex-first-order transitions of the corresponding weak and strong first-order
transitions in the 3d three-state and four-state Potts models.Comment: 12 pages, 12 figure
A Numerical Study of Improved Quark Actions on Anisotropic Lattices
Tadpole improved Wilson quark actions with clover terms on anisotropic
lattices are studied numerically.
Using asymmetric lattice volumes, the pseudo-scalar meson dispersion
relations are measured for 8 lowest lattice momentum modes with quark mass
values ranging from the strange to the charm quark with various values of the
gauge coupling and 3 different values of the bare speed of light
parameter . These results can be utilized to extrapolate or interpolate to
obtain the optimal value for the bare speed of light parameter
at a given gauge coupling for all bare quark mass values . In particular,
the optimal values of at the physical strange and charm quark mass are
given for various gauge couplings.
The lattice action with these optimized parameters can then be used to study
physical properties of hadrons involving either light or heavy quarks.Comment: 22 pages, 7 figures, 2 tables. Analysis greatly modified compared
with previous versio
Constraints on Dark Energy state equation with varying pivoting redshift
We assume the DE state equations w(a) = w_0+w_a(a_p-a), and study the
dependence of the constraints on w_0 and w_a coefficients on the pivoting
redshift 1+z_p=1/a_p. Coefficients are fitted to data including WMAP7, SNIa
(Union 2.1), BAO's (including WiggleZ and SDSS results) and H_0 constraints.
The fitting algorithm is CosmoMC. We find specific differences between the
cases when neutrino mass is allowed or disregarded. More in detail: i) The z_p
value yielding uncorrelated constraints on w_0 and w_a is different in the two
cases, holding ~0.25 and ~0.35, respectively. (ii) If we consider the intervals
allowed to w_0, we find that they shift when z_p increases, in opposite
directions for vanishing or allowed neutrino mass. This leads to no overlap
between 1sigma intervals already at z_p >~0.4. (iii) The known effect that a
more negative state parameter is required to allow for neutrino mass displays
its effects on w_a, rather than on w_0. (iv) The w_0-w_a constraints found by
using any pivot z_p can be translated into constraints holding at a specific
z_p value (0 or the z_p where errors are uncorrelated). When we do so, error
ellipses exhibit a satisfactory overlap.Comment: 13 pages, 7 figures, 2 table
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