163 research outputs found
Loosely Coupled Formulations for Automated Planning: An Integer Programming Perspective
We represent planning as a set of loosely coupled network flow problems,
where each network corresponds to one of the state variables in the planning
domain. The network nodes correspond to the state variable values and the
network arcs correspond to the value transitions. The planning problem is to
find a path (a sequence of actions) in each network such that, when merged,
they constitute a feasible plan. In this paper we present a number of integer
programming formulations that model these loosely coupled networks with varying
degrees of flexibility. Since merging may introduce exponentially many ordering
constraints we implement a so-called branch-and-cut algorithm, in which these
constraints are dynamically generated and added to the formulation when needed.
Our results are very promising, they improve upon previous planning as integer
programming approaches and lay the foundation for integer programming
approaches for cost optimal planning
ROSAT PSPC observations of 36 high-luminosity clusters of galaxies: constraints on the gas fraction
We present a detailed and homogeneous analysis of the ROSAT PSPC surface
brightness profiles of 36 clusters of galaxies with high X-ray luminosity (L_X
> 10^{45} erg s^{-1}) and redshifts between 0.05 and 0.44. Using recent ASCA
estimates of the temperature of the gas for most of the clusters in the sample,
we apply both the deprojection technique and model fitting to the surface
brightness profiles to constrain the gas and dark matter distributions under
the assumption that the gas is both isothermal and hydrostatic.
Applying robust estimators, we find that the gas fraction within r_{500} of
the clusters in our sample has a distribution centred on f_gas(r_{500}) = 0.168
h_{50}^{-1.5}. The gas fraction ranges from 0.101 to 0.245 at the 95 per cent
confidence level. The values of f_gas show highly significant variations
between individual clusters, which may be explained if the dark matter has a
significant baryonic component. Within a cluster, the average radial dependence
of the gas mass fraction increases outward as r^s, with s~0.20. Combining these
results with those of primordial nucleosynthesis calculations and the current
estimate of H_0, the above central location implies \Omega_{0, m} < 0.56 at the
95 per cent confidence level. This upper limit decreases to 0.34 if we take the
highest significant estimates for f_gas.
A significant decrease in cluster gas fraction with redshift from the local
value, f_{gas, 0}, of 0.21, found assuming \Omega_{0, m} =1, is also reduced if
\Omega_{0, m} is low.Comment: 17 pages, 15 figures, MNRAS in press. Also available at
http://xalph3.ast.cam.ac.uk/~settori/paper.htm
Spatially resolved spectroscopy of Coma cluster early -- type galaxies: III. The stellar population gradients
We derive central values and logarithmic gradients for the Hbeta, Mg and Fe
indices of 35 early-type galaxies in the Coma cluster. We find that pure
elliptical galaxies have on average slightly higher velocity dispersions, lower
Hbeta, and higher metallic line-strengths than galaxies with disks (S0). The
gradients strongly correlate with the gradients of sigma, but only weakly with
the central index values and galaxy velocity dispersion. Using stellar
population models with variable element abundance ratios from Thomas, Maraston
& Bender (2003a) we derive average ages, metallicities and [alpha/Fe] ratios in
the center and at the effective radius. We find that the [alpha/Fe] ratio
correlates with velocity dispersion and drives 30% of the Mg-sigma relation,
the remaining 70% being caused by metallicity variations. We derive negative
metallicity gradients (-0.16 dex per decade) that are significantly flatter
than what is expected from gaseous monolithic collapse models, pointing to the
importance of mergers in the galaxy formation history. The gradients in age are
negligible, implying that no significant residual star formation has occurred
either in the center or in the outer parts of the galaxies, and that the
stellar populations at different radii must have formed at a common epoch. For
the first time we derive the gradients of the [alpha/Fe] ratio and find them
very small on the mean. Hence, [alpha/Fe] enhancement is not restricted to
galaxy centers but it is a global phenomenon. Our results imply that the
Mg-sigma local relation inside a galaxy, unlike the global Mg-sigma relation,
must be primarily driven by metallicity variations alone.Comment: 19 pages, 8 figures, Accepted for publication in A&
Inverse Compton Scattering as the Source of Diffuse EUV Emission in the Coma Cluster of Galaxies
We have examined the hypothesis that the majority of the diffuse EUV flux in
the Coma cluster is due to inverse Compton scattering of low energy cosmic ray
electrons (0.16 < epsilon < 0.31 GeV) against the 3K black-body background. We
present data on the two-dimensional spatial distribution of the EUV flux and
show that these data provide strong support for a non-thermal origin for the
EUV flux. However, we show that this emission cannot be produced by an
extrapolation to lower energies of the observed synchrotron radio emitting
electrons and an additional component of low energy cosmic ray electrons is
required.Comment: 11 pages, 5 figure
3C 295, a cluster and its cooling flow at z=0.46
We present ROSAT HRI data of the distant and X-ray luminous (L_x(bol)=2.6^
{+0.4}_{-0.2} 10^{45}erg/sec) cluster of galaxies 3C 295. We fit both a
one-dimensional and a two-dimensional isothermal beta-model to the data, the
latter one taking into account the effects of the point spread function (PSF).
