1,489 research outputs found
Large-scale Correlation of Mass and Galaxies with the Lyman-alpha Forest Transmitted Flux
We present predictions of the correlation between the Lyman-alpha forest
absorption in quasar spectra and the mass within \sim 5 Mpc/h (comoving) of the
line of sight, using fully hydrodynamic and hydro-PM numerical simulations of
the cold dark matter model supported by present observations. The observed
correlation based on galaxies and the Lya forest can be directly compared to
our theoretical results, assuming that galaxies are linearly biased on large
scales. Specifically, we predict the average value of the mass fluctuation,
, conditioned to a fixed value of the Lya forest transmitted flux
delta_F, after they have been smoothed over a 10 Mpc/h cube and line of sight
interval, respectively. We find that /sigma_m as a function of
delta_F/sigma_F has a slope of 0.6 at this smoothing scale, where sigma_m and
sigma_F are the rms dispersions (this slope should decrease with the smoothing
scale). We show that this value is largely insensitive to the cosmological
model and other Lya forest parameters. Comparison of our predictions to
observations should provide a fundamental test of our ideas on the nature of
the Lya forest and the distribution of galaxies, and can yield a measurement of
the bias factor of any type of galaxies that are observed in the vicinity of
Lya forest lines of sight.Comment: Submitted to ApJ, 41 page
Simulating Reionization: Character and Observability
In recent years there has been considerable progress in our understanding of
the nature and properties of the reionization process. In particular, the
numerical simulations of this epoch have made a qualitative leap forward,
reaching sufficiently large scales to derive the characteristic scales of the
reionization process and thus allowing for realistic observational predictions.
Our group has recently performed the first such large-scale radiative transfer
simulations of reionization, run on top of state-of-the-art simulations of
early structure formation. This allowed us to make the first realistic
observational predictions about the Epoch of Reionization based on detailed
radiative transfer and structure formation simulations. We discuss the basic
features of reionization derived from our simulations and some recent results
on the observational implications for the high-redshift Ly-alpha sources.Comment: 3 pages, to appear in the Proceedings of First Stars III, Santa Fe,
July 2007, AIP Conference Serie
How to measure redshift-space distortions without sample variance
We show how to use multiple tracers of large-scale density with different
biases to measure the redshift-space distortion parameter
beta=f/b=(dlnD/dlna)/b (where D is the growth rate and a the expansion factor),
to a much better precision than one could achieve with a single tracer, to an
arbitrary precision in the low noise limit. In combination with the power
spectrum of the tracers this allows a much more precise measurement of the
bias-free velocity divergence power spectrum, f^2 P_m - in fact, in the low
noise limit f^2 P_m can be measured as well as would be possible if velocity
divergence was observed directly, with rms improvement factor ~[5.2(beta^2+2
beta+2)/beta^2]^0.5 (e.g., ~10 times better than a single tracer for beta=0.4).
This would allow a high precision determination of f D as a function of
redshift with an error as low as 0.1%. We find up to two orders of magnitude
improvement in Figure of Merit for the Dark Energy equation of state relative
to Stage II, a factor of several better than other proposed Stage IV Dark
Energy surveys. The ratio b_2/b_1 will be determined with an even greater
precision than beta, producing, when measured as a function of scale, an
exquisitely sensitive probe of the onset of non-linear bias. We also extend in
more detail previous work on the use of the same technique to measure
non-Gaussianity. Currently planned redshift surveys are typically designed with
signal to noise of unity on scales of interest, and are not optimized for this
technique. Our results suggest that this strategy may need to be revisited as
there are large gains to be achieved from surveys with higher number densities
of galaxies.Comment: 22 pages, 13 figure
Dark energy and curvature from a future baryonic acoustic oscillation survey using the Lyman-alpha forest
We explore the requirements for a Lyman-alpha forest (LyaF) survey designed
to measure the angular diameter distance and Hubble parameter at 2~<z~<4 using
the standard ruler provided by baryonic acoustic oscillations (BAO). The goal
would be to obtain a high enough density of sources to probe the
three-dimensional density field on the scale of the BAO feature. A
percent-level measurement in this redshift range can almost double the Dark
Energy Task Force Figure of Merit, relative to the case with only a similar
precision measurement at z~1, if the Universe is not assumed to be flat. This
improvement is greater than the one obtained by doubling the size of the z~1
survey, with Planck and a weak SDSS-like z=0.3 BAO measurement assumed in each
case. Galaxy BAO surveys at z~1 may be able to make an effective LyaF
measurement simultaneously at minimal added cost, because the required number
density of quasars is relatively small. We discuss the constraining power as a
function of area, magnitude limit (density of quasars), resolution, and
signal-to-noise of the spectra. For example, a survey covering 2000 sq. deg.
and achieving S/N=1.8 per Ang. at g=23 (~40 quasars per sq. deg.) with an
R~>250 spectrograph is sufficient to measure both the radial and transverse
oscillation scales to 1.4% from the LyaF (or better, if fainter magnitudes and
possibly Lyman-break galaxies can be used). At fixed integration time and in
the sky-noise-dominated limit, a wider, noisier survey is generally more
efficient; the only fundamental upper limit on noise being the need to identify
a quasar and find a redshift. Because the LyaF is much closer to linear and
generally better understood than galaxies, systematic errors are even less
likely to be a problem.Comment: 18 pages including 6 figures, submitted to PR
Gaussian Behavior of the Number of Summands in Zeckendorf Decompositions in Small Intervals
Zeckendorf's theorem states that every positive integer can be written
uniquely as a sum of non-consecutive Fibonacci numbers , with initial
terms . We consider the distribution of the number of
summands involved in such decompositions. Previous work proved that as the distribution of the number of summands in the Zeckendorf
decompositions of , appropriately normalized, converges
to the standard normal. The proofs crucially used the fact that all integers in
share the same potential summands.
