261 research outputs found
High Dynamic-Range Radio-Interferometric Images at 327 MHz
Radio astronomical imaging using aperture synthesis telescopes requires
deconvolution of the point spread function as well as calibration of the
instrumental characteristics (primary beam) and foreground
(ionospheric/atmospheric) effects. These effects vary in time and also across
the field of view, resulting in directionally-dependent (DD), time-varying
gains. The primary beam will deviate from the theoretical estimate in real
cases at levels that will limit the dynamic range of images if left
uncorrected. Ionospheric electron density variations cause time and position
variable refraction of sources. At low frequencies and sufficiently high
dynamic range this will also defocus the images producing error patterns that
vary with position and also with frequency due to the chromatic aberration of
synthesis telescopes. Superposition of such residual sidelobes can lead to
spurious spectral signals. Field-based ionospheric calibration as well as
"peeling" calibration of strong sources leads to images with higher dynamic
range and lower spurious signals but will be limited by sensitivity on the
necessary short-time scales. The results are improved images although some
artifacts remain.Comment: to appear in Comptes Rendus Physique (2011
Full Resolution Deconvolution of Complex Faraday Spectra
Polarized synchrotron emission from multiple Faraday depths can be separated
by calculating the complex Fourier transform of the Stokes' parameters as a
function of the wavelength squared, known as Faraday Synthesis. As commonly
implemented, the transform introduces an additional term , which
broadens the real and imaginary spectra, but not the amplitude spectrum. We use
idealized tests to investigate whether additional information can be recovered
with a clean process restoring beam set to the narrower width of the peak in
the real ``full" resolution spectrum with . We find that the
choice makes no difference, except for the use of a smaller
restoring beam. With this smaller beam, the accuracy and phase stability are
unchanged for single Faraday components. However, using the smaller restoring
beam for multiple Faraday components we find a) better discrimination of the
components, b) significant reductions in blending of structures in tomography
images, and c) reduction of spurious features in the Faraday spectra and
tomography maps. We also discuss the limited accuracy of information on scales
comparable to the width of the amplitude spectrum peak, and note a clean-bias,
reducing the recovered amplitudes. We present examples using MeerKAT L-band
data. We also revisit the maximum width in Faraday depth to which surveys are
sensitive, and introduce the variable , the width for which the power
drops by a factor of 2. We find that most surveys cannot resolve continuous
Faraday distributions unless the narrower full restoring beam is used.Comment: 17 pages, 23 figures, accepted for publication in MNRAS, 4 April,
202
Recent developments in optical interferometry data standards
A working group on interferometry data standards has been established within IAU Commission 54 (Optical/ Infrared Interferometry). The working group includes members representing the major optical interferometry projects worldwide, and aims to enhance existing standards and develop new ones to satisfy the broad interests of the optical interferometry community. We present the initial work of the group to enhance the OIFITS data exchange standard, and outline the software packages and libraries now available which implement the standard
MeerKAT view of the dancing ghosts : peculiar galaxy pair PKS 2130-538 in Abell 3785
We present MeerKAT L-band (886–1682 MHz) observations of the extended radio structure of the peculiar galaxy pair PKS 2130−538 known as the ‘Dancing Ghosts’. The complex of bending and possibly interacting jets and lobes originate from two active galactic nuclei hosts in the Abell 3785 galaxy cluster, one of which is the brightest cluster galaxy. The radio properties of the PKS 2130−538 – flux density, spectral index, and polarization – are typical for large, bent-tail galaxies. We also investigate a number of thin extended low surface brightness filaments originating from the lobes. South-east from the Dancing Ghosts, we detect a region of low surface brightness emission that has no clear origin. While it could originate from the Abell 3785 radio halo, we investigate the possibility that it is associated with the two PKS 2130−538 hosts. We find no evidence of interaction between the two PKS 2130−538 hosts
The MeerKAT Galaxy Clusters Legacy Survey: star formation in massive clusters at 0.15 < z < 0.35
We investigate dust-unbiased star formation rates (SFR) as a function of the
environment in 20 massive clusters ()
between using radio luminosities () from the
recently released MeerKAT Galaxy Cluster Legacy Survey catalogue. We use
optical data from the Dark Energy Camera Legacy Survey to estimate photo-s
and assign cluster membership. We observe a steady decline in the fraction
() of star-forming galaxies from to the cluster centres
in our full cluster sample, but notice a significant difference in
gradients between clusters hosting large-scale extended radio emission in the
form of haloes and relics (associated with ongoing merger activity) and
non-radio-halo/relic hosting clusters. For star-forming galaxies within
, the in clusters hosting radio haloes and relics
() is higher than in non-radio-halo/relic hosting
clusters (). We observe a difference between the total
SFR normalised by cluster mass for non-radio-halo/relic hosting clusters
( Myr/M) and for clusters with
