12,513 research outputs found
The Consistent Newtonian Limit of Einstein's Gravity with a Cosmological Constant
We derive the `exact' Newtonian limit of general relativity with a positive
cosmological constant . We point out that in contrast to the case with
, the presence of a positive in Einsteins's equations
enforces, via the condition , on the potential , a range
, within which the
Newtonian limit is valid. It also leads to the existence of a maximum mass,
. As a consequence we cannot put the boundary
condition for the solution of the Poisson equation at infinity. A boundary
condition suitably chosen now at a finite range will then get reflected in the
solution of provided the mass distribution is not spherically symmetric.Comment: Latex, 15 pages, no figures, errors correcte
Quasars, their host galaxies, and their central black holes
We present the final results from our deep HST imaging study of the hosts of
radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and radio galaxies (RGs).
We describe new WFPC2 R-band observations for 14 objects and model these images
in conjunction with the data already reported in McLure et al (1999). We find
that spheroidal hosts become more prevalent with increasing nuclear luminosity
such that, for nuclear luminosities M_V < -23.5, the hosts of both radio-loud
and radio-quiet AGN are virtually all massive ellipticals. Moreover we
demonstrate that the basic properties of these hosts are indistinguishable from
those of quiescent, evolved, low-redshift ellipticals of comparable mass. This
result kills any lingering notion that radio-loudness is determined by
host-galaxy morphology, and also sets severe constraints on evolutionary
schemes which attempt to link low-z ULIRGs with RQQs. Instead, we show that our
results are as expected given the relationship between black-hole and spheroid
mass established for nearby galaxies, and apply this relation to estimate the
mass of the black hole in each object. The results agree very well with
completely-independent estimates based on nuclear emission-line widths; all the
quasars in our sample have M(bh) > 5 x 10^8 solar masses, while the radio-loud
objects are confined to M(bh) > 10^9 solar masses. This apparent mass-threshold
difference, which provides a natural explanation for why RQQs outnumber RLQs by
a factor of 10, appears to reflect the existence of a minimum and maximum level
of black-hole radio output which is a strong function of black-hole mass.
Finally, we use our results to estimate the fraction of massive
spheroids/black-holes which produce quasar-level activity. This fraction is
\~0.1% at the present day, rising to > 10% at z = 2-3.Comment: Revised version accepted for publication in Monthly Notices of the
Royal Astronomical Society. 46 pages, the final 19 of which comprise an
Appendix. 15 figures in main text. A further 14 4-panel greyscale plots and
14 line plots which appear in the Appendix have been reproduced here with
reduced quality due to space limitations. A full resolution copy of the
manuscript can be obtained via ftp://ftp.roe.ac.uk/pub/jsd/dunlop2002.ps.g
Improving predictive power of physically based rainfall-induced shallow landslide models: a probabilistic approach
Distributed models to forecast the spatial and temporal occurrence of
rainfall-induced shallow landslides are based on deterministic laws. These
models extend spatially the static stability models adopted in geotechnical
engineering, and adopt an infinite-slope geometry to balance the resisting and
the driving forces acting on the sliding mass. An infiltration model is used to
determine how rainfall changes pore-water conditions, modulating the local
stability/instability conditions. A problem with the operation of the existing
models lays in the difficulty in obtaining accurate values for the several
variables that describe the material properties of the slopes. The problem is
particularly severe when the models are applied over large areas, for which
sufficient information on the geotechnical and hydrological conditions of the
slopes is not generally available. To help solve the problem, we propose a
probabilistic Monte Carlo approach to the distributed modeling of
rainfall-induced shallow landslides. For the purpose, we have modified the
Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability
Analysis (TRIGRS) code. The new code (TRIGRS-P) adopts a probabilistic approach
to compute, on a cell-by-cell basis, transient pore-pressure changes and
related changes in the factor of safety due to rainfall infiltration.
Infiltration is modeled using analytical solutions of partial differential
equations describing one-dimensional vertical flow in isotropic, homogeneous
materials. Both saturated and unsaturated soil conditions can be considered.
