4,936 research outputs found
Foreground removal for Square Kilometre Array observations of the Epoch of Reionization with the Correlated Component Analysis
We apply the Correlated Component Analysis (CCA) method on simulated data of
the Square Kilometre Array, with the aim of accurately cleaning the 21 cm
reionization signal from diffuse foreground contamination. The CCA has been
developed for the Cosmic Microwave Background, but the application of the
Fourier-domain implementation of this method to the reionization signal is
straightforward.
The CCA is a parametric method to estimate the frequency behaviour of the
foregrounds from the data by using second-order statistics. We test its
performance on foreground simulations of increasing complexity, designed to
challenge the parametric models adopted. We also drop the assumption of
spectral smoothness that most of the methods rely upon. We are able to clean
effectively the simulated data across the explored frequency range (100-200
MHz) for all the foreground simulations. This shows that the CCA method is very
promising for EoR component separation.Comment: 12 pages, 15 figures, accepted by MNRA
A new model of the microwave polarized sky for CMB experiments
We present a new model of the microwave sky in polarization that can be used
to simulate data from CMB polarization experiments. We exploit the most recent
results from the Planck satellite to provide an accurate description of the
diffuse polarized foreground synchrotron and thermal dust emission. Our model
can include the two mentioned foregrounds, and also a constructed template of
Anomalous Microwave Emission (AME). Several options for the frequency
dependence of the foregrounds can be easily selected, to reflect our
uncertainties and to test the impact of different assumptions. Small angular
scale features can be added to the foreground templates to simulate
high-resolution observations. We present tests of the model outputs to show the
excellent agreement with Planck and WMAP data. We determine the range within
which the foreground spectral indices can be varied to be consistent with the
current data. We also show forecasts for a high-sensitivity, high-resolution
full-sky experiment such as the Cosmic ORigin Explorer (COrE). Our model is
released as a python script that is quick and easy to use, available at
\url{http://www.jb.man.ac.uk/~chervias}.Comment: 12 pages, 14 Figures, 1 Table, Accepted by MNRA
SKA Weak Lensing III: Added Value of Multi-Wavelength Synergies for the Mitigation of Systematics
In this third paper of a series on radio weak lensing for cosmology with the
Square Kilometre Array, we scrutinise synergies between cosmic shear
measurements in the radio and optical/near-IR bands for mitigating systematic
effects. We focus on three main classes of systematics: (i) experimental
systematic errors in the observed shear; (ii) signal contamination by intrinsic
alignments; and (iii) systematic effects due to an incorrect modelling of
non-linear scales. First, we show that a comprehensive, multi-wavelength
analysis provides a self-calibration method for experimental systematic
effects, only implying <50% increment on the errors on cosmological parameters.
We also illustrate how the cross-correlation between radio and optical/near-IR
surveys alone is able to remove residual systematics with variance as large as
0.00001, i.e. the same order of magnitude of the cosmological signal. This also
opens the possibility of using such a cross-correlation as a means to detect
unknown experimental systematics. Secondly, we demonstrate that, thanks to
polarisation information, radio weak lensing surveys will be able to mitigate
contamination by intrinsic alignments, in a way similar but fully complementary
to available self-calibration methods based on position-shear correlations.
Lastly, we illustrate how radio weak lensing experiments, reaching higher
redshifts than those accessible to optical surveys, will probe dark energy and
the growth of cosmic structures in regimes less contaminated by non-linearities
in the matter perturbations. For instance, the higher-redshift bins of radio
catalogues peak at z~0.8-1, whereas their optical/near-IR counterparts are
limited to z<0.5-0.7. This translates into having a cosmological signal 2 to 5
times less contaminated by non-linear perturbations.Comment: 16 pages, 10 figures, 2 tables; improved discussion of experimental
systematics in Sec. 2; updated to match published versio
SKA Weak Lensing II: Simulated Performance and Survey Design Considerations
We construct a pipeline for simulating weak lensing cosmology surveys with
the Square Kilometre Array (SKA), taking as inputs telescope sensitivity
curves; correlated source flux, size and redshift distributions; a simple
ionospheric model; source redshift and ellipticity measurement errors. We then
use this simulation pipeline to optimise a 2-year weak lensing survey performed
with the first deployment of the SKA (SKA1). Our assessments are based on the
total signal-to-noise of the recovered shear power spectra, a metric that we
find to correlate very well with a standard dark energy figure of merit. We
first consider the choice of frequency band, trading off increases in number
counts at lower frequencies against poorer resolution; our analysis strongly
prefers the higher frequency Band 2 (950-1760 MHz) channel of the SKA-MID
telescope to the lower frequency Band 1 (350-1050 MHz). Best results would be
obtained by allowing the centre of Band 2 to shift towards lower frequency,
around 1.1 GHz. We then move on to consider survey size, finding that an area
of 5,000 square degrees is optimal for most SKA1 instrumental configurations.
