21,847 research outputs found
Metastability in Markov processes
We present a formalism to describe slowly decaying systems in the context of
finite Markov chains obeying detailed balance. We show that phase space can be
partitioned into approximately decoupled regions, in which one may introduce
restricted Markov chains which are close to the original process but do not
leave these regions. Within this context, we identify the conditions under
which the decaying system can be considered to be in a metastable state.
Furthermore, we show that such metastable states can be described in
thermodynamic terms and define their free energy. This is accomplished showing
that the probability distribution describing the metastable state is indeed
proportional to the equilibrium distribution, as is commonly assumed. We test
the formalism numerically in the case of the two-dimensional kinetic Ising
model, using the Wang--Landau algorithm to show this proportionality
explicitly, and confirm that the proportionality constant is as derived in the
theory. Finally, we extend the formalism to situations in which a system can
have several metastable states.Comment: 30 pages, 5 figures; version with one higher quality figure available
at http://www.fis.unam.mx/~dsanders
Multiphoton Coincidence Spectroscopy
We extend the analysis of photon coincidence spectroscopy beyond bichromatic
excitation and two-photon coincidence detection to include multichromatic
excitation and multiphoton coincidence detection. Trichromatic excitation and
three-photon coincidence spectroscopy are studied in detail, and we identify an
observable signature of a triple resonance in an atom-cavity system.Comment: 6 page, REVTeXs, 6 Postscript figures. The abstract appeared in the
Proceedings of ACOLS9
Bars from the Inside Out: An HST Study of their Dusty Circumnuclear Regions
The results of bar-driven mass inflow are directly observable in
high-resolution HST observations of their circumnuclear regions. These
observations reveal a wealth of structures dominated by dust lanes, often with
a spiral-like morphology, and recent star formation. Recent work has shown that
some of these structures are correlated with the presence or absence of a bar.
I extend this work with an investigation of circumnuclear morphology as a
function of bar strength for a sample of 48 galaxies with both measured bar
strengths and ``structure maps'' computed from HST images. The structure maps
for these galaxies, which have projected spatial resolutions of 2 - 15 pc, show
that the fraction of galaxies with grand-design (GD) circumnuclear dust spirals
increases significantly with bar strength, while tightly wound dust spirals are
only present in the most axisymmetric galaxies. GD structure is only found at
the centers of galaxies classified as SB(s) or SB(rs) and not SB(r). SB(s)
galaxies on average have stronger bars than SB(r) galaxies. There is also a
modest increase in the fraction of loosely wound dust spirals at later
morphological types, which may reflect an increase in the fraction of galaxies
with circumnuclear, gaseous disks. (abridged)Comment: 10 pages, 6 figures. To appear in "Penetrating Bars through Masks of
Cosmic Dust: The Hubble Tuning Fork strikes a New Note" held June 7-12th,
2004 in Pilanesberg National Park, South Africa. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/safrica04.pd
The development of structural adhesives systems suitable for use with liquid oxygen Annual summary report, 1 Jul. 1963 - 30 Jun. 1964
Fluorinated, chlorinated, and halogenated polymer adhesives prepared and tested for compatibility with liquid oxyge
Bayes-X: a Bayesian inference tool for the analysis of X-ray observations of galaxy clusters
We present the first public release of our Bayesian inference tool, Bayes-X,
for the analysis of X-ray observations of galaxy clusters. We illustrate the
use of Bayes-X by analysing a set of four simulated clusters at z=0.2-0.9 as
they would be observed by a Chandra-like X-ray observatory. In both the
simulations and the analysis pipeline we assume that the dark matter density
follows a spherically-symmetric Navarro, Frenk and White (NFW) profile and that
the gas pressure is described by a generalised NFW (GNFW) profile. We then
perform four sets of analyses. By numerically exploring the joint probability
distribution of the cluster parameters given simulated Chandra-like data, we
show that the model and analysis technique can robustly return the simulated
cluster input quantities, constrain the cluster physical parameters and reveal
the degeneracies among the model parameters and cluster physical parameters. We
then analyse Chandra data on the nearby cluster, A262, and derive the cluster
physical profiles. To illustrate the performance of the Bayesian model
selection, we also carried out analyses assuming an Einasto profile for the
matter density and calculated the Bayes factor. The results of the model
selection analyses for the simulated data favour the NFW model as expected.
