11,005 research outputs found
A comparison of aircraft-based surface-layer observations over Denmark Strait and the Irminger sea with meteorological analyses and QuikSCAT winds
A compilation of aircraft observations of the atmospheric surface layer is compared with several meteorological analyses and QuikSCAT wind products. The observations are taken during the Greenland Flow Distortion Experiment, in February and March 2007, during cold-air outbreak conditions and moderate to high wind speeds. About 150 data points spread over six days are used, with each data point derived from a 2-min run (equivalent to a 12 km spatial average). The observations were taken 30–50 m above the sea surface and are adjusted to standard heights. Surface-layer temperature, humidity and wind, as well as sea-surface temperature (SST) and surface turbulent fluxes are compared against co-located data from the ECMWF operational analyses, NCEP Global Reanalyses, NCEP North American Regional Reanalyses (NARR), Met Office North Atlantic European (NAE) operational analyses, two MM5 hindcasts, and two QuikSCAT products. In general, the limited-area models are better at capturing the mesoscale high wind speed features and their associated structure; often the models underestimate the highest wind speeds and gradients. The most significant discrepancies are: a poor simulation of relative humidity by the NCEP global and MM5 models, a cold bias in 2 m air temperature near the sea-ice edge in the NAE model, and an overestimation of wind speed above 20 m s-1 in the QuikSCAT wind products. In addition, the NCEP global, NARR and MM5 models all have significant discrepancies associated with the parametrisation of surface turbulent heat fluxes. A high-resolution prescription of the SST field is crucial in this region, although these were not generally used at this time
Radio-Optical Galaxy Shape Correlations in the COSMOS Field
We investigate the correlations in galaxy shapes between optical and radio
wavelengths using archival observations of the COSMOS field. Cross-correlation
studies between different wavebands will become increasingly important for
precision cosmology as future large surveys may be dominated by systematic
rather than statistical errors. In the case of weak lensing, galaxy shapes must
be measured to extraordinary accuracy (shear systematics of ) in
order to achieve good constraints on dark energy parameters. By using shape
information from overlapping surveys in optical and radio bands, robustness to
systematics may be significantly improved without loss of constraining power.
Here we use HST-ACS optical data, VLA radio data, and extensive simulations to
investigate both our ability to make precision measurements of source shapes
from realistic radio data, and to constrain the intrinsic astrophysical scatter
between the shapes of galaxies as measured in the optical and radio wavebands.
By producing a new image from the VLA-COSMOS L-band radio visibility data that
is well suited to galaxy shape measurements, we are able to extract precise
measurements of galaxy position angles. Comparing to corresponding measurements
from the HST optical image, we set a lower limit on the intrinsic astrophysical
scatter in position angles, between the optical and radio bands, of
radians (or ) at a confidence
level.Comment: 17 pages, 13 figure, 5 tables. Updated to match published version
with a number of typographical correction
Stellar Populations of Bulges in 14 Cluster Disc Galaxies
‘The definitive version is available at www.blackwell-synergy.com.’ Copyright Blackwell Publishing / RAS. DOI: 10.1111/j.1365-2966.2008.13566.xPeer reviewe
The near-IR - L and - n relations
We present near-IR surface photometry (2D-profiling) for a sample of 29
nearby galaxies for which super-massive black hole (SMBH) masses are
constrained. The data is derived from the UKIDSS-LASS survey representing a
significant improvement in image quality and depth over previous studies based
on 2MASS data. We derive the spheroid luminosity and spheroid S\'ersic index
for each galaxy with GALFIT3 and use these data to construct SMBH mass -bulge
luminosity (--) and SMBH - S\'ersic index (--)
relations. The best fit K-band relation for elliptical and disk galaxies is
with an intrinsic scatter of 0.4dex whilst for elliptical
galaxies we find with an intrinsic scatter of 0.31dex. Our
revised -- relation agrees closely with the previous near-IR
constraint by \citet{tex:G07}. The lack of improvement in the intrinsic scatter
in moving to higher quality near-IR data suggests that the SMBH relations are
not currently limited by the quality of the imaging data but is either
intrinsic or a result of uncertainty in the precise number of required
components required in the profiling process. Contrary to expectation (see
\citealt{tex:GD07a}) a relation between SMBH mass and the S\'ersic index was
not found at near-IR wavelengths. This latter outcome is believed to be
explained by the generic inconsistencies between 1D and 2D galaxy profiling
which are currently under further investigation.Comment: 35 pages, 37 figures, MNRAS accepte
L2 Boosting on generalized Hoeffding decomposition for dependent variables. Application to Sensitivity Analysis
This paper is dedicated to the study of an estimator of the generalized
Hoeffding decomposition. We build such an estimator using an empirical
Gram-Schmidt approach and derive a consistency rate in a large dimensional
settings.
Then, we apply a greedy algorithm with these previous estimators to
Sensitivity Analysis. We also establish the consistency of this -boosting up to sparsity assumptions on the signal to analyse. We end the
paper with numerical experiments, which demonstrates the low computational cost
of our method as well as its efficiency on standard benchmark of Sensitivity
Analysis.Comment: 48 pages, 7 Figure
Interpolating point spread function anisotropy
Planned wide-field weak lensing surveys are expected to reduce the
statistical errors on the shear field to unprecedented levels. In contrast,
systematic errors like those induced by the convolution with the point spread
function (PSF) will not benefit from that scaling effect and will require very
accurate modeling and correction. While numerous methods have been devised to
carry out the PSF correction itself, modeling of the PSF shape and its spatial
variations across the instrument field of view has, so far, attracted much less
attention. This step is nevertheless crucial because the PSF is only known at
star positions while the correction has to be performed at any position on the
sky. A reliable interpolation scheme is therefore mandatory and a popular
approach has been to use low-order bivariate polynomials. In the present paper,
we evaluate four other classical spatial interpolation methods based on splines
(B-splines), inverse distance weighting (IDW), radial basis functions (RBF) and
ordinary Kriging (OK). These methods are tested on the Star-challenge part of
the GRavitational lEnsing Accuracy Testing 2010 (GREAT10) simulated data and
are compared with the classical polynomial fitting (Polyfit). We also test all
our interpolation methods independently of the way the PSF is modeled, by
interpolating the GREAT10 star fields themselves (i.e., the PSF parameters are
known exactly at star positions). We find in that case RBF to be the clear
winner, closely followed by the other local methods, IDW and OK. The global
methods, Polyfit and B-splines, are largely behind, especially in fields with
(ground-based) turbulent PSFs. In fields with non-turbulent PSFs, all
interpolators reach a variance on PSF systematics better than
the upper bound expected by future space-based surveys, with
the local interpolators performing better than the global ones
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