146 research outputs found
A Snapshot Survey for Gravitational Lenses Among z>=4.0 Quasars: I. The z>5.7 Sample
Over the last few years, the Sloan Digital Sky Survey (SDSS) has discovered
several hundred quasars with redshift between 4.0 and 6.4. Including the
effects of magnification bias, one expects a priori that an appreciable
fraction of these objects are gravitationally lensed. We have used the Advanced
Camera for Surveys on the Hubble Space Telescope to carry out a snapshot
imaging survey of high-redshift SDSS quasars to search for gravitationally
split lenses. This paper, the first in a series reporting the results of the
survey, describes snapshot observations of four quasars at z = 5.74, 5.82, 5.99
and 6.30, respectively. We find that none of these objects has a lensed
companion within 5 magnitudes with a separation larger than 0.3 arcseconds;
within 2.5 magnitudes, we can rule out companions within 0.1 arcseconds. Based
on the non-detection of strong lensing in these four systems, we constrain the
z~6 luminosity function to a slope of beta>-4.63 (3 sigma), assuming a break in
the quasar luminosity function at M_{1450}^*=-24.0. We discuss the implications
of this constraint on the ionizing background due to quasars in the early
universe. Given that these quasars are not highly magnified, estimates of the
masses of their central engines by the Eddington argument must be taken
seriously, possibly challenging models of black hole formation.Comment: 23 pages, 8 figures, 2 tables, submitted to A
Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization
Bright point sources associated with extragalactic AGN and radio galaxies are
an important foreground for low frequency radio experiments aimed at detecting
the redshifted 21cm emission from neutral hydrogen during the epoch of
reionization. The frequency dependence of the synthesized beam implies that the
sidelobes of these sources will move across the field of view as a function of
observing frequency, hence frustrating line-of-sight foreground subtraction
techniques. We describe a method for subtracting these point sources from dirty
maps produced by an instrument such as the MWA. This technique combines matched
filters with an iterative centroiding scheme to locate and characterize point
sources in the presence of a diffuse background. Simulations show that this
technique can improve the dynamic range of EOR maps by 2-3 orders of magnitude.Comment: 11 pages, 8 figures, 1 table, submitted to PAS
Improved Conformal Mapping of the Borel Plane
The conformal mapping of the Borel plane can be utilized for the analytic
continuation of the Borel transform to the entire positive real semi-axis and
is thus helpful in the resummation of divergent perturbation series in quantum
field theory. We observe that the rate of convergence can be improved by the
application of Pad\'{e} approximants to the Borel transform expressed as a
function of the conformal variable, i.e. by a combination of the analytic
continuation via conformal mapping and a subsequent numerical approximation by
rational approximants. The method is primarily useful in those cases where the
leading (but not sub-leading) large-order asymptotics of the perturbative
coefficients are known.Comment: 6 pages, LaTeX, 2 tables; certain numerical examples adde
PMN J1632-0033: A new gravitationally lensed quasar
We report the discovery of a gravitationally lensed quasar resulting from our
survey for lenses in the southern sky. Radio images of PMN J1632-0033 with the
VLA and ATCA exhibit two compact, flat-spectrum components with separation
1.47" and flux density ratio 13.2. Images with the HST reveal the optical
counterparts to the radio components and also the lens galaxy. An optical
spectrum of the bright component, obtained with the first Magellan telescope,
reveals quasar emission lines at redshift 3.42. Deeper radio images with MERLIN
and the VLBA reveal a faint third radio component located near the center of
the lens galaxy, which is either a third image of the background quasar or
faint emission from the lens galaxy.Comment: 21 pp., including 4 figures; thoroughly revised in light of new
MERLIN/HST data; accepted for publication in A
Modelling and peeling extended sources with shapelets: a Fornax A case study
To make a power spectrum (PS) detection of the 21 cm signal from the Epoch of
Reionisation (EoR), one must avoid/subtract bright foreground sources. Sources
such as Fornax A present a modelling challenge due to spatial structures
spanning from arc seconds up to a degree. We compare modelling with multi-scale
(MS) CLEAN components to 'shapelets', an alternative set of basis functions. We
introduce a new image-based shapelet modelling package, SHAMFI. We also
introduce a new CUDA simulation code (WODEN) to generate point source,
Gaussian, and shapelet components into visibilities. We test performance by
modelling a simulation of Fornax A, peeling the model from simulated
visibilities, and producing a residual PS. We find the shapelet method
consistently subtracts large-angular-scale emission well, even when the
angular-resolution of the data is changed. We find that when increasing the
angular-resolution of the data, the MS CLEAN model worsens at large
angular-scales. When testing on real MWA data, the expected improvement is not
seen in real data because of the other dominating systematics still present.
Through further simulation we find the expected differences to be lower than
obtainable through current processing pipelines. We conclude shapelets are
worthwhile for subtracting extended galaxies, and may prove essential for an
EoR detection in the future, once other systematics have been addressed.Comment: 17 pages, 11 Figures, accepted for publication in Publications of the
Astronomical Society of Australia (18/05/2020). "For the SHAMFI code, see:
https://github.com/JLBLine/SHAMFI" . "For the SHAMFI documentation, see:
https://shamfi.readthedocs.io/" . "For the WODEN code and documentation see:
https://github.com/JLBLine/WODEN
Discovery of Two Gravitationally Lensed Quasars with Image Separations of 3 Arcseconds from the Sloan Digital Sky Survey
We report the discovery of two doubly-imaged quasars, SDSS
J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and
1.789 and with image separations of 2.86'' and 2.90'', respectively. The
objects were selected as lens candidates from the Sloan Digital Sky Survey
(SDSS). Based on the identical nature of the spectra of the two quasars in each
pair and the identification of the lens galaxies, we conclude that the objects
are gravitational lenses. The lenses are complicated; in both systems there are
several galaxies in the fields very close to the quasars, in addition to the
lens galaxies themselves. The lens modeling implies that these nearby galaxies
contribute significantly to the lens potentials. On larger scales, we have
detected an enhancement in the galaxy density near SDSS J100128.61+502756.9.
The number of lenses with image separation of ~3'' in the SDSS already exceeds
the prediction of simple theoretical models based on the standard
Lambda-dominated cosmology and observed velocity function of galaxies.Comment: 24 pages, 9 figures, accepted for publication in Ap
SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar
We report the discovery of SDSSJ115517.35+634622.0, a previously unknown
gravitationally lensed quasar. The lens system exhibits two images of a quasar, with an image separation of 1{\farcs}832 \pm 0.007 . Near-IR
imaging of the system reveals the presence of the lensing galaxy between the
two quasar images. Based on absorption features seen in the Sloan Digital Sky
Survey (SDSS) spectrum, we determine a lens galaxy redshift of .
The lens is rather unusual in that one of the quasar images is only
0{\farcs}22\pm0{\farcs}07 () from the center of the
lens galaxy and photometric modeling indicates that this image is significantly
brighter than predicted by a SIS model. This system was discovered in the
course of an ongoing search for strongly lensed quasars in the dataset from the
SDSS.Comment: 18 pages, 6 figures. Accepted for publication in A
Calibration and Stokes Imaging with Full Embedded Element Primary Beam Model for the Murchison Widefield Array
15 pages, 11 figures. Accepted for publication in PASA. © Astronomical Society of Australia 2017The Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science program, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programs for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescope's primary beam. Recent results from the MWA GaLactic Extragalactic All-sky MWA (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10-20 % in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWA's primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 x 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200 - 231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.Peer reviewedFinal Accepted Versio
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