However, we find that the Einasto profile is preferred in the analysis of A262.
The Bayes-X software, which is implemented in Fortran 90, is available at
http://www.mrao.cam.ac.uk/facilities/software/bayesx/.Comment: 22 pages, 11 figure
Recommended from our members
Energy use for urban water management by utilities and Households in Los Angeles
Reducing energy consumption for urban water management may yield economic and environmental benefits. Few studies provide comprehensive assessments of energy needs for urban water sectors that include both utility operations and household use. Here, we evaluate the energy needs for urban water management in metropolitan Los Angeles (LA) County. Using planning scenarios that include both water conservation and alternative supply options, we estimate energy requirements of water imports, groundwater pumping, distribution in pipes, water and wastewater treatment, and residential water heating across more than one hundred regional water agencies covering over 9 million people. Results show that combining water conservation with alternative local supplies such as stormwater capture and water reuse (nonpotable or indirect potable) can reduce the energy consumption and intensity of water management in LA. Further advanced water treatment for direct potable reuse could increase energy needs. In aggregate, water heating represents a major source of regional energy consumption. The heating factor associated with grid-supplied electricity drives the relative contribution of energy-for-water by utilities and households. For most scenarios of grid operations, energy for household water heating significantly outweighs utility energy consumption. The study demonstrates how publicly available and detailed data for energy and water use supports sustainability planning. The method is applicable to cities everywhere
Continuous-variable quantum teleportation of entanglement
Entangled coherent states can be used to determine the entanglement fidelity
for a device that is designed to teleport coherent states. This entanglement
fidelity is universal, in that the calculation is independent of the use of
entangled coherent states and applies generally to the teleportation of
entanglement using coherent states. The average fidelity is shown to be a poor
indicator of the capability of teleporting entanglement; i.e., very high
average fidelity for the quantum teleportation apparatus can still result in
low entanglement fidelity for one mode of the two-mode entangled coherent
state.Comment: 5 pages, 1 figure, published versio
The luminosity function of the brightest galaxies in the IRAS survey
Results from a study of the far infrared properties of the brightest galaxies in the IRAS survey are described. There is a correlation between the infrared luminosity and the infrared to optical luminosity ratio and between the infrared luminosity and the far infrared color temperature in these galaxies. The infrared bright galaxies represent a significant component of extragalactic objects in the local universe, being comparable in space density to the Seyferts, optically identified starburst galaxies, and more numerous than quasars at the same bolometric luminosity. The far infrared luminosity in the local universe is approximately 25% of the starlight output in the same volume
Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured
Mergers of galaxies are thought to cause significant gas inflows to the inner
parsecs, which can activate rapid accretion onto supermassive black holes
(SMBHs), giving rise to Active Galactic Nuclei (AGN). During a significant
fraction of this process, SMBHs are predicted to be enshrouded by gas and dust.
Studying 52 galactic nuclei in infrared-selected local Luminous and
Ultra-luminous infrared galaxies in different merger stages in the hard X-ray
band, where radiation is less affected by absorption, we find that the amount
of material around SMBHs increases during the last phases of the merger. We
find that the fraction of Compton-thick (CT, ) AGN in late merger galaxies is higher
() than in local hard X-ray selected AGN
(), and that obscuration reaches its maximum when the
nuclei of the two merging galaxies are at a projected distance of
kiloparsecs (). We also
find that all AGN of our sample in late merger galaxies have , which implies that the obscuring material covers
of the X-ray source. These observations show that the material
is most effectively funnelled from the galactic scale to the inner tens of
parsecs during the late stages of galaxy mergers, and that the close
environment of SMBHs in advanced mergers is richer in gas and dust with respect
to that of SMBHs in isolated galaxies, and cannot be explained by the classical
AGN unification model in which the torus is responsible for the obscuration.Comment: Final version matching the article published in MNRAS - 30 pages, 16
figure
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