For the error analysis of the parameters of the two-dimensional model we
introduce a Monte-Carlo technique. Applying a substructure analysis, by
subtracting a cluster model from the data, we find no evidence for a merger,
but we see a decrement in emission South-East of the center of the cluster,
which might be due to absorption. We confirm previous results by Henry &
Henriksen(1986) that 3C 295 hosts a cooling flow. The equations for the simple
and idealized cooling flow analysis presented here are solely based on the
isothermal beta-model, which fits the data very well, including the center of
the cluster. We determine a cooling flow radius of 60-120kpc and mass accretion
rates of dot{M}=400-900 Msun/y, depending on the applied model and temperature
profile. We also investigate the effects of the ROSAT PSF on our estimate of
dot{M}, which tends to lead to a small overestimate of this quantity if not
taken into account. This increase of dot{M} (10-25%) can be explained by a
shallower gravitational potential inferred by the broader overall profile
caused by the PSF, which diminishes the efficiency of mass accretion. We also
determine the total mass of the cluster using the hydrostatic approach. At a
radius of 2.1 Mpc, we estimate the total mass of the cluster (M{tot}) to be
(9.2 +/- 2.7) 10^{14}Msun. For the gas to total mass ratio we get M{gas}/M{tot}
=0.17-0.31, in very good agreement with the results for other clusters of
galaxies, giving strong evidence for a low density universe.Comment: 26 pages, 7 figures, accepted for publication in Ap
Can Neutron Star Mergers Alone Explain the r-process Enrichment of the Milky Way?
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article under the terms of the Creative Commons Attribution License, https://creativecommons.org/licenses/by/4.0/Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole–neutron star mergers, depending on the black hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this Letter, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.Peer reviewe
Dark matter distribution in the Coma cluster from galaxy kinematics: breaking the mass-anisotropy degeneracy
We study velocity moments of elliptical galaxies in the Coma cluster using
Jeans equations. The dark matter distribution in the cluster is modelled by a
generalised formula based upon the results of cosmological N-body simulations.
