6,841 research outputs found
Comprehensive Two-Point Analyses of Weak Gravitational Lensing Surveys
We present a framework for analyzing weak gravitational lensing survey data,
including lensing and source-density observables, plus spectroscopic redshift
calibration data. All two-point observables are predicted in terms of
parameters of a perturbed Robertson-Walker metric, making the framework
independent of the models for gravity, dark energy, or galaxy properties. For
Gaussian fluctuations the 2-point model determines the survey likelihood
function and allows Fisher-matrix forecasting. The framework includes nuisance
terms for the major systematic errors: shear measurement errors, magnification
bias and redshift calibration errors, intrinsic galaxy alignments, and
inaccurate theoretical predictions. We propose flexible parameterizations of
the many nuisance parameters related to galaxy bias and intrinsic alignment.
For the first time we can integrate many different observables and systematic
errors into a single analysis. As a first application of this framework, we
demonstrate that: uncertainties in power-spectrum theory cause very minor
degradation to cosmological information content; nearly all useful information
(excepting baryon oscillations) is extracted with ~3 bins per decade of angular
scale; and the rate at which galaxy bias varies with redshift substantially
influences the strength of cosmological inference. The framework will permit
careful study of the interplay between numerous observables, systematic errors,
and spectroscopic calibration data for large weak-lensing surveys.Comment: submitted to Ap
A proposal on the galaxy intrinsic alignment self-calibration in weak lensing surveys
The galaxy intrinsic alignment causes the galaxy ellipticity-ellipticity
power spectrum between two photometric redshifts to decrease faster with
respect to the redshift separation , for fixed mean redshift. This
offers a valuable diagnosis on the intrinsic alignment. We show that the
distinctive dependences of the GG, II and GI correlations on over
the range |\Delta z^P|\la 0.2 can be understood robustly without strong
assumptions on the intrinsic alignment. This allows us to measure the intrinsic
alignment within each conventional photo-z bin of typical size \ga 0.2,
through lensing tomography of photo-z bin size . Both the
statistical and systematical errors in the lensing cosmology can be reduced by
this self-calibration technique.Comment: v2: minor revisions. 5 pages, 4 figures. MNRAS letters in pres
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
2-Amino-4-methylpyridinium 3-carboxy-4-hydroxybenzenesulfonate monohydrate
In the crystal structure of the title salt, C6H9N2
+·C7H5O6S−·H2O, the water molecule acts as an acceptor of bifurcated N—H⋯O hydrogen bonds from the pyridinium H atom and one H atom of the 2-amino group, forming an R
2
1(6) ring. The 3-carboxy-4-hydroxybenzenesulfonate anions self-assemble via O—H⋯O hydrogen bonds, leading to supramolecular chains along the a axis. These chains and R
2
1(6) motifs are linked via O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds, forming a layer parallel to the ac plane. There is also an intramolecular O—H⋯O hydrogen bond in the 3-carboxy-4-hydroxybenzenesulfonate anion, generating an S(6) ring motif
Catastrophic photometric redshift errors: weak lensing survey requirements
We study the sensitivity of weak lensing surveys to the effects of
catastrophic redshift errors - cases where the true redshift is misestimated by
a significant amount. To compute the biases in cosmological parameters, we
adopt an efficient linearized analysis where the redshift errors are directly
related to shifts in the weak lensing convergence power spectra. We estimate
the number Nspec of unbiased spectroscopic redshifts needed to determine the
catastrophic error rate well enough that biases in cosmological parameters are
below statistical errors of weak lensing tomography. While the straightforward
estimate of Nspec is ~10^6 we find that using only the photometric redshifts
with z<=2.5 leads to a drastic reduction in Nspec to ~30,000 while negligibly
increasing statistical errors in dark energy parameters. Therefore, the size of
spectroscopic survey needed to control catastrophic errors is similar to that
previously deemed necessary to constrain the core of the z_s-z_p distribution.
