1,216 research outputs found
Escape of Ionizing Radiation from High Redshift Galaxies
We model the escape of ionizing radiation from high-redshift galaxies using
high-resolution Adaptive Mesh Refinement N-body + hydrodynamics simulations.
Our simulations include time-dependent and spatially-resolved transfer of
ionizing radiation in three dimensions, including effects of dust absorption.
For galaxies of total mass M > 10^11 Msun and star formation rates SFR ~ 1-5
Msun/yr, we find angular averaged escape fractions of 0.01-0.03 over the entire
redshift interval studied (3<z<9). In addition, we find that the escape
fraction varies by more than an order of magnitude along different
lines-of-sight within individual galaxies, from the largest values near
galactic poles to the smallest along the galactic disk. The escape fraction
declines steeply at lower masses and SFR. We show that the low values of escape
fractions are due to a small fraction of young stars located just outside the
edge of HI disk. We compare our predicted escape fraction of ionizing photons
with previous results, and find a general agreement with both other simulation
results and available direct detection measurements at z ~ 3. We also compare
our simulations with a novel method to estimate the escape fraction in galaxies
from the observed distribution of neutral hydrogen column densities along the
lines of sights to long duration gamma-ray bursts. Using this method we find
escape fractions of the GRB host galaxies of 2-3%, consistent with our
theoretical predictions. [abridged]Comment: submitted to Ap
AGN Obscuring Tori Supported by Infrared Radiation Pressure
Explicit 2-d axisymmetric solutions are found to the hydrostatic equilibrium,
energy balance, and photon diffusion equations within obscuring tori around
active galactic nuclei. These solutions demonstrate that infrared radiation
pressure can support geometrically thick structures in AGN environments subject
to certain constraints: the bolometric luminosity must be roughly 0.03--1 times
the Eddington luminosity; and the Compton optical depth of matter in the
equatorial plane should be order unity, with a tolerance of about an order of
magnitude up or down. Both of these constraints are at least roughly consistent
with observations. In addition, angular momentum must be redistributed so that
the fractional rotational support against gravity rises from the inner edge of
the torus to the outer in a manner specific to the detailed shape of the
gravitational potential. This model also predicts that the column densities
observed in obscured AGN should range from about 10^{22} to about 10^{24}
cm^{-2}.Comment: ApJ, in pres
A long-term survivor of Bland-White-Garland syndrome with systemic collateral supply: A case report and review of the literature
BACKGROUND: land-White-Garland syndrome (anomalous origin of the left coronary artery from the pulmonary artery) is a rare disease which may result in myocardial infarction, congestive heart failure and sometimes death during the early infantile period. Case presentation: A succesfully treated case of a 45-year-old mother of 2 children with Bland-White-Garland syndrome and concomitant severe mitral regurgitation is presented. Subsequent therapy consisted of ligation of the anomalous origin of the left coronary artery, anastomosis of the left internal mammary artery to the left anterior descending branch and mitral valve replacement. Continuous blood flow from the left coronary artery ostium during extracorporeal circulation and aorta clamping suggested systemic collateral supply. Conclusions: Recognition and diagnosis of Bland-White-Garland syndrome is important due to its potentially life-threatening complications
An investigation of herpes simplex virus promoter activity compatible with latency establishment reveals VP16-independent activation of immediate-early promoters in sensory neurones
Herpes simplex virus (HSV) type-1 establishes lifelong latency in sensory neurones and it is widely assumed that latency is the consequence of a failure to initiate virus immediate-early (IE) gene expression. However, using a Ore reporter mouse system in conjunction with Ore-expressing HSV-1 recombinants we have previously shown that activation of the IE ICPO promoter can precede latency establishment in at least 30 % of latently infected cells. During productive infection of non-neuronal cells, IE promoter activation is largely dependent on the transactivator VP16 a late structural component of the virion. Of significance, VP16 has recently been shown to exhibit altered regulation in neurones; where its de novo synthesis is necessary for IE gene expression during both lytic infection and reactivation from latency. In the current study, we utilized the Ore reporter mouse model system to characterize the full extent of viral promoter activity compatible with cell survival and latency establishment. In contrast to the high frequency activation of representative IE promoters prior to latency establishment, cell marking using a virus recombinant expressing Ore under VP16 promoter control was very inefficient. Furthermore, infection of neuronal cultures with VP16 mutants reveals a strong VP16 requirement for IE promoter activity in non-neuronal cells, but not sensory neurones. We conclude that only IE promoter activation can efficiently precede latency establishment and that this activation is likely to occur through a VP16-independent mechanism
The Arecibo Dual-Beam Survey: The HI Mass Function of Galaxies
We use the HI-selected galaxy sample from the Arecibo Dual-Beam Survey
(Rosenberg & Schneider 2000) to determine the shape of the HI mass function of
galaxies in the local universe using both the step-wise maximum likelihood and
the 1/V_tot methods. Our survey region spanned all 24 hours of right ascension
at selected declinations between 8 and 29 degrees covering ~430 deg^2 of sky in
the main beam. The survey is not as deep as some previous Arecibo surveys, but
it has a larger total search volume and samples a much larger area of the sky.
