593 research outputs found
Improved Parameters and New Lensed Features for Q0957+561 from WFPC2 Imaging
New HST WFPC2 observations of the lensed double QSO 0957+561 will allow
improved constraints on the lens mass distribution and hence will improve the
derived value of H. We first present improved optical positions and
photometry for the known components of this lens. The optical separation
between the A and B quasar images agrees with VLBI data at the 10 mas level,
and the optical center of the primary lensing galaxy G1 coincides with the VLBI
source G' to within 10 mas. The best previous model for this lens (Grogin and
Narayan 1996) is excluded by these data and must be reevaluated.
Several new resolved features are found within 10\arcsec of G1, including an
apparent fold arc with two bright knots. Several other small galaxies are
detected, including two which may be multiple images of each other. We present
positions and crude photometry of these objects.Comment: 7 pages including 2 postscript figures, LaTeX, emulateapj style. Also
available at
http://www.astro.lsa.umich.edu:80/users/philf/www/papers/list.htm
Size of spectroscopic calibration samples for cosmic shear photometric redshifts
Weak gravitational lensing surveys using photometric redshifts can have their
cosmological constraints severely degraded by errors in the photo-z scale. We
explore the cosmological degradation vs the size of the spectroscopic survey
required to calibrate the photo-z probability distribution. Previous work has
assumed a simple Gaussian distribution of photo-z errors; here we describe a
method for constraining an arbitrary parametric photo-z error model. As an
example we allow the photo-z probability distribution to be the sum of
Gaussians. To limit cosmological degradation to a fixed level, photo-z models
with multiple Gaussians require up to 5 times larger calibration sample than
one would estimate from assuming a single-Gaussian model. This degradation
saturates at . Assuming a single Gaussian when the photo-z
distribution has multiple parameters underestimates cosmological parameter
uncertainties by up to 35%. The size of required calibration sample also
depends upon the shape of the fiducial distribution, even when the RMS photo-z
error is held fixed. The required calibration sample size varies up to a factor
of 40 among the fiducial models studied, but this is reduced to a factor of a
few if the photo-z parameters are forced to be slowly varying with redshift.
Finally we show that the size of the required calibration sample can be
substantially reduced by optimizing its redshift distribution. We hope this
study will help stimulate work on better understanding of photo-z errors.Comment: 9 pages 4 figures, minor changes, match the published versio
(4aR,6aS,10aR,10bS)-7,7,10a-Trimethyl-1,4,4a,5,6,6a,7,8,9,10,10a,10b-dodecahydro-2H-naphtho[2,1-c]pyran (Pyamber)
The crystal structure of the title compound, C16H28O, features C—H⋯O hydrogen bonds making C(6) zigzag chains along one 21 screw axis. Within the limits of the data collection affected by crystal quality, the Hooft parameter gave correct indications of the known molecular chirality based on the single O atom anomalous dispersion in contrast to the indeterminate Flack value. Synthetic steps starting from manool are reported
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
7-(5-Methylsulfanyl-β-d-erythrofuranosyl)-7H-pyrrolo[2,3-d]pyrimidin-4-amine monohydrate (MT-tubercidin·H2O)
The title compound, C12H16N4O3S·H2O, which has potential as a possible antimalarial drug, was studied when small deviations in melting points, for two differently aged preparations, were observed. The unexpected existence of a water molecule of crystallization is considered to be the cause of this variation. The 7H-pyrrolo[2,3-d]pyrimidine unit is very slightly puckered with a total puckering amplitude of 0.035 (2) Å; its mean plane makes an angle of 88.40 (12)° with the mean plane through the ribofuranosyl unit. In the crystal, the molecules are bound by strong O—H⋯N and N—H⋯O hydrogen bonds, utilizing all available protons and linking mainly through the water of crystallization
e-Social Science and Evidence-Based Policy Assessment : Challenges and Solutions
Peer reviewedPreprin
Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales
Most of the matter in the universe is not luminous and can be observed
directly only through its gravitational effect. An emerging technique called
weak gravitational lensing uses background galaxies to reveal the foreground
dark matter distribution on large scales. Light from very distant galaxies
travels to us through many intervening overdensities which gravitationally
distort their apparent shapes. The observed ellipticity pattern of these
distant galaxies thus encodes information about the large-scale structure of
the universe, but attempts to measure this effect have been inconclusive due to
systematic errors. We report the first detection of this ``cosmic shear'' using
145,000 background galaxies to reveal the dark matter distribution on angular
scales up to half a degree in three separate lines of sight. The observed
angular dependence of this effect is consistent with that predicted by two
leading cosmological models, providing new and independent support for these
models.Comment: 18 pages, 5 figures: To appear in Nature. (This replacement fixes tex
errors and typos.
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