We also study the efficacy of the recent proposal to measure redshift errors by
cross-correlation between the photo-z and spectroscopic samples. We find that
this method requires ~10% a priori knowledge of the bias and stochasticity of
the outlier population, and is also easily confounded by lensing magnification
bias. The cross-correlation method is therefore unlikely to supplant the need
for a complete spectroscopic redshift survey of the source population.Comment: 14 pages, 3 figure
Diagnosing space telescope misalignment and jitter using stellar images
Accurate knowledge of the telescope's point spread function (PSF) is
essential for the weak gravitational lensing measurements that hold great
promise for cosmological constraints. For space telescopes, the PSF may vary
with time due to thermal drifts in the telescope structure, and/or due to
jitter in the spacecraft pointing (ground-based telescopes have additional
sources of variation). We describe and simulate a procedure for using the
images of the stars in each exposure to determine the misalignment and jitter
parameters, and reconstruct the PSF at any point in that exposure's field of
view. The simulation uses the design of the SNAP (http://snap.lbl.gov)
telescope. Stellar-image data in a typical exposure determines secondary-mirror
positions as precisely as . The PSF ellipticities and size, which
are the quantities of interest for weak lensing are determined to and accuracies respectively in each exposure,
sufficient to meet weak-lensing requirements. We show that, for the case of a
space telescope, the PSF estimation errors scale inversely with the square root
of the total number of photons collected from all the usable stars in the
exposure.Comment: 20 pages, 6 figs, submitted to PAS
Balancing Producer Fairness and Efficiency via Prior-Weighted Rating System Design
Online marketplaces use rating systems to promote the discovery of
high-quality products. However, these systems also lead to high variance in
producers' economic outcomes: a new producer who sells high-quality items, may
unluckily receive one low rating early on, negatively impacting their future
popularity. We investigate the design of rating systems that balance the goals
of identifying high-quality products (efficiency) and minimizing the variance
in economic outcomes of producers of similar quality (individual producer
fairness).
We show that there is a trade-off between these two goals: rating systems
that promote efficiency are necessarily less individually fair to producers. We
introduce prior-weighted rating systems as an approach to managing this
trade-off. Informally, the system we propose sets a system-wide prior for the
quality of an incoming product; subsequently, the system updates that prior to
a posterior for each producer's quality based on user-generated ratings over
time. We show theoretically that in markets where products accrue reviews at an
equal rate, the strength of the rating system's prior determines the operating
point on the identified trade-off: the stronger the prior, the more the
marketplace discounts early ratings data (increasing individual fairness), but
the slower the platform is in learning about true item quality (so efficiency
suffers). We further analyze this trade-off in a responsive market where
customers make decisions based on historical ratings. Through calibrated
simulations, we show that the choice of prior strength mediates the same
efficiency-consistency trade-off in this setting. Overall, we demonstrate that
by tuning the prior as a design choice in a prior-weighted rating system,
platforms can be intentional about the balance between efficiency and producer
fairness.Comment: 12 pages, 8 figures, submitted to TheWebConf 202
2-Methyl-3-(4-nitrophenyl)acrylic acid
The title compound, C10H9NO4, forms R
2
2(8) dimers due to intermolecular O—H⋯O hydrogen bonding in the crystal structure. Two dimers are further linked to each other through two intermolecular C—H⋯O hydrogen bonds, forming an R
3
3(7) ring motif. The nitro groups form an intramolecular C—H⋯O hydrogen bond mimicking a five-membered ring. As a result of these hydrogen bonds, polymeric sheets are formed. The aromatic ring makes a dihedral angle of 42.84 (8)° with the carboxylate group and an angle of 8.01 (14)° with the nitro group. There is a π-interaction (N—O⋯π) between the nitro group and the aromatic ring, with a distance of 3.7572 (14) Å between the N atom and the centroid of the aromatic ring
The Impact of Non-Gaussian Errors on Weak Lensing Surveys
The weak lensing power spectrum carries cosmological information via its
dependence on the growth of structure and on geometric factors. Since much of
the cosmological information comes from scales affected by nonlinear
clustering, measurements of the lensing power spectrum can be degraded by
non-Gaussian covariances. Recently there have been conflicting studies about
the level of this degradation. We use the halo model to estimate it and include
new contributions related to the finite size of lensing surveys, following
Rimes and Hamilton's study of 3D simulations. We find that non-Gaussian
correlations between different multipoles can degrade the cumulative
signal-to-noise for the power spectrum amplitude by up to a factor of 2 (or 5
for a worst-case model that exceeds current N-body simulation predictions).
However, using an eight-parameter Fisher analysis we find that the marginalized
errors on individual parameters are degraded by less than 10% (or 20% for the
worst-case model). The smaller degradation in parameter accuracy is primarily
because: individual parameters in a high-dimensional parameter space are
degraded much less than the volume of the full Fisher ellipsoid; lensing
involves projections along the line of sight, which reduce the non-Gaussian
effect; some of the cosmological information comes from geometric factors which
are not degraded at all. We contrast our findings with those of Lee & Pen
(2008) who suggested a much larger degradation in information content. Finally,
our results give a useful guide for exploring survey design by giving the
cosmological information returns for varying survey area, depth and the level
of some systematic errors.Comment: To appear in MNRAS, 22 pages, 12 figures. Minor modifications made
according to the referee comment
- …