4,404 research outputs found
Noise Estimates for Measurements of Weak Lensing from the Lyman-alpha Forest
We have proposed a method for measuring weak lensing using the Lyman-alpha
forest. Here we estimate the noise expected in weak lensing maps and power
spectra for different sets of observational parameters. We find that surveys of
the size and quality of the ones being done today and ones planned for the
future will be able to measure the lensing power spectrum at a source redshift
of z~2.5 with high precision and even be able to image the distribution of
foreground matter with high fidelity on degree scales. For example, we predict
that Lyman-alpha forest lensing measurement from the Dark Energy Spectroscopic
Instrument survey should yield the mass fluctuation amplitude with statistical
errors of 1.5%. By dividing the redshift range into multiple bins some
tomographic lensing information should be accessible as well. This would allow
for cosmological lensing measurements at higher redshift than are accessible
with galaxy shear surveys and correspondingly better constraints on the
evolution of dark energy at relatively early times.Comment: 8 pages, 8 figures, submitted to MNRA
Weak lensing of the Lyman-alpha forest
The angular positions of quasars are deflected by the gravitational lensing
effect of foreground matter. The Lyman-alpha forest seen in the spectra of
these quasars is therefore also lensed. We propose that the signature of weak
gravitational lensing of the forest could be measured using similar techniques
that have been applied to the lensed Cosmic Microwave Background, and which
have also been proposed for application to spectral data from 21cm radio
telescopes. As with 21cm data, the forest has the advantage of spectral
information, potentially yielding many lensed "slices" at different redshifts.
We perform an illustrative idealized test, generating a high resolution angular
grid of quasars (of order arcminute separation), and lensing the
Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field.
We find that standard quadratic estimators can be used to reconstruct images of
the foreground mass distribution at z~1. There currently exists a wealth of Lya
forest data from quasar and galaxy spectral surveys, with smaller sightline
separations expected in the future. Lyman-alpha forest lensing is sensitive to
the foreground mass distribution at redshifts intermediate between CMB lensing
and galaxy shear, and avoids the difficulties of shape measurement associated
with the latter. With further refinement and application of mass reconstruction
techniques, weak gravitational lensing of the high redshift Lya forest may
become a useful new cosmological probe.Comment: 9 pages, 7 figures, submitted to MNRA
Two-Level Systems in Evaporated Amorphous Silicon
In -beam evaporated amorphous silicon (-Si), the densities of two-level
systems (TLS), and , determined from specific heat
and internal friction measurements, respectively, have been shown to
vary by over three orders of magnitude. Here we show that and
are proportional to each other with a constant of
proportionality that is consistent with the measurement time dependence
proposed by Black and Halperin and does not require the introduction of
additional anomalous TLS. However, and depend strongly
on the atomic density of the film () which depends on both film
thickness and growth temperature suggesting that the -Si structure is
heterogeneous with nanovoids or other lower density regions forming in a dense
amorphous network. A review of literature data shows that this atomic density
dependence is not unique to -Si. These findings suggest that TLS are not
intrinsic to an amorphous network but require a heterogeneous structure to
form
An Optimal Design for Universal Multiport Interferometers
Universal multiport interferometers, which can be programmed to implement any
linear transformation between multiple channels, are emerging as a powerful
tool for both classical and quantum photonics. These interferometers are
typically composed of a regular mesh of beam splitters and phase shifters,
allowing for straightforward fabrication using integrated photonic
architectures and ready scalability. The current, standard design for universal
multiport interferometers is based on work by Reck et al (Phys. Rev. Lett. 73,
58, 1994). We demonstrate a new design for universal multiport interferometers
based on an alternative arrangement of beam splitters and phase shifters, which
outperforms that by Reck et al. Our design occupies half the physical footprint
