807 research outputs found
Modified Gravity Makes Galaxies Brighter
We investigate the effect of modifed gravity with screening mechanisms, such
as the chameleon or symmetron models, upon the structure of main sequence
stars. We find that unscreened stars can be significantly more luminous and
ephemeral than their screened doppelgangers. By embedding these stars into
dwarf galaxies, which can be unscreened for values of the parameters not yet
ruled out observationally, we show that the cumulative effect of their
increased luminosity can enhance the total galactic luminosity. We estimate
this enhancement and find that it can be considerable given model parameters
that are still under experimental scrutiny. By looking for systematic offsets
between screened dwarf galaxies in clusters and unscreened galaxies in voids,
these effects could form the basis of an independent observational test that
can potentially lower the current experimental bounds on the model independent
parameters of these theories by and order of magnitude or more.Comment: 16 pages, six figure
The Chameleonic Contribution to the SZ Radial Profile of the Coma Cluster
We constrain the chameleonic Sunyaev--Zel'dovich (CSZ) effect in the Coma
cluster from measurements of the Coma radial profile presented in the WMAP
7-year results. The CSZ effect arises from the interaction of a scalar (or
pseudoscalar) particle with the cosmic microwave background in the magnetic
field of galaxy clusters. We combine this radial profile data with SZ
measurements towards the centre of the Coma cluster in different frequency
bands, to find Delta T_{SZ,RJ}(0)=-400+/-40 microKelvin and Delta T_{CSZ}^{204
GHz}(0)=-20+/-15 microKelvin (68% CL) for the thermal SZ and CSZ effects in the
cluster respectively. The central value leads to an estimate of the photon to
scalar (or pseudoscalar) coupling strength of g = (5.2 - 23.8) x 10^{-10}
GeV^{-1}, while the 95% confidence bound is estimated to be g < (8.7 - 39.4) x
10^{-10} GeV^{-1}.Comment: 13 pages, 3 figure
Accuracy and feasibility of an android-based digital assessment tool for post stroke visual disorders - The StrokeVision App
Background: Visual impairment affects up to 70% of stroke survivors. We designed an app (StrokeVision) to facilitate screening for common post stroke visual issues (acuity, visual fields and visual inattention). We sought to describe the test-time, feasibility, acceptability and accuracy of our app based digital visual assessments against a) current methods used for bedside screening, and b) gold standard measures.
Methods: Patients were prospectively recruited from acute stroke settings. Index tests were app based assessments of fields and inattention performed by a trained researcher. We compared against usual clinical screening practice of visual fields to confrontation including inattention assessment (simultaneous stimuli). We also compared app to gold standard assessments of formal kinetic perimetry (Goldman or Octopus Visual Field Assessment); and pencil and paper based tests of inattention (Albert’s, Star Cancellation, and Line Bisection). Results of inattention and field tests were adjudicated by a specialist Neuro-Ophthalmologist. All assessors were masked to each other’s results. Participants and assessors graded acceptability using a bespoke scale that ranged from 0 (completely unacceptable) to 10 (perfect acceptability).
Results: Of 48 stroke survivors recruited, the complete battery of index and reference tests for fields was successfully completed in 45. Similar acceptability scores were observed for app-based (assessor median score 10 [IQR:9-10]; patient 9 [IQR:8-10]) and traditional bedside testing (assessor 10 [IQR:9-10; patient 10 [IQR:9-10]). Median test time was longer for app-based testing (combined time-to-completion of all digital tests 420 seconds [IQR:390-588]) when compared with conventional bedside testing (70 seconds, [IQR:40-70]) but shorter than gold standard testing (1260 seconds, [IQR:1005-1620]). Compared with gold standard assessments, usual screening practice demonstrated 79% sensitivity and 82% specificity for detection of a stroke-related field defect. This compares with 79% sensitivity and 88% specificity for StrokeVision digital assessment.
Conclusion: StrokeVision shows promise as a screening tool for visual complications in the acute phase of stroke. The app is at least as good as usual screening and offers other functionality that may make it attractive for use in acute stroke
Molecular Gas in Candidate Double Barred Galaxies III. A Lack of Molecular Gas?
