1,361 research outputs found
Inhomogeneous systematic signals in cosmic shear observations
We calculate the systematic errors in the weak gravitational lensing power
spectrum which would be caused by spatially varying calibration (i.e.
multiplicative) errors, such as might arise from uncorrected seeing or
extinction variations. The systematic error is fully described by the angular
two-point correlation function of the systematic in the case of the 2D lensing
that we consider here. We investigate three specific cases: Gaussian,
``patchy'' and exponential correlation functions. In order to keep systematic
errors below statistical errors in future LSST-like surveys, the spatial
variation of calibration should not exceed 3% rms. This conclusion is
independently true for all forms of correlation function we consider. The
relative size the E- and B-mode power spectrum errors does, however, depend
upon the form of the correlation function, indicating that one cannot repair
the E-mode power spectrum systematics by means of the B-mode measurements.Comment: 8 pages, 3 figures. Changes reflect PRD published versio
Resampling images in Fourier domain
When simulating sky images, one often takes a galaxy image defined by
a set of pixelized samples and an interpolation kernel, and then wants to
produce a new sampled image representing this galaxy as it would appear with a
different point-spread function, a rotation, shearing, or magnification, and/or
a different pixel scale. These operations are sometimes only possible, or most
efficiently executed, as resamplings of the Fourier transform of
the image onto a -space grid that differs from the one produced by a
discrete Fourier transform (DFT) of the samples. In some applications it is
essential that the resampled image be accurate to better than 1 part in ,
so in this paper we first use standard Fourier techniques to show that
Fourier-domain interpolation with a wrapped sinc function yields the exact
value of in terms of the input samples and kernel. This operation
scales with image dimension as and can be prohibitively slow, so we next
investigate the errors accrued from approximating the sinc function with a
compact kernel. We show that these approximations produce a multiplicative
error plus a pair of ghost images (in each dimension) in the simulated image.
Standard Lanczos or cubic interpolators, when applied in Fourier domain,
produce unacceptable artifacts. We find that errors part in can be
obtained by (1) 4-fold zero-padding of the original image before executing the
DFT, followed by (2) resampling to the desired grid using
a 6-point, piecewise-quintic interpolant that we design expressly to minimize
the ghosts, then (3) executing the DFT back to domain.Comment: Typographical and one algebraic correction, to appear in PASP March
201
Detectability of CMB tensor B modes via delensing with weak lensing galaxy surveys
We analyze the possibility of delensing CMB polarization maps using
foreground weak lensing (WL) information. We build an estimator of the CMB
lensing potential out of optimally combined projected potential estimators to
different source redshift bins. Our estimator is most sensitive to the redshift
depth of the WL survey, less so to the shape noise level. Estimators built
using galaxy surveys like LSST and SNAP yield a 30-50% reduction in the lensing
B-mode power. We illustrate the potential advantages of a 21-cm survey by
considering a fiducial WL survey for which we take the redshift depth zmax and
the effective angular concentration of sources n as free parameters. For a
noise level of 1 muK arcmin in the polarization map itself, as projected for a
CMBPol experiment, and a beam with FWHM=10 arcmin, we find that going to
zmax=20 at n=100 gal/sqarcmin yields a delensing performance similar to that of
a quadratic lensing potential estimator applied to small-scale CMB maps: the
lensing B-mode contamination is reduced by almost an order of magnitude. In
this case, there is also a reduction by a factor of ~4 in the detectability
threshold of the tensor B-mode power. At this CMB noise level, there is little
gain from sources with zmax>20. The delensing gains are lost if the CMB beam
exceeds ~20 arcmin. The delensing efficiency and useful zmax depend acutely on
the CMB map noise level, but beam sizes below 10 arcmin do not help. Delensing
via foreground sources does not require arcminute-resolution CMB observations,
a substantial practical advantage over the use of CMB observables for
delensing.Comment: 10 pages, 5 figures; accepted for publication in Physical Review
Photometric Redshift Biases from Galaxy Evolution
Proposed cosmological surveys will make use of photometric redshifts of
galaxies that are significantly fainter than any complete spectroscopic
redshift surveys that exist to train the photo-z methods. We investigate the
photo-z biases that result from known differences between the faint and bright
populations: a rise in AGN activity toward higher redshift, and a metallicity
difference between intrinsically luminous and faint early-type galaxies. We
find that even very small mismatches between the mean photometric target and
the training set can induce photo-z biases large enough to corrupt derived
cosmological parameters significantly. A metallicity shift of ~0.003dex in an
old population, or contamination of any galaxy spectrum with ~0.2% AGN flux, is
sufficient to induce a 10^-3 bias in photo-z. These results highlight the
danger in extrapolating the behavior of bright galaxies to a fainter
population, and the desirability of a spectroscopic training set that spans all
of the characteristics of the photo-z targets, i.e. extending to the 25th mag
or fainter galaxies that will be used in future surveys
Digital Marketing: A Deep Dive & Path Forward
My project was a deep dive and a path forward into the Commonwealth Associates, Inc.\u27s digital marketing presence using the tools, analytics, and current practices acquired from MBA 631. This included critical assessments, key take-aways with recommendations, and also included additional funds for Commonwealth’s 2021 marketing budget.https://ecommons.udayton.edu/grad_showcase/1000/thumbnail.jp
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