818 research outputs found
Precision of diffuse 21-cm lensing
We study the limits of accuracy for weak lensing maps of dark matter using
diffuse 21-cm radiation from the pre-reionization epoch using simulations. We
improve on previous "optimal" quadratic lensing estimators by using shear and
convergence instead of deflection angles. We find that non-Gaussianity provides
a limit to the accuracy of weak lensing reconstruction, even if instrumental
noise is reduced to zero. The best reconstruction result is equivalent to
Gaussian sources with effectively independent cell of side length 2.0/h Mpc.
Using a source full map from z=10-20, this limiting sensitivity allows mapping
of dark matter at a Signal-to-Noise ratio (S/N) greater than 1 out to l < 6000,
which is better than any other proposed technique for large area weak lensing
mapping.Comment: 16 pages, 7 figures, submitted to MNRA
Towards optimal parallel PM N-body codes: PMFAST
We present a new parallel PM N-body code named PMFAST that is freely
available to the public. PMFAST is based on a two-level mesh gravity solver
where the gravitational forces are separated into long and short range
components. The decomposition scheme minimizes communication costs and allows
tolerance for slow networks. The code approaches optimality in several
dimensions. The force computations are local and exploit highly optimized
vendor FFT libraries. It features minimal memory overhead, with the particle
positions and velocities being the main cost. The code features support for
distributed and shared memory parallelization through the use of MPI and
OpenMP, respectively.
The current release version uses two grid levels on a slab decomposition,
with periodic boundary conditions for cosmological applications. Open boundary
conditions could be added with little computational overhead. We present timing
information and results from a recent cosmological production run of the code
using a 3712^3 mesh with 6.4 x 10^9 particles. PMFAST is cost-effective,
memory-efficient, and is publicly available.Comment: 18 pages, 11 figure
Self calibration of photometric redshift scatter in weak lensing surveys
Photo-z errors, especially catastrophic errors, are a major uncertainty for
precision weak lensing cosmology. We find that the shear-(galaxy number)
density and density-density cross correlation measurements between photo-z
bins, available from the same lensing surveys, contain valuable information for
self-calibration of the scattering probabilities between the true-z and photo-z
bins. The self-calibration technique we propose does not rely on cosmological
priors nor parameterization of the photo-z probability distribution function,
and preserves all of the cosmological information available from shear-shear
measurement. We estimate the calibration accuracy through the Fisher matrix
formalism. We find that, for advanced lensing surveys such as the planned stage
IV surveys, the rate of photo-z outliers can be determined with statistical
uncertainties of 0.01-1% for galaxies. Among the several sources of
calibration error that we identify and investigate, the {\it galaxy
distribution bias} is likely the most dominant systematic error, whereby
photo-z outliers have different redshift distributions and/or bias than
non-outliers from the same bin. This bias affects all photo-z calibration
techniques based on correlation measurements. Galaxy bias variations of
produce biases in photo-z outlier rates similar to the statistical
errors of our method, so this galaxy distribution bias may bias the
reconstructed scatters at several- level, but is unlikely to completely
invalidate the self-calibration technique.Comment: v2: 19 pages, 10 figures. Added one figure. Expanded discussions.
Accepted to MNRA
Out-of-Core Hydrodynamic Simulations for Cosmological Applications
We present an out-of-core hydrodynamic code for high resolution cosmological
simulations that require terabytes of memory. Out-of-core computation refers to
the technique of using disk space as virtual memory and transferring data in
and out of main memory at high I/O bandwidth. The code is based on a two-level
mesh scheme where short-range physics is solved on a high-resolution, localized
mesh while long-range physics is captured on a lower resolution, global mesh.
The two-level mesh gravity solver allows FFTs to operate on data stored
entirely in memory, which is much faster than the alternative of computing the
transforms out-of-core through non-sequential disk accesses. We also describe
an out-of-core initial conditions generator that is used to prepare large data
sets for cosmological simulations. The out-of-core code is accurate,
cost-effective, and memory-efficient and the current version is implemented to
run in parallel on shared-memory machines. I/O overhead is significantly
reduced down to less than 10% by performing disk operations concurrently with
numerical calculations. The current computational setup, which includes a 32
processor Alpha server and a 3 TB striped SCSI disk array, allows us to run
cosmological simulations with up to 4000^3 grid cells and 2000^3 dark matter
particles.Comment: 19 pages, 10 figures; accepted by New Astronom
The three dimensional power spectrum of dark and luminous matter from the VIRMOS-DESCART cosmic shear survey
We present the first optimal power spectrum estimation and three dimensional
deprojections for the dark and luminous matter and their cross correlations.
