12,315 research outputs found
How to find real-world applications for compressive sensing
The potential of compressive sensing (CS) has spurred great interest in the
research community and is a fast growing area of research. However, research
translating CS theory into practical hardware and demonstrating clear and
significant benefits with this hardware over current, conventional imaging
techniques has been limited. This article helps researchers to find those niche
applications where the CS approach provides substantial gain over conventional
approaches by articulating lessons learned in finding one such application; sea
skimming missile detection. As a proof of concept, it is demonstrated that a
simplified CS missile detection architecture and algorithm provides comparable
results to the conventional imaging approach but using a smaller FPA. The
primary message is that all of the excitement surrounding CS is necessary and
appropriate for encouraging our creativity but we all must also take off our
"rose colored glasses" and critically judge our ideas, methods and results
relative to conventional imaging approaches.Comment: 10 page
Early Science Result from the Japanese Virtual Observatory: AGN and Galaxy Clustering at z = 0.3 to 3.0
We present the result of projected cross correlation analysis of AGNs and
galaxies at redshifts from 0.3 to 3.0. The Japanese Virtual Observatory (JVO)
was used to obtain the Subaru Suprime-Cam images and UKIDSS catalog data around
AGNs. We investigated 1,809 AGNs, which is about ten times larger a sample than
that used in previous studies on AGN-galaxy clustering at redshifts larger than
0.6. 90% of the AGN samples are optically-selected AGN from the SDSS and 2dF
catalogs. The galaxy samples at low redshift includes many redder objects from
UKIDSS survey, while at higher redshift they are mainly blue galaxies from
Suprime-Cam. We found significant excess of galaxies around the AGNs at
redshifts from 0.3 to 1.8. For the low redshift samples (), we obtained
correlation length of 5--6 Mpc (), which
indicates that the AGNs at this redshift range reside in a similar environment
around typical local galaxies. We also found that AGNs at higher redshift
ranges reside in a denser environment than lower redshift AGNs; For AGNs, the cross correlation length was measured as 11
Mpc (). Considering that our galaxies sample is based on
optical observations with Suprime-Cam at the redshift range, it is expected
that blue star-forming galaxies comprise the majority of objects that are
observed to be clustered around the AGNs. It is successfully demonstrated that
the use of the archive through the Virtual Observatory system can provide a
powerful tool for investigating the small scale environment of the intermediate
redshift AGNs.Comment: accepted to PASJ, 36 pages, 21 figures, this is an accepted versio
VLT near-infrared spectra of hard serendipitous Chandra sources
We present near-infrared long-slit spectra of eight optically-dim X-ray
sources obtained with ISAAC on the Very Large Telescope. Six of the sources
have hard X-ray emission with a significant fraction of the counts emerging
above 2 keV. All were discovered serendipitously in the fields of three nearby
galaxy clusters observed with Chandra, and identified through near-infrared
imaging. The X-ray fluxes lie close to the break in the source counts. Two of
the sources show narrow emission lines, and a third has a broad line. One of
the narrow line-emitting sources has a clear redshift identification at z=2.18,
while the other has a tentative determination based on the highest redshift
detection of He I 10830 at z=1.26. The remainder have featureless spectra to
deep limiting equivalent widths of 20--60 angstroms and line flux approx= 5 x
10^{-17} erg/s/cm^2 in the K-band. High-quality J, H and Ks--band images of the
sources were combined with archival optical detections or limits to estimate a
photometric redshift for six. Two sources show complex double morphology. The
hard sources have spectral count ratios consistent with heavily obscured AGN,
while the host galaxy emits much of the optical and near-infrared flux. The
most likely explanation for the featureless continua is that the line photons
are being scattered or destroyed by optically-thick gas and associated dust
with large covering fractions.Comment: Replaced in response to problems with the PDF version of Fig 4 at
arxiv.org, but not at the mirror sites (lanl.gov, soton.ac.uk). No content
change
Bright Ultraviolet Regions and Star Formation Characteristics in Nearby Dwarf Galaxies
We compare star formation in the inner and outer disks of 11 dwarf Irregular
galaxies (dIm) within 3.6 Mpc. The regions are identified on GALEX near-UV
images, and modeled with UV, optical, and near-IR colors to determine masses
and ages. A few galaxies have made 10^5-10^6 Msun complexes in a starburst
phase, while others have not formed clusters in the last 50 Myrs. The maximum
region mass correlates with the number of regions as expected from the
size-of-sample effect. We find no radial gradients in region masses and ages,
even beyond the realm of Halpha emission, although there is an exponential
decrease in the luminosity density and number density of the regions with
radius. Halpha is apparently lacking in the outer parts only because nebular
emission around massive stars is too faint to see. The outermost regions for
the 5 galaxies with HI data formed at average gas surface densities of 1.9-5.9
Msun/pc2. These low average densities imply either that local gas densities are
high or sub-threshold star formation is possible. The distribution of regions
on a log Mass - log age plot is is usually uniform along log age for equal
intervals of log Mass. This uniformity results from either an individual region
mass that varies as 1/age or a region disruption probability that varies as
1/age. A correlation between fading-corrected surface brightness and age
suggests the former.Comment: Astronomical Journal, in press for November 2009. 34 pages, 18
figures, 5 table
Laser Guide Star for Large Segmented-Aperture Space Telescopes, Part I: Implications for Terrestrial Exoplanet Detection and Observatory Stability
Precision wavefront control on future segmented-aperture space telescopes
presents significant challenges, particularly in the context of high-contrast
exoplanet direct imaging. We present a new wavefront control architecture that
translates the ground-based artificial guide star concept to space with a laser
source aboard a second spacecraft, formation flying within the telescope
field-of-view. We describe the motivating problem of mirror segment motion and
develop wavefront sensing requirements as a function of guide star magnitude
and segment motion power spectrum. Several sample cases with different values
for transmitter power, pointing jitter, and wavelength are presented to
illustrate the advantages and challenges of having a non-stellar-magnitude
noise limited wavefront sensor for space telescopes. These notional designs
allow increased control authority, potentially relaxing spacecraft stability
requirements by two orders of magnitude, and increasing terrestrial exoplanet
discovery space by allowing high-contrast observations of stars of arbitrary
brightness.Comment: Submitted to A
Improving Sensitivity to Weak Pulsations with Photon Probability Weighting
All gamma-ray telescopes suffer from source confusion due to their inability
to focus incident high-energy radiation, and the resulting background
contamination can obscure the periodic emission from faint pulsars. In the
context of the Fermi Large Area Telescope, we outline enhanced statistical
tests for pulsation in which each photon is weighted by its probability to have
originated from the candidate pulsar. The probabilities are calculated using
the instrument response function and a full spectral model, enabling powerful
background rejection. With Monte Carlo methods, we demonstrate that the new
tests increase the sensitivity to pulsars by more than 50% under a wide range
of conditions. This improvement may appreciably increase the completeness of
the sample of radio-loud gamma-ray pulsars. Finally, we derive the asymptotic
null distribution for the H-test, expanding its domain of validity to
arbitrarily complex light curves.Comment: 10 pages, 11 figures, published by ApJ; v2 fixes an error in Eq.
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