radio haloes and relics (
Myr/M). There is a decline
in the mass normalised total SFR of clusters for galaxies with SFR above the
luminous infrared galaxies (LIRGs) SFR limit at our redshift slice,
corresponding to 2 Gyr in look-back time. This is consistent with the rapid
decline in SF activity with decreasing redshift amongst cluster LIRGs seen by
previous studies using infrared-derived SFR.Comment: 13 pages, 9 figures. To be published in MNRA
Multiwavelength Observations of the Second Largest Known FR II Radio Galaxy, NVSS 2146+82
We present multi-frequency VLA, multicolor CCD imaging, optical spectroscopy,
and ROSAT HRI observations of the giant FR II radio galaxy NVSS 2146+82. This
galaxy, which was discovered by the NRAO VLA Sky Survey (NVSS), has an angular
extent of nearly 20' from lobe to lobe. The radio structure is normal for an FR
II source except for its large size and regions in the lobes with unusually
flat radio spectra. Our spectroscopy indicates that the optical counterpart of
the radio core is at a redshift of z=0.145, so the linear size of the radio
structure is ~4 h_50^-1 Mpc. This object is therefore the second largest FR II
known (3C 236 is ~6 h_50^-1 Mpc). Optical imaging of the field surrounding the
host galaxy reveals an excess number of candidate galaxy cluster members above
the number typically found in the field surrounding a giant radio galaxy. WIYN
HYDRA spectra of a sample of the candidate cluster members reveal that six
share the same redshift as NVSS 2146+82, indicating the presence of at least a
``rich group'' containing the FR II host galaxy. ROSAT HRI observations of NVSS
2146+82 place upper limits on the X-ray flux of 1.33 x 10^-13 ergs cm^-2 s^-1
for any hot IGM and 3.52 x 10^-14 ergs cm^-2 s^-1 for an X-ray AGN, thereby
limiting any X-ray emission at the distance of the radio galaxy to that typical
of a poor group or weak AGN. Several other giant radio galaxies have been found
in regions with overdensities of nearby galaxies, and a separate study has
shown that groups containing FR IIs are underluminous in X-rays compared to
groups without radio sources. We speculate that the presence of the host galaxy
in an optically rich group of galaxies that is underluminous in X-rays may be
related to the giant radio galaxy phenomenon.Comment: 46 pages, 15 figures, AASTeX aaspp4 style, accepted for publication
in A
HSD3B1 genotype identifies glucocorticoid responsiveness in severe asthma
Asthma resistance to glucocorticoid treatment is a major health problem with unclear etiology. Glucocorticoids inhibit adrenal androgen production. However, androgens have potential benefits in asthma. HSD3B1 encodes for 3β-hydroxysteroid dehydrogenase-1 (3β-HSD1), which catalyzes peripheral conversion from adrenal dehydroepiandrosterone (DHEA) to potent androgens and has a germline missense-encoding polymorphism. The adrenal restrictive HSD3B1(1245A) allele limits conversion, whereas the adrenal permissive HSD3B1(1245C) allele increases DHEA metabolism to potent androgens. In the Severe Asthma Research Program (SARP) III cohort, we determined the association between DHEA-sulfate and percentage predicted forced expiratory volume in 1 s (FEV1PP). HSD3B1(1245) genotypes were assessed, and association between adrenal restrictive and adrenal permissive alleles and FEV1PP in patients with (GC) and without (noGC) daily oral glucocorticoid treatment was determined (n = 318). Validation was performed in a second cohort (SARP I&II; n = 184). DHEA-sulfate is associated with FEV1PP and is suppressed with GC treatment. GC patients homozygous for the adrenal restrictive genotype have lower FEV1PP compared with noGC patients (54.3% vs. 75.1%; P < 0.001). In patients with the homozygous adrenal permissive genotype, there was no FEV1PP difference in GC vs. noGC patients (73.4% vs. 78.9%; P = 0.39). Results were independently confirmed: FEV1PP for homozygous adrenal restrictive genotype in GC vs. noGC is 49.8 vs. 63.4 (P < 0.001), and for homozygous adrenal permissive genotype, it is 66.7 vs. 67.7 (P = 0.92). The adrenal restrictive HSD3B1(1245) genotype is associated with GC resistance. This effect appears to be driven by GC suppression of 3β-HSD1 substrate. Our results suggest opportunities for prediction of GC resistance and pharmacologic intervention
The 2012 Interferometric Imaging Beauty Contest
We present the results of the fifth Interferometric Imaging Beauty Contest.
The contest consists in blind imaging of test data sets derived from model
sources and distributed in the OIFITS format. Two scenarios of imaging with
CHARA/MIRC-6T were offered for reconstruction: imaging a T Tauri disc and
imaging a spotted red supergiant. There were eight different teams competing
this time: Monnier with the software package MACIM; Hofmann, Schertl and
Weigelt with IRS; Thi\'ebaut and Soulez with MiRA ; Young with BSMEM; Mary and
Vannier with MIROIRS; Millour and Vannier with independent BSMEM and MiRA
entries; Rengaswamy with an original method; and Elias with the radio-astronomy
package CASA. The contest model images, the data delivered to the contestants
and the rules are described as well as the results of the image reconstruction
obtained by each method. These results are discussed as well as the strengths
and limitations of each algorithm
Focal endoscopic mucosal resection before radiofrequency ablation is equally effective and safe compared with radiofrequency ablation alone for the eradication of Barrett's esophagus with advanced neoplasia
Endoscopic mucosal resection (EMR) is commonly performed before radiofrequency ablation (RFA) for nodular dysplastic Barrett’s esophagus (BE)
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