We generalize these results to subintervals of as ; the analysis is significantly more involved here as different integers
have different sets of potential summands. Explicitly, fix an integer sequence
. As , for almost all the distribution of the number of summands in the Zeckendorf
decompositions of integers in the subintervals ,
appropriately normalized, converges to the standard normal. The proof follows
by showing that, with probability tending to , has at least one
appropriately located large gap between indices in its decomposition. We then
use a correspondence between this interval and to obtain
the result, since the summands are known to have Gaussian behavior in the
latter interval. % We also prove the same result for more general linear
recurrences.Comment: Version 1.0, 8 page
Benford Behavior of Zeckendorf Decompositions
A beautiful theorem of Zeckendorf states that every integer can be written
uniquely as the sum of non-consecutive Fibonacci numbers . A set is said to satisfy Benford's law if
the density of the elements in with leading digit is
; in other words, smaller leading digits are more
likely to occur. We prove that, as , for a randomly selected
integer in the distribution of the leading digits of the
Fibonacci summands in its Zeckendorf decomposition converge to Benford's law
almost surely. Our results hold more generally, and instead of looking at the
distribution of leading digits one obtains similar theorems concerning how
often values in sets with density are attained.Comment: Version 1.0, 12 pages, 1 figur
Evidence for Quadratic Tidal Tensor Bias from the Halo Bispectrum
The relation between the clustering properties of luminous matter in the form
of galaxies and the underlying dark matter distribution is of fundamental
importance for the interpretation of ongoing and upcoming galaxy surveys. The
so called local bias model, where galaxy density is a function of local matter
density, is frequently discussed as a means to infer the matter power spectrum
or correlation function from the measured galaxy correlation. However,
gravitational evolution generates a term quadratic in the tidal tensor and thus
non-local in the density field, even if this term is absent in the initial
conditions (Lagrangian space). Because the term is quadratic, it contributes as
a loop correction to the power spectrum, so the standard linear bias picture
still applies on large scales, however, it contributes at leading order to the
bispectrum for which it is significant on all scales. Such a term could also be
present in Lagrangian space if halo formation were influenced by the tidal
field. We measure the corresponding coupling strengths from the
matter-matter-halo bispectrum in numerical simulations and find a non-vanishing
coefficient for the tidal tensor term. We find no scale dependence of the bias
parameters up to k=0.1 h/Mpc and that the tidal effect is increasing with halo
mass. While the Lagrangian bias picture is a better description of our results
than the Eulerian bias picture, our results suggest that there might be a tidal
tensor bias already in the initial conditions. We also find that the
coefficients of the quadratic density term deviate quite strongly from the
theoretical predictions based on the spherical collapse model and a universal
mass function. Both quadratic density and tidal tensor bias terms must be
included in the modeling of galaxy clustering of current and future surveys if
one wants to achieve the high precision cosmology promise of these datasets.Comment: 14 pages, 4 figures, 1 tabl
Seasonal and Spatial Patterns Differ Between Intracellular and Extracellular Antibiotic Resistance Genes in Urban Stormwater Runoff
Antibiotic resistance is a public health threat that is exacerbated by the dispersion of antibiotic resistance genes (ARGs) into aquatic environments. Urban stormwater runoff has been recognized as a source of and a mechanism by which intracellular ARGs (iARGs) can be transported into receiving environments. The prevalence and behavior of extracellular ARGs (eARGs) in stormwater, however, has yet to be considered. This study quantified four iARGs and eARGs under baseflow and stormflow conditions. Urban stormwater runoff was found to be a source of all the ARGs examined, with the absolute concentration of all iARGs and two eARGs (ermF and tetC) increasing significantly (p \u3c 0.05) between baseflow and stormflow. The occurrence of iARGs and eARGs in stormwater runoff was also investigated across three seasons to differentiate temporal trends. All eARGs were found to be most abundant in the fall, while the iARGs did not display a consistent seasonal trend. Following, spatial patterns of the ARGs were elucidated by targeting four sampling locations, including direct runoff from a curbside storm inlet and a stormwater outfall, and two receiving environments, the Menomonee River and Lake Michigan. Stormwater was found to have the largest impact, in terms of the percent increase in ARG concentrations between baseflow and stormflow, on the outfall location where on average the iARGs and eARGs increased 16% and 12.3%, respectively. The variability in seasonal and spatial trends between the iARGs and eARGs suggests a difference in sources and transport mechanisms of the ARGs into the environment. Moreover, the results of this study revealed that eARGs are relevant contaminants to consider when determining the threat of antibiotic resistance originating from stormwater runoff
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Implementing a mobile diagnostic unit to increase access to imaging and laboratory services in western Kenya.
Access to basic imaging and laboratory services remains a major challenge in rural, resource-limited settings in sub-Saharan Africa. In 2016, the Academic Model Providing Access to Healthcare programme in western Kenya implemented a mobile diagnostic unit (MDU) outfitted with a generator-powered X-ray machine and basic laboratory tests to address the lack of these services at rural, low-resource, public health facilities. The objective of this paper is to describe the design, implementation, preliminary impact and operational challenges of the MDU in western Kenya. Since implementing the MDU at seven rural health facilities serving a catchment of over half a million people, over 4500 chest radiographs have been performed, with one or more abnormalities detected in approximately 30% of radiographs. We observed favorable feedback and uptake of MDU services by healthcare workers and patients. However, various operational challenges in the design and construction of the MDU and the transmission and reporting of radiographs in remote areas were encountered. Our experience supports the feasibility of deploying an MDU to increase access to basic radiology and laboratory services in rural, resource-limited settings
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