11 research outputs found
Radio Weak Lensing Shear Measurement in the Visibility Domain - II. Source Extraction
This paper extends the method introduced in Rivi et al. (2016b) to measure
galaxy ellipticities in the visibility domain for radio weak lensing surveys.
In that paper we focused on the development and testing of the method for the
simple case of individual galaxies located at the phase centre, and proposed to
extend it to the realistic case of many sources in the field of view by
isolating visibilities of each source with a faceting technique. In this second
paper we present a detailed algorithm for source extraction in the visibility
domain and show its effectiveness as a function of the source number density by
running simulations of SKA1-MID observations in the band 950-1150 MHz and
comparing original and measured values of galaxies' ellipticities. Shear
measurements from a realistic population of 10^4 galaxies randomly located in a
field of view of 1 deg^2 (i.e. the source density expected for the current
radio weak lensing survey proposal with SKA1) are also performed. At SNR >= 10,
the multiplicative bias is only a factor 1.5 worse than what found when
analysing individual sources, and is still comparable to the bias values
reported for similar measurement methods at optical wavelengths. The additive
bias is unchanged from the case of individual sources, but is significantly
larger than typically found in optical surveys. This bias depends on the shape
of the uv coverage and we suggest that a uv-plane weighting scheme to produce a
more isotropic shape could reduce and control additive bias.Comment: 11 pages, 8 figures, MNRAS accepte
RadioLensfit: Bayesian weak lensing measurement in the visibility domain
Observationally, weak lensing has been served so far by optical surveys due
to the much larger number densities of background galaxies achieved, which is
typically by two to three orders of magnitude compared to radio. However, the
high sensitivity of the new generation of radio telescopes such as the Square
Kilometre Array (SKA) will provide a density of detected galaxies that is
comparable to that found at optical wavelengths, and with significant source
shape measurements to make large area radio surveys competitive for weak
lensing studies. This will lead weak lensing to become one of the primary
science drivers in radio surveys too, with the advantage that they will access
the largest scales in the Universe going beyond optical surveys, like LSST and
Euclid, in terms of redshifts that are probed. RadioLensfit is an adaptation to
radio data of "lensfit", a model-fitting approach for galaxy shear measurement,
originally developed for optical weak lensing surveys. Its key advantage is
working directly in the visibility domain, which is the natural approach to
adopt with radio data, avoiding systematics due to the imaging process. We
present results on galaxy shear measurements, including investigation of
sensitivity to instrumental parameters such as the visibilities gridding size,
based on simulations of individual galaxy visibilities performed by using
SKA1-MID baseline configuration. We get an amplitude of the shear bias in the
method comparable with SKA1 requirements for a population of galaxies with
realistic flux and scalelength distributions estimated from the VLA SWIRE
catalog.Comment: 4 pages, 4 figures, The many facets of extragalactic radio surveys:
towards new scientific challenges, Bologna 20-23, 201
Radio Galaxy Detection in the Visibility Domain
We explore a new Bayesian method of detecting galaxies from radio
interferometric data of the faint sky. Working in the Fourier domain, we fit a
single, parameterised galaxy model to simulated visibility data of star-forming
galaxies. The resulting multimodal posterior distribution is then sampled using
a multimodal nested sampling algorithm such as MultiNest. For each galaxy, we
construct parameter estimates for the position, flux, scale-length and
ellipticities from the posterior samples. We first test our approach on
simulated SKA1-MID visibility data of up to 100 galaxies in the field of view,
considering a typical weak lensing survey regime (SNR ) where 98% of
the input galaxies are detected with no spurious source detections. We then
explore the low SNR regime, finding our approach reliable in galaxy detection
and providing in particular high accuracy in positional estimates down to SNR
. The presented method does not require transformation of visibilities
to the image domain, and requires no prior knowledge of the number of galaxies
in the field of view, thus could become a useful tool for constructing accurate
radio galaxy catalogs in the future.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
GPU Accelerated Particle Visualization with Splotch
Splotch is a rendering algorithm for exploration and visual discovery in
particle-based datasets coming from astronomical observations or numerical
simulations. The strengths of the approach are production of high quality
imagery and support for very large-scale datasets through an effective mix of
the OpenMP and MPI parallel programming paradigms. This article reports our
experiences in re-designing Splotch for exploiting emerging HPC architectures
nowadays increasingly populated with GPUs. A performance model is introduced
for data transfers, computations and memory access, to guide our re-factoring
of Splotch. A number of parallelization issues are discussed, in particular
relating to race conditions and workload balancing, towards achieving optimal
performances. Our implementation was accomplished by using the CUDA programming
paradigm. Our strategy is founded on novel schemes achieving optimized data
organisation and classification of particles. We deploy a reference simulation
to present performance results on acceleration gains and scalability. We
finally outline our vision for future work developments including possibilities
for further optimisations and exploitation of emerging technologies.Comment: 25 pages, 9 figures. Astronomy and Computing (2014
Some bounds for the genus of
Starting from a -symmetric crystallization of a closed n-manifolds , we give an algorithm to build a crystallization OF . This algorithm allows to give a formula for the calculation of the regular genus of , in the cases , and some bounds for the genus of the product-manifolds represented
Contribution of parallel NSGA-II in optimal design of water distribution networks
Optimization of water distribution networks is a NP-hard problem that researchers have tried to deal with using different formulations and algorithmic approaches. Among these, multi-objective heuristic algorithms are interesting because of their capacity for dealing with separate objectives that allow us to choose a posteriori the best compromise, but one of their main drawbacks is the long time required to obtain good solutions. Parallel processing is the most promising way to reduce the computing time and can make the convergence to adequate solutions faster. This paper intends to investigate the possibility of improving the efficacy and efficiency of an NSGA-II algorithm by parallelization of the optimization process at the same time. Results of different parallel implementations of NSGA-II applied to optimal design of small-and medium-size water distribution networks are presented. Good speed-up can be reached with a global model, hence improving the algorithm efficiency. Unlike the global model, the island model (or the hierarchical parallelization) can also improve its efficacy because it introduces fundamental changes in the algorithm exploration method. Possibilities offered by parallel island models have been investigated showing that some parameter configurations can find better solutions compared with the serial version of the algorithm. © IWA Publishing 2012