71,088 research outputs found
Correction of distortion for optimal image stacking in Wide Field Adaptive Optics: Application to GeMS data
The advent of Wide Field Adaptive Optics (WFAO) systems marks the beginning
of a new era in high spatial resolution imaging. The newly commissioned Gemini
South Multi-Conjugate Adaptive Optics System (GeMS) combined with the infrared
camera Gemini South Adaptive Optics Imager (GSAOI), delivers quasi
diffraction-limited images over a field of 2 arc-minutes across. However,
despite this excellent performance, some variable residues still limit the
quality of the analyses. In particular, distortions severely affect GSAOI and
become a critical issue for high-precision astrometry and photometry. In this
paper, we investigate an optimal way to correct for the distortion following an
inverse problem approach. Formalism as well as applications on GeMS data are
presented.Comment: 10 pages, 6 figure
The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
We present the first proper motion measurements for the galaxy M31. We
obtained new V-band imaging data with the HST ACS/WFC and WFC3/UVIS of a
spheroid field near the minor axis, an outer disk field along the major axis,
and a field on the Giant Southern Stream. The data provide 5-7 year time
baselines with respect to pre-existing deep first-epoch observations. We
measure the positions of thousands of M31 stars and hundreds of compact
background galaxies in each field. High accuracy and robustness is achieved by
building and fitting a unique template for each individual object. The average
proper motion for each field is obtained from the average motion of the M31
stars between the epochs with respect to the background galaxies. For the three
fields, the observed proper motions (mu_W,mu_N) are (-0.0458, -0.0376),
(-0.0533, -0.0104), and (-0.0179,-0.0357) mas/yr, respectively. The ability to
average over large numbers of objects and over the three fields yields a final
accuracy of 0.012 mas/yr. The robustness of the proper-motion measurements and
uncertainties are supported by the fact that data from different instruments,
taken at different times and with different telescope orientations, as well as
measurements of different fields, all yield statistically consistent results.
Papers II and III explore the implications for our understanding of the
history, future, and mass of the Local Group. (Abridged)Comment: 42 pages, 13 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
Mapping Cluster Mass Distributions via Gravitational Lensing of Background Galaxies
We present a new method for measuring the projected mass distributions of
galaxy clusters. The gravitational amplification is measured by comparing the
joint distribution in redshift and magnitude of galaxies behind the cluster
with that of field galaxies. We show that the total amplification is directly
related to the surface mass density in the weak field limit, and so it is
possible to map the mass distribution of the cluster. The method is shown to be
limited by discreteness noise and galaxy clustering behind the lens. Galaxy
clustering sets a lower limit to the error along the redshift direction, but a
clustering independent lensing signature may be obtained from the magnitude
distribution at fixed redshift. Statistical techniques are developed for
estimating the surface mass density of the cluster. We extend these methods to
account for any obscuration by cluster halo dust, which may be mapped
independently of the dark matter. We apply the method to a series of numerical
simulations and show the feasibility of the approach. We consider approximate
redshift information, and show how the mass estimates are degraded.Comment: ApJ in press. 23 pages of LaTeX plus figs. Text & figs available by
anonymous ftp from resun03.roe.ac.uk in directory /pub/jap/lens (you need
btp.tex and apj.sty
Optimizing Lossy Compression Rate-Distortion from Automatic Online Selection between SZ and ZFP
With ever-increasing volumes of scientific data produced by HPC applications,
significantly reducing data size is critical because of limited capacity of
storage space and potential bottlenecks on I/O or networks in writing/reading
or transferring data. SZ and ZFP are the two leading lossy compressors
available to compress scientific data sets. However, their performance is not
consistent across different data sets and across different fields of some data
sets: for some fields SZ provides better compression performance, while other
fields are better compressed with ZFP. This situation raises the need for an
automatic online (during compression) selection between SZ and ZFP, with a
minimal overhead. In this paper, the automatic selection optimizes the
rate-distortion, an important statistical quality metric based on the
signal-to-noise ratio. To optimize for rate-distortion, we investigate the
principles of SZ and ZFP. We then propose an efficient online, low-overhead
selection algorithm that predicts the compression quality accurately for two
compressors in early processing stages and selects the best-fit compressor for
each data field. We implement the selection algorithm into an open-source
library, and we evaluate the effectiveness of our proposed solution against
plain SZ and ZFP in a parallel environment with 1,024 cores. Evaluation results
on three data sets representing about 100 fields show that our selection
algorithm improves the compression ratio up to 70% with the same level of data
distortion because of very accurate selection (around 99%) of the best-fit
compressor, with little overhead (less than 7% in the experiments).Comment: 14 pages, 9 figures, first revisio
The Weak Lensing Signal and the Clustering of BOSS Galaxies I: Measurements
A joint analysis of the clustering of galaxies and their weak gravitational
lensing signal is well-suited to simultaneously constrain the galaxy-halo
connection as well as the cosmological parameters by breaking the degeneracy
between galaxy bias and the amplitude of clustering signal. In a series of two
papers, we perform such an analysis at the highest redshift () in
the literature using CMASS galaxies in the Sloan Digital Sky Survey-III Baryon
Oscillation Spectroscopic Survey Eleventh Data Release (SDSS-III/BOSS DR11)
catalog spanning 8300~deg. In this paper, we present details of the
clustering and weak lensing measurements of these galaxies. We define a
subsample of 400,916 CMASS galaxies based on their redshifts and stellar mass
estimates so that the galaxies constitute an approximately volume-limited and
similar population over the redshift range . We obtain a
signal-to-noise ratio for the galaxy clustering measurement. We
also explore the redshift and stellar mass dependence of the clustering signal.
For the weak lensing measurement, we use existing deeper imaging data from the
CFHTLS with publicly available shape and photometric redshift catalogs from
CFHTLenS, but only in a 105~deg area which overlaps with BOSS. This
restricts the lensing measurement to only 5,084 CMASS galaxies. After careful
systematic tests, we find a highly significant detection of the CMASS weak
lensing signal, with total . These measurements form the basis of
the halo occupation distribution and cosmology analysis presented in More et
al. (Paper II).Comment: 15 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
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