Most models of double-barred galaxies suggest that a molecular gas component
is crucial for maintaining long-lived nuclear bars. We have undertaken a CO
survey in an attempt to determine the gas content of these systems and to
locate double barred galaxies with strong CO emission that could be candidates
for high resolution mapping. We observed 10 galaxies in CO J=2-1 and J=3-2 and
did not detect any galaxies that had not already been detected in previous CO
surveys. We preferentially detect emission from galaxies containing some form
of nuclear activity. Simulations of these galaxies require that they contain 2%
to 10% gas by mass in order to maintain long-lived nuclear bars. The fluxes for
the galaxies for which we have detections suggest that the gas mass fraction is
in agreement with these models requirements. The lack of emission in the other
galaxies suggests that they contain as little as 7 x 10^6 solar masses of
molecular material which corresponds to < 0.1% gas by mass. This result
combined with the wide variety of CO distributions observed in double barred
galaxies suggests the need for models of double-barred galaxies that do not
require a large, well ordered molecular gas component.Comment: 17 pages (3 figures embedded on pg 17). To appear in the March 10
issue of the Astrophysical Journa
f(R) Gravity and Chameleon Theories
We analyse f(R) modifications of Einstein's gravity as dark energy models in
the light of their connection with chameleon theories. Formulated as
scalar-tensor theories, the f(R) theories imply the existence of a strong
coupling of the scalar field to matter. This would violate all experimental
gravitational tests on deviations from Newton's law. Fortunately, the existence
of a matter dependent mass and a thin shell effect allows one to alleviate
these constraints. The thin shell condition also implies strong restrictions on
the cosmological dynamics of the f(R) theories. As a consequence, we find that
the equation of state of dark energy is constrained to be extremely close to -1
in the recent past. We also examine the potential effects of f(R) theories in
the context of the Eot-wash experiments. We show that the requirement of a thin
shell for the test bodies is not enough to guarantee a null result on
deviations from Newton's law. As long as dark energy accounts for a sizeable
fraction of the total energy density of the Universe, the constraints which we
deduce also forbid any measurable deviation of the dark energy equation of
state from -1. All in all, we find that both cosmological and laboratory tests
imply that f(R) models are almost coincident with a Lambda-CDM model at the
background level.Comment: 18 pages, 5 figure
Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations
We have performed a multi-transition CO study of the centers of seven
double-barred galaxies that exhibit a variety of molecular gas morphologies to
determine if the molecular gas properties are correlated with the nuclear
morphology and star forming activity. Near infrared galaxy surveys have
revealed the existence of nuclear stellar bars in a large number of barred or
lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide
range of morphologies. Recent simulations of double-barred galaxies suggest
that variations in the gas properties may allow it to respond differently to
similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio
is lower in galaxies with centrally concentrated gas distributions and higher
in galaxies with CO emission dispersed around the galactic center in rings and
peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which
indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas
temperature and density, rather than variations in optical depth. There is
evidence that the galaxies which contain more centralized CO distributions are
comprised of molecular gas that is cooler and less dense. Observations suggest
that the star formation rates are higher in the galaxies containing the warmer,
denser, less centrally concentrated gas. It is possible that either the bar
dynamics are responsible for the variety of gas distributions and densities
(and hence the star formation rates) or that the star formation alone is
responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap
Mutations in CHMP2B in lower motor neuron predominant amyotrophic lateral sclerosis (ALS)
Background: Amyotrophic lateral sclerosis (ALS), a common late-onset neurodegenerative disease, is associated with fronto-temporal dementia (FTD) in 3-10% of patients. A mutation in CHMP2B was recently identified in a Danish pedigree with autosomal dominant FTD. Subsequently, two unrelated patients with familial ALS, one of whom also showed features of FTD, were shown to carry missense mutations in CHMP2B. The initial aim of this study was to determine whether mutations in CHMP2B contribute more broadly to ALS pathogenesis.
Methodology/Principal Findings: Sequencing of CHMP2B in 433 ALS cases from the North of England identified 4 cases carrying 3 missense mutations, including one novel mutation, p. Thr104Asn, none of which were present in 500 neurologically normal controls. Analysis of clinical and neuropathological data of these 4 cases showed a phenotype consistent with the lower motor neuron predominant (progressive muscular atrophy (PMA)) variant of ALS. Only one had a recognised family history of ALS and none had clinically apparent dementia. Microarray analysis of motor neurons from CHMP2B cases, compared to controls, showed a distinct gene expression signature with significant differential expression predicting disassembly of cell structure; increased calcium concentration in the ER lumen; decrease in the availability of ATP; down-regulation of the classical and p38 MAPK signalling pathways, reduction in autophagy initiation and a global repression of translation. Transfection of mutant CHMP2B into HEK-293 and COS-7 cells resulted in the formation of large cytoplasmic vacuoles, aberrant lysosomal localisation demonstrated by CD63 staining and impairment of autophagy indicated by increased levels of LC3-II protein. These changes were absent in control cells transfected with wild-type CHMP2B.
Conclusions/Significance: We conclude that in a population drawn from North of England pathogenic CHMP2B mutations are found in approximately 1% of cases of ALS and 10% of those with lower motor neuron predominant ALS. We provide a body of evidence indicating the likely pathogenicity of the reported gene alterations. However, absolute confirmation of pathogenicity requires further evidence, including documentation of familial transmission in ALS pedigrees which might be most fruitfully explored in cases with a LMN predominant phenotype
ATR maintains chromosomal integrity during postnatal cerebellar neurogenesis and is required for medulloblastoma formation
Microcephaly and medulloblastoma may both result from mutations that compromise genomic stability. We report that ATR, which is mutated in the microcephalic disorder Seckel syndrome, sustains cerebellar growth by maintaining chromosomal integrity during postnatal neurogenesis. Atr deletion in cerebellar granule neuron progenitors (CGNPs) induced proliferation-associated DNA damage, p53 activation, apoptosis and cerebellar hypoplasia in mice. Co-deletions of either p53 or Bax and Bak prevented apoptosis in Atr-deleted CGNPs, but failed to fully rescue cerebellar growth. ATR-deficient CGNPs had impaired cell cycle checkpoint function and continued to proliferate, accumulating chromosomal abnormalities. RNA-Seq demonstrated that the transcriptional response to ATR-deficient proliferation was highly p53 dependent and markedly attenuated by p53 co-deletion. Acute ATR inhibition in vivo by nanoparticle-formulated VE-822 reproduced the developmental disruptions seen with Atr deletion. Genetic deletion of Atr blocked tumorigenesis in medulloblastoma-prone SmoM2 mice. Our data show that p53-driven apoptosis and cell cycle arrest – and, in the absence of p53, non-apoptotic cell death – redundantly limit growth in ATR-deficient progenitors. These mechanisms may be exploited for treatment of CGNP-derived medulloblastoma using ATR inhibition
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