We conducted extensive tests on all aspects of the galaxy detection process,
allowing us to empirically correct for our sensitivity limits, unlike the
previous surveys. The mass function for the entire sample is quite steep, with
a power-law slope of \alpha ~ -1.5. We find indications that the slope of the
HI mass function is flatter near the Virgo cluster, suggesting that
evolutionary effects in high density environments may alter the shape of the HI
mass function. These evolutionary effects may help to explain differences in
the HI mass function derived by different groups. We are sensitive to the most
massive sources (log M > 5x10^10 M\solar) over most of the declination range,
\~1 sr, and do not detect any massive low surface brightness galaxies. These
statistics restrict the population of Malin 1-like galaxies to <5.5x10^-6
Mpc^-3.Comment: ApJ accepted, 12 page
Algorithms for computational argumentation in artificial intelligence
Argumentation is a vital aspect of intelligent behaviour by humans. It provides the means for comparing
information by analysing pros and cons when trying to make a decision. Formalising argumentation in
computational environment has become a topic of increasing interest in artificial intelligence research
over the last decade.
Computational argumentation involves reasoning with uncertainty by making use of logic in order
to formalize the presentation of arguments and counterarguments and deal with conflicting information.
A common assumption for logic-based argumentation is that an argument is a pair where Φ is
a consistent set which is minimal for entailing a claim α. Different logics provide different definitions
for consistency and entailment and hence give different options for formalising arguments and counterarguments.
The expressivity of classical propositional logic allows for complicated knowledge to be
represented but its computational cost is an issue. This thesis is based on monological argumentation
using classical propositional logic [12] and aims in developing algorithms that are viable despite the
computational cost. The proposed solution adapts well established techniques for automated theorem
proving, based on resolution and connection graphs. A connection graph is a graph where each node is
a clause and each arc denotes there exist complementary disjuncts between nodes. A connection graph
allows for a substantially reduced search space to be used when seeking all the arguments for a claim
from a given knowledgebase. In addition, its structure provides information on how its nodes can be linked
with each other by resolution, providing this way the basis for applying algorithms which search for
arguments by traversing the graph. The correctness of this approach is supported by theoretical results,
while experimental evaluation demonstrates the viability of the algorithms developed. In addition, an
extension of the theoretical work for propositional logic to first-order logic is introduced
Constraints on the Abundance of Highly Ionized Proto-Cluster Regions from the Absence of Large Voids in the Lyman Alpha Forest
Energetic feedback processes during the formation of galaxy clusters may have
heated and ionized a large fraction of the intergalactic gas in proto-cluster
regions. When such a highly ionized hot ``super-bubble'' falls along the
sightline to a background quasar, it would be seen as a large void, with little
or no absorption, in the Lyman alpha forest. We examine the spectra of 137
quasars in the Sloan Digital Sky Survey, to search for such voids, and find no
clear evidence of their existence. The size distribution of voids in the range
5-70 Angstrom (corresponding to physical sizes of approximately 3-35 comoving
Mpc/h) is consistent with the standard model for the Lyman alpha forest without
additional hot bubbles. We adapt a physical model for HII bubble growth during
cosmological reionization (Furlanetto, Zaldarriaga and Hernquist 2004), to
describe the expected size-distribution of hot super-bubbles at redshift around
z = 3. This model incorporates the conjoining of bubbles around individual
neighboring galaxies. Using the non-detection of voids, we find that models in
which the volume filling factor of hot bubbles exceeds approximately 20 percent
at z=3 can be ruled out, primarily because they overproduce the number of large
(40-50 Angstrom) voids. We conclude that any pre-heating mechanism that
explains galaxy cluster observations must avoid heating the low-density gas in
the proto-cluster regions, either by operating relatively recently (z<3) or by
depositing entropy in the high-density regions.Comment: submitted to ApJ, 9 emulateapj pages with 3 figure
The Imprint of Gravitational Waves on the Cosmic Microwave Background
Long-wavelength gravitational waves can induce significant temperature
anisotropy in the cosmic microwave background. Distinguishing this from
anisotropy induced by energy density fluctuations is critical for testing
inflationary cosmology and theories of large-scale structure formation. We
describe full radiative transport calculations of the two contributions and
show that they differ dramatically at angular scales below a few degrees. We
show how anisotropy experiments probing large- and small-angular scales can
combine to distinguish the imprint due to gravitational waves.Comment: 11 pages, Penn Preprint-UPR-
Predictions of the causal entropic principle for environmental conditions of the universe
The causal entropic principle has been proposed as a superior alternative to
the anthropic principle for understanding the magnitude of the cosmological
constant. In this approach, the probability to create observers is assumed to
be proportional to the entropy production \Delta S in a maximal causally
connected region -- the causal diamond. We improve on the original treatment by
better quantifying the entropy production due to stars, using an analytic model
for the star formation history which accurately accounts for changes in
cosmological parameters. We calculate the dependence of \Delta S on the density
contrast Q=\delta\rho/\rho, and find that our universe is much closer to the
most probable value of Q than in the usual anthropic approach and that
probabilities are relatively weakly dependent on this amplitude. In addition,
we make first estimates of the dependence of \Delta S on the baryon fraction
and overall matter abundance. Finally, we also explore the possibility that
decays of dark matter, suggested by various observed gamma ray excesses, might
produce a comparable amount of entropy to stars.Comment: RevTeX4, 13pp, 10 figures; v2. clarified introduction, added ref
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