Its inner slope (cuspy or flat), concentration, and mass within the virial
radius are kept as free parameters, as well as the velocity anisotropy, assumed
independent of position. We show that the study of line-of-sight velocity
dispersion alone does not allow to constrain the parameters. By a joint
analysis of the observed profiles of velocity dispersion and kurtosis we are
able to break the degeneracy between the mass distribution and velocity
anisotropy. We determine the dark matter distribution at radial distances
larger than 3% of the virial radius and we find that the galaxy orbits are
close to isotropic. Due to limited resolution, different inner slopes are found
to be consistent with the data and we observe a strong degeneracy between the
inner slope and concentration : the best-fitting profiles have the
two parameters related with . Our best-fitting NFW profile
has concentration , which is 50% higher than standard values found in
cosmological simulations for objects of similar mass. The total mass within the
virial radius of Mpc is 1.4 \times 10^{15} h_{70}^{-1}
M_{\sun} (with 30% accuracy), 85% of which is dark. At this distance from the
cluster centre, the mass-to-light ratio in the blue band is solar
units. The total mass within the virial radius leads to estimates of the
density parameter of the Universe, assuming that clusters trace the
mass-to-light ratio and baryonic fraction of the Universe, with .Comment: 13 pages, 10 figures, revised version accepted by MNRAS with
discussion on substructure and equilibrium of elliptical galaxies adde
Extragalactic Foregrounds of the Cosmic Microwave Background: Prospects for the MAP Mission
(Abridged) While the major contribution to the Cosmic Microwave Background
(CMB) anisotropies are the sought-after primordial fluctuations produced at the
surface of last scattering, other effects produce secondary fluctuations at
lower redshifts. Here, we study the extragalactic foregrounds of the CMB in the
context of the upcoming MAP mission. We first survey the major extragalactic
foregrounds and show that discrete sources, the Sunyaev-Zel'dovich (SZ) effect,
and gravitational lensing are the most dominant ones for MAP. We then show that
MAP will detect (>5 sigma) about 46 discrete sources and 10 SZ clusters
directly with 94 GHz fluxes above 2 Jy. The mean SZ fluxes of fainter clusters
can be probed by cross-correlating MAP with cluster positions extracted from
existing catalogs. For instance, a MAP-XBACs cross-correlation will be
sensitive to clusters with S(94GHz)>200mJy, and will thus provide a test of
their virialization state and a measurement of their gas fraction. Finally, we
consider probing the hot gas on supercluster scales by cross-correlating the
CMB with galaxy catalogs. Assuming that galaxies trace the gas, we show that a
cross-correlation between MAP and the APM catalog should yield a marginal
detection, or at least a four-fold improvement on the COBE upper limits for the
rms Compton y-parameter.Comment: 27 LaTeX pages, including 5 ps figures and 2 tables. To appear in
ApJ. Minor revisions to match accepted version. Color figures and further
links available at http://www.astro.princeton.edu/~refreg
X-Ray Emission Line Ratios and Multiphase Gas in Elliptical Galaxies and Galaxy Clusters
We examine the K shell emission lines produced by isothermal and simple
multiphase models of the hot gas in elliptical galaxies and galaxy clusters to
determine the most effective means for constraining the width of the
differential emission measure (xi(T)) in these systems which we characterize by
a dimensionless parameter, sigma_xi. Comparison of line ratios of
two-temperature (sigma_xi << 1) and cooling flow (sigma_xi ~1) models is
presented in detail. We find that a two-temperature model can approximate very
accurately a cooling flow spectrum over 0.5-10 keV.
We have re-analyzed the ASCA spectra of three of the brightest galaxy
clusters to assess the evidence for multiphase gas in their cores: M87 (Virgo),
the Centaurus cluster, and the Perseus cluster. K-alpha emission line blends of
Si, S, Ar, Ca, and Fe are detected in each system as is significant Fe K-beta
emission. The Fe K-beta/K-alpha ratios are consistent with optically thin
plasma models and do not suggest resonance scattering in these systems.
Consideration of both the ratios of H-like to He-like K-alpha lines and the
local continuum temperatures clearly rules out isothermal gas in each case. To
obtain more detailed constraints we fitted plasma models over 1.6-9 keV where
the emission is dominated by these K shell lines and by continuum. In each case
the ASCA spectra cannot determine whether the gas emits at only two
temperatures or over a continuous range of temperatures as expected for a
cooling flow. The metal abundances are near solar for all of the multiphase
models. We discuss the implications of these results and examine the prospects
for determining the temperature structure in these systems with upcoming X-ray
missions.Comment: 30 pages (18 figures), To Appear in MNRAS. Major revision of the
initially posted version: (1) The section on ASCA data of ellipticals was
expanded and moved to astro-ph/9811080; (2) The ASCA data of M87, Centaurus,
and Perseus have been re-analyzed. We find evidence for Fe K-beta emission in
these systems, but the Fe K-beta/K-alpha ratios do not suggest resonance
scattering; (3) The metal abundances of the cores of these systems are
consistent with the meteoritic solar value
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