Finally, we forecast the performance of a weak lensing survey with the second
deployment of the SKA. The increased survey size (3\,steradian) and
sensitivity improves both the signal-to-noise and the dark energy metrics by
two orders of magnitude.Comment: 15 pages, 11 figures, 1 table. Comments welcome. Updated to match
published versio
Thurstonian Scaling of Compositional Questionnaire Data
To prevent response biases, personality questionnaires may use comparative response formats. These include forced choice, where respondents choose among a number of items, and quantitative comparisons, where respondents indicate the extent to which items are preferred to each other. The present article extends Thurstonian modeling of binary choice data (Brown & Maydeu-Olivares, 2011a) to “proportion-of-total” (compositional) formats. Following Aitchison (1982), compositional item data are transformed into log-ratios, conceptualized as differences of latent item utilities. The mean and covariance structure of the log-ratios is modelled using Confirmatory Factor Analysis (CFA), where the item utilities are first-order factors, and personal attributes measured by a questionnaire are second-order factors. A simulation study with two sample sizes, N=300 and N=1000, shows that the method provides very good recovery of true parameters and near-nominal rejection rates. The approach is illustrated with empirical data from N=317 students, comparing model parameters obtained with compositional and Likert scale versions of a Big Five measure. The results show that the proposed model successfully captures the latent structures and person scores on the measured traits
Species-specific differences in the expression of the HNF1A, HNF1B and HNF4A genes
addresses: Institute of Biomedical and Clinical Sciences, Peninsula Medical School, University of Exeter, Exeter, United Kingdom. [email protected]: PMCID: PMC2773013types: Journal Article; Research Support, Non-U.S. Gov'tCopyright: © 2009 Harries et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.The HNF1A, HNF1B and HNF4A genes are part of an autoregulatory network in mammalian pancreas, liver, kidney and gut. The layout of this network appears to be similar in rodents and humans, but inactivation of HNF1A, HNF1B or HNF4A genes in animal models cause divergent phenotypes to those seen in man. We hypothesised that some differences may arise from variation in the expression profile of alternatively processed isoforms between species
Influence of Context on Item Parameters in Forced-Choice Personality Assessments
A fundamental assumption in computerized adaptive testing (CAT) is that item parameters are invariant with respect to context – items surrounding the administered item. This assumption, however, may not hold in forced-choice (FC) assessments, where explicit comparisons are made between items included in the same block. We empirically examined the influence of context on item parameters by comparing parameter estimates from two FC instruments. The first instrument was compiled of blocks of three items, whereas in the second, the context was manipulated by adding one item to each block, resulting in blocks of four. The item parameter estimates were highly similar. However, a small number of significant deviations were observed, confirming the importance of context when designing adaptive FC assessments. Two patterns of such deviations were identified, and methods to reduce their occurrences in a FC CAT setting were proposed. It was shown that with a small proportion of violations of the parameter invariance assumption, score estimation remained stable
The Tiered Radio Extragalactic Continuum Simulation (T-RECS)
We present the Tiered Radio Extragalactic Continuum Simulation (T-RECS): a
new simulation of the radio sky in continuum, over the 150 MHz-20 GHz range.
T-RECS models two main populations of radio galaxies: Active Galactic Nuclei
(AGNs) and Star-Forming Galaxies (SFGs), and corresponding sub-populations. Our
model also includes polarized emission over the full frequency range, which has
been characterised statistically for each population using the available
information. We model the clustering properties in terms of probability
distributions of hosting halo masses, and use lightcones extracted from a
high-resolution cosmological simulation to determine the positions of haloes.
This limits the sky area for the simulations including clustering to a 25deg2
field of view. We compare luminosity functions, number counts in total
intensity and polarization, and clustering properties of our outputs to
up-to-date compilations of data and find a very good agreement. We deliver a
set of simulated catalogues, as well as the code to produce them, which can be
used for simulating observations and predicting results from deep radio surveys
with existing and forthcoming radio facilities, such as the Square Kilometre
Array (SKA).Comment: 20 pages, 11 figures, accepted by MNRA
Bismuth-Based Nano- and Microparticles in X-Ray Contrast, Radiation Therapy, and Radiation Shielding Applications
Bismuth has gained attention in preclinical research because of its ability to attenuate X-rays and high biocompatibility, which make it an excellent element for use in a biomedical agent or in radiation shielding. Developments in the synthesis of elemental bismuth nano- and microparticles, their X-radiation interactions, and their biological interactions will be reviewed in this chapter. The chapter will pay special focus to emerging medical applications of elemental bismuth nano- and microparticles, including the possibility of targeted molecular X-ray imaging, photo-thermal and X-radiation dose enhancing therapies for cancer treatment, and the construction of flexible radiation shielding materials and X-ray opaque devices
How water limits influence conservation in Massachusetts towns
Thesis (M.C.P.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 2006.Includes bibliographical references (p. 54-58).It's surprising to learn that the water-rich state of Massachusetts experiences incidences of water stress, where rivers go dry for stretches of the year and where municipalities struggle to meet water demand. Water conservation and demand management is one important part of reducing pressures on water supplies and alleviating ecosystem stress. Although the state has recently revisited water management policies and has promoted revised measures to increase conservation, the actual measures taken are implemented on the scale of the municipality. This thesis examines three affluent suburban municipalities located in stressed river basins in eastern Massachusetts that have taken different degrees of conservation and demand management efforts. The stories reveal that the decisions to curb water demand have been influenced by the degree to which towns have experienced a perceptible limit to their supply. These towns also show us how a crisis, or an event of water shortage, can bring focus to the limits of water, providing an opportunity for town managers to redefine the problem in such a way that conservation is the solution.(cont.) As towns continue to face increasing pressure on water supplies, some municipalities would like to turn to regional water. Although these regional systems can help offset some ecological pressures as well as promote economy of scales, it raises questions as to whether such a system would remove the perceptible limit that gives impetus for conservation.by Anna L. Brown.M.C.P
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