The results are obtained using a new optimal fast estimator (Pen 2003)
deprojected using minimum variance and SVD techniques. We show the resulting
3-D power spectra for dark matter and galaxies, and their covariance for the
VIRMOS-DESCART weak lensing shear and galaxy data. The survey is most sensitive
to nonlinear scales k_NL ~ 1 h Mpc^-1. On these scales, our 3-D power spectrum
of dark matter is in good agreement with the RCS 3-D power spectrum found by
(Hoekstra et al 2002). Our galaxy power is similar to that found by the 2MASS
survey, and larger than that of SDSS, APM and RCS, consistent with the expected
difference in galaxy population.
We find an average bias b=1.24+/-0.18 for the I selected galaxies, and a
cross correlation coefficient r=0.75+/-0.23. Together with the power spectra,
these results optimally encode the entire two point information about dark
matter and galaxies, including galaxy-galaxy lensing. We address some of the
implications regarding galaxy halos and mass-to-light ratios. The best fit
``halo'' parameter h=r/b=0.57+/-0.16, suggesting that dynamical masses
estimated using galaxies systematically underestimate total mass.
Ongoing surveys, such as the Canada-France-Hawaii-Telescope-Legacy survey
will significantly improve on the dynamic range, and future photometric
redshift catalogs will allow tomography along the same principles.Comment: 17 pages, 19 figures, submitted to mnra
Changes in Retinol-Binding Protein Concentrations and Thyroid Homeostasis with Nonoccupational Exposure to DDT
BACKGROUND: The insecticide dichlorodiphenyltrichloroethane (DDT) has been used for malaria vector control in the northern and eastern parts of the Vhembe District of Limpopo Province, South Africa, since 1945. Bioaccumulation of DDT raises concern because it reportedly affects thyroid function.
OBJECTIVE: Our objective was to investigate the association between DDT uptake (as reflected in plasma concentrations) and thyroid homeostasis while considering related factors.
METHODS: We compared dietary intake, serum retinol-binding protein (RBP), transthyretin (TTR) and albumin concentrations, and liver and thyroid function between cases with evidence of a body burden of DDT in the circulation (concentration of any DDT isomer ≥ 0.02 μg/g lipid; n = 278) and controls (concentration of all DDT isomers < 0.02 μg/g lipid; n = 40) in a cross-sectional study. Further analyses were performed to assess the relevance of changes in RBP status associated with DDT uptake.
RESULTS: RBP concentrations below the reference range were more prevalent in cases (54% vs. 10% in controls; χ2 = 27.4; p < 0.001), which could not be explained by nutrient intake. We observed significantly lower thyroid hormone concentrations among cases (p ≤ 0.01). We also observed a significant linear trend for serum concentrations of free thyroxine and free triiodothyronine (p < 0.001) and a significant quadratic trend for serum thyroid-stimulating hormone (p = 0.025) and TTR (p < 0.001) across the control group and case groups with normal and relatively low RBP concentrations. Relatively low RBP concentrations were associated with significantly higher DDT and 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (DDE) isomer concentrations and with a higher DDE/DDT ratio (p ≤ 0.01), which signifies long-term exposure. Inadequate intake of vitamin A and zinc were observed in 84% and 58%, respectively, of the total study population.
CONCLUSION: RBP concentrations appear to decrease in the presence of long-term DDT uptake, which may have deleterious effects on thyroid function and vitamin A nutritional status. This is of major concern in a population with poor vitamin A and zinc intake
Limits on the ultra-bright Fast Radio Burst population from the CHIME Pathfinder
We present results from a new incoherent-beam Fast Radio Burst (FRB) search
on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Pathfinder. Its
large instantaneous field of view (FoV) and relative thermal insensitivity
allow us to probe the ultra-bright tail of the FRB distribution, and to test a
recent claim that this distribution's slope, , is quite small. A 256-input incoherent beamformer was
deployed on the CHIME Pathfinder for this purpose. If the FRB distribution were
described by a single power-law with , we would expect an FRB
detection every few days, making this the fastest survey on sky at present. We
collected 1268 hours of data, amounting to one of the largest exposures of any
FRB survey, with over 2.4\,\,10\,deg\,hrs. Having seen no
bursts, we have constrained the rate of extremely bright events to
\,sky\,day above \,220 Jy\,ms
for between 1.3 and 100\,ms, at 400--800\,MHz. The non-detection also
allows us to rule out with 95 confidence, after
marginalizing over uncertainties in the GBT rate at 700--900\,MHz, though we
show that for a cosmological population and a large dynamic range in flux
density, is brightness-dependent. Since FRBs now extend to large
enough distances that non-Euclidean effects are significant, there is still
expected to be a dearth of faint events and relative excess of bright events.