TRIGRS-P copes with the natural variability inherent to the mechanical and
hydrological properties of the slope materials by allowing values of the TRIGRS
model input parameters to be sampled randomly from a given probability
distribution. [..]Comment: 25 pages, 14 figures, 9 tables. Revised version; accepted for
publication in Geoscientific Model Development on 13 February 201
Nonthermal Emission Associated with Strong AGN Outbursts at the Centers of Galaxy Clusters
Recently, strong AGN outbursts at the centers of galaxy clusters have been
found. Using a simple model, we study particle acceleration around a shock
excited by an outburst and estimate nonthermal emission from the accelerated
particles. We show that emission from secondary electrons is consistent with
the radio observations of the minihalo in the Perseus cluster, if there was a
strong AGN outburst >~10^8 yrs ago with an energy of ~1.8x10^62 erg. The
validity of our model depends on the frequency of the large outbursts. We also
estimate gamma-ray emission from the accelerated particles and show that it
could be detected with GLAST.Comment: Accepted for publication in ApJ
“Cyst-ained” research into Heterodera parasitism
Nematodes are roundworms that constitute the phylum Nematoda. Only a small fraction of nematode genera contains plant-parasitic or animal-parasitic species, while the majority of nematodes are free-living [1]. Heterodera glycines, the soybean cyst nematode, is a plant-parasitic nematode causing major damage to soybean production worldwide. Annual United States yield loss estimates due to H. glycines range up to $1.2 billion, likely making this nematode the most serious pathogen threat to sustainable soybean production [2]. While cyst nematode-resistant soybean cultivars are available, they do not control all H. glycines biotypes present in a given field and, therefore, select for virulent nematode populations that can overcome available resistance genes, leading to a slow but steady erosion of resistance efficacy [3]. Clearly, long-term management of the soybean cyst nematode in modern soybean production will need additional tools, and it is likely that such new tools will be developed from detailed molecular knowledge of the complex Heterodera cyst nematode-plant interactions. This short review provides a snapshot of currently unfolding research discoveries from the genus Heterodera, which also includes other cyst nematodes, particularly the sugar beet cyst nematode H. schachtii, which can infect Arabidopsis and therefore has been used as a model system. Since nematode effectors (the proteins delivered into host plant tissues to mediate parasitism) are at the forefront of nematode–plant interactions, their identification and functional characterization are heavily emphasized in this manuscript
Control of multiferroic domains by external electric fields in TbMnO3
The control of multiferroic domains through external electric fields has been
studied by dielectric measurements and by polarized neutron diffraction on
single-crystalline TbMnO. Full hysteresis cycles were recorded by varying
an external field of the order of several kV/mm and by recording the chiral
magnetic scattering as well as the charge in a sample capacitor. Both methods
yield comparable coercive fields that increase upon cooling.Comment: 12 pages, 6 figure
Evidence for the convergence of β-adrenergic and muscarinic signalling systems at a post-receptor site
AbstractThe β-adrenergic agonist isoproterenol stimulates inositol trisphosphate (IP3) formation and cytosolic Ca2+ ([Ca2+]i) mobilization in rat parotid acini via a cAMP-dependent process. Atropine, a muscarinic antagonist, inhibited these isoproterenol responses without affecting isoproterenol-induced amylase secretion or peak [Ca2+]i and IP3 responses elicited by α1-adrenergic stimulation with epinephrine. Atropine had no effect on isoproterenol-induced [Ca2+]i responses in a cell line which lacked muscarinic receptors and did not alter β-adrenoreceptor ligand binding. These results suggest that the inhibition by atropine results from a post-receptor effect on cAMP-mediated stimulation of phosphatidylinositol 4,5 bisphosphate (PIP2) hydrolysis
Handwritten digit recognition by bio-inspired hierarchical networks
The human brain processes information showing learning and prediction
abilities but the underlying neuronal mechanisms still remain unknown.
Recently, many studies prove that neuronal networks are able of both
generalizations and associations of sensory inputs. In this paper, following a
set of neurophysiological evidences, we propose a learning framework with a
strong biological plausibility that mimics prominent functions of cortical
circuitries. We developed the Inductive Conceptual Network (ICN), that is a
hierarchical bio-inspired network, able to learn invariant patterns by
Variable-order Markov Models implemented in its nodes. The outputs of the
top-most node of ICN hierarchy, representing the highest input generalization,
allow for automatic classification of inputs. We found that the ICN clusterized
MNIST images with an error of 5.73% and USPS images with an error of 12.56%
6C radio galaxies at z~1: The influence of radio power on the alignment effect
Powerful radio galaxies often display enhanced optical/UV continuum emission
and extended emission line regions, elongated and aligned with the radio jet
axis. The expansion of the radio source strongly affects the gas clouds in the
surrounding IGM, and the kinematic and ionization properties of the extended
emission line regions display considerable variation over the lifetime of
individual sources, as well as with cosmic epoch. We present the results of
deep rest-frame UV and optical imaging and UV spectroscopy of high redshift 6C
radio galaxies. The interdependence of the host galaxy and radio source
properties are discussed, considering: (i) the relative contribution of shocks
associated with the expanding radio source to the observed emission line gas
kinematics, and their effect on the ionization state of the gas; (ii) the
similarities and differences between the morphologies of the host galaxies and
aligned emission for a range of radio source powers; and (iii) the influence of
radio power on the strength of the observed alignment effect.Comment: LaTeX, 6 pages, 5 figures, Elsevier Science format. To appear in
"Radio galaxies: past, present & future". eds. M. Jarvis et al., Leiden, Nov
200
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