of the Reck design and is significantly more robust to optical losses.Comment: 8 pages, 4 figure
Impact of a large wildfire on water-soluble organic aerosol in a major urban area: the 2009 Station Fire in Los Angeles County
This study examines the nature of water-soluble organic aerosol measured in Pasadena, CA, under typical conditions and under the influence of a large wildfire (the 2009 Station Fire). During non-fire periods, water-soluble organic carbon (WSOC) variability was driven by photochemical production processes and sea breeze transport, resulting in an average diurnal cycle with a maximum at 15:00 local time (up to 4.9 μg C m^(−3)). During the Station Fire, primary production was a key formation mechanism for WSOC. High concentrations of WSOC (up to 41 μg C m^(−3)) in smoke plumes advected to the site in the morning hours were tightly correlated with nitrate and chloride, numerous aerosol mass spectrometer (AMS) organic mass spectral markers, and total non-refractory organic mass. Processed residual smoke was transported to the measurement site by the sea breeze later in the day, leading to higher afternoon WSOC levels than on non-fire days. Parameters representing higher degrees of oxidation of organics, including the ratios of the organic metrics m/z 44:m/z 57 and m/z 44:m/z 43, were elevated in those air masses. Intercomparisons of relative amounts of WSOC, organics, m/z 44, and m/z 43 show that the fraction of WSOC comprising acid-oxygenates increased as a function of photochemical aging owing to the conversion of aliphatic and non-acid oxygenated organics to more acid-like organics. The contribution of water-soluble organic species to the organic mass budget (10th–90th percentile values) ranged between 27 %–72 % and 27 %–68 % during fire and non-fire periods, respectively. The seasonal incidence of wildfires in the Los Angeles Basin greatly enhances the importance of water-soluble organics, which has implications for the radiative and hygroscopic properties of the regional aerosol
Structural dichroism in the antiferromagnetic insulating phase of V_2O_3
We performed near-edge x-ray absorption spectroscopy (XANES) at V K edge in
the antiferromagnetic insulating (AFI) phase of a 2.8% Cr-doped V_2O_3 single
crystal. Linear dichroism of several percent is measured in the hexagonal plane
and found to be in good agreement with ab-initio calculations based on multiple
scattering theory. This experiment definitively proves the structural origin of
the signal and therefore solves a controversy raised by previous
interpretations of the same dichroism as non-reciprocal. It also calls for a
further investigation of the role of the magnetoelectric annealing procedure in
cooling to the AFI phase.Comment: 4 pages 3 figures. To be published in Phys. Rev. B (2005
The Population of Dark Matter Subhaloes: Mass Functions and Average Mass Loss Rates
Using a cosmological N-Body simulation and a sample of re-simulated
cluster-like haloes, we study the mass loss rates of dark matter subhaloes, and
interpret the mass function of subhaloes at redshift zero in terms of the
evolution of the mass function of systems accreted by the main halo progenitor.
When expressed in terms of the ratio between the mass of the subhalo at the
time of accretion and the present day host mass the unevolved subhalo mass
function is found to be universal. However, the subhalo mass function at
redshift zero clearly depends on , in that more massive host haloes host
more subhaloes. To relate the unevolved and evolved subhalo mass functions, we
measure the subhalo mass loss rate as a function of host mass and redshift. We
find that the average, specific mass loss rate of dark matter subhaloes depends
mainly on redshift. These results suggest a pleasingly simple picture for the
evolution and mass dependence of the evolved subhalo mass function. Less
massive host haloes accrete their subhaloes earlier, which are thus subjected
to mass loss for a longer time. In addition, their subhaloes are typically
accreted by denser hosts, which causes an additional boost of the mass loss
rate. To test the self-consistency of this picture, we use a merger trees
constructed using the extended Press-Schechter formalism, and evolve the
subhalo populations using the average mass loss rates obtained from our
simulations, finding the subhalo mass functions to be in good agreement with
the simulations. [abridged]Comment: 12 pages, 12 figures; submitted to MNRA
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