Nevertheless we have constrained the allowed number of ultra-intense FRBs.
While this does not have significant implications for deeper, large-FoV surveys
like full CHIME and APERTIF, it does have important consequences for other
wide-field, small dish experiments
First Detection of Cosmic Structure in the 21-cm Intensity Field
We present the first statistically significant detection of cosmic structure
using broadly distributed hydrogen radio emission. This is accomplished using a
cross correlation with optical galaxies. Statistical noise levels of K
are achieved, unprecedented in this frequency band. This lends support to the
idea that large volumes of the universe can be rapidly mapped without the need
to resolve individual faint galaxies, enabling precise constraints to dark
energy models. We discuss strategies for improved intensity mapping.Comment: 5 pages, 4 figures, submitted to MNRA
Absolute estimation of initial concentrations of amplicon in a real-time RT-PCR process
<p>Abstract</p> <p>Background</p> <p>Since real time PCR was first developed, several approaches to estimating the initial quantity of template in an RT-PCR reaction have been tried. While initially only the early thermal cycles corresponding to exponential duplication were used, lately there has been an effort to use all of the cycles in a PCR. The efforts have included both fitting empirical sigmoid curves and more elaborate mechanistic models that explore the chemical reactions taking place during each cycle. The more elaborate mechanistic models require many more parameters than can be fit from a single amplification, while the empirical models provide little insight and are difficult to tailor to specific reactants.</p> <p>Results</p> <p>We directly estimate the initial amount of amplicon using a simplified mechanistic model based on chemical reactions in the annealing step of the PCR. The basic model includes the duplication of DNA with the digestion of Taqman probe and the re-annealing between previously synthesized DNA strands of opposite orientation. By modelling the amount of Taqman probe digested and matching that with the observed fluorescence, the conversion factor between the number of fluorescing dye molecules and observed fluorescent emission can be estimated, along with the absolute initial amount of amplicon and the rate parameter for re-annealing. The model is applied to several PCR reactions with known amounts of amplicon and is shown to work reasonably well. An expanded version of the model allows duplication of amplicon without release of fluorescent dye, by adding 1 more parameter to the model. The additional process is helpful in most cases where the initial primer concentration exceeds the initial probe concentration. Software for applying the algorithm to data may be downloaded at <url>http://www.niehs.nih.gov/research/resources/software/pcranalyzer/</url></p> <p>Conclusion</p> <p>We present proof of the principle that a mechanistically based model can be fit to observations from a single PCR amplification. Initial amounts of amplicon are well estimated without using a standard solution. Using the ratio of the predicted initial amounts of amplicon from 2 PCRs is shown to work well even when the absolute amounts of amplicon are underestimated in the individual PCRs.</p
Bone refilling in cortical bone multicellular units: Insights into tetracycline double labelling from a computational model
Bone remodelling is carried out by `bone multicellular units' (BMUs) in which
active osteoclasts and active osteoblasts are spatially and temporally coupled.
The refilling of new bone by osteoblasts towards the back of the BMU occurs at
a rate that depends both on the number of osteoblasts and on their secretory
activity. In cortical bone, a linear phenomenological relationship between
matrix apposition rate (MAR) and BMU cavity radius is found experimentally. How
this relationship emerges from the combination of complex, nonlinear
regulations of osteoblast number and secretory activity is unknown.
Here, we extend our previous mathematical model of cell development within a
single BMU to investigate how osteoblast number and osteoblast secretory
activity vary along the BMU's closing cone. MARs predicted by the model are
compared with data from tetracycline double labelling experiments. We find that
the linear phenomenological relationship observed in these experiments between
MAR and BMU cavity radius holds for most of the refilling phase simulated by
our model, but not near the start and end of refilling. This suggests that at a
particular bone site undergoing remodelling, bone formation starts and ends
rapidly. Our model also suggests that part of the observed cross-sectional
variability in tetracycline data may be due to different bone sites being
refilled by BMUs at different stages of their lifetime. The different stages of
a BMU's lifetime depend on whether the cell populations within the BMU are
still developing or have reached a quasi-steady state while travelling through
bone. We find that due to their longer lifespan, active osteoblasts reach a
quasi-steady distribution more slowly than active osteoclasts. We suggest that
this fact may locally enlarge the Haversian canal diameter (due to a local lack
of osteoblasts compared to osteoclasts) near the BMU's point of origin.Comment: 16 pages, 6 figures, 3 tables. V3: minor changes: added 2 paragraphs
(BMU cavity in Section 2 and Model Robustness in Section 4), references
[52,54
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