18,565 research outputs found
A Note on Methods of Producing Corrected Side Scan Sonar Displays
Existing methods of display of side scan sonar information are reviewed. A proposal is made for a new type of display which will enable substantial areas to be surveyed and the results displayed in a form corrected for the major errors introduced by ship motion
K+A Galaxies as the Aftermath of Gas-Rich Mergers: Simulating the Evolution of Galaxies as Seen by Spectroscopic Surveys
Models of poststarburst (or "K+A") galaxies are constructed by combining
fully three-dimensional hydrodynamic simulations of galaxy mergers with
radiative transfer calculations of dust attenuation. Spectral line catalogs are
generated automatically from moderate-resolution optical spectra calculated as
a function of merger progress in each of a large suite of simulations. The
mass, gas fraction, orbital parameters, and mass ratio of the merging galaxies
are varied systematically, showing that the lifetime and properties of the K+A
phase are strong functions of merger scenario. K+A durations are generally less
than ~0.1-0.3 Gyr, significantly shorter than the commonly assumed 1 Gyr, which
is obtained only in rare cases, owing to a wide variation in star formation
histories resulting from different orbital and progenitor configurations.
Combined with empirical merger rates, the model lifetimes predict
rapidly-rising K+A fractions as a function of redshift that are consistent with
results of large spectroscopic surveys, resolving tension between the observed
K+A abundance and that predicted when one assumes the K+A duration is the
lifetime of A stars (~1 Gyr). The effects of dust attenuation, viewing angle,
and aperture bias on our models are analyzed. In some cases, the K+A features
are longer-lived and more pronounced when AGN feedback removes dust from the
center, uncovering the young stars formed during the burst. In this picture,
the K+A phase begins during or shortly after the bright starburst/AGN phase in
violent mergers, and thus offers a unique opportunity to study the effects of
quasar and star formation feedback on the gas reservoir and evolution of the
remnant. Analytic fitting formulae are provided for the estimates of K+A
incidence as a function of merger scenario.Comment: 26 pages, 13 figures; ApJ; minor changes to reflect accepted versio
A new source detection algorithm using FDR
The False Discovery Rate (FDR) method has recently been described by Miller
et al (2001), along with several examples of astrophysical applications. FDR is
a new statistical procedure due to Benjamini and Hochberg (1995) for
controlling the fraction of false positives when performing multiple hypothesis
testing. The importance of this method to source detection algorithms is
immediately clear. To explore the possibilities offered we have developed a new
task for performing source detection in radio-telescope images, Sfind 2.0,
which implements FDR. We compare Sfind 2.0 with two other source detection and
measurement tasks, Imsad and SExtractor, and comment on several issues arising
from the nature of the correlation between nearby pixels and the necessary
assumption of the null hypothesis. The strong suggestion is made that
implementing FDR as a threshold defining method in other existing
source-detection tasks is easy and worthwhile. We show that the constraint on
the fraction of false detections as specified by FDR holds true even for highly
correlated and realistic images. For the detection of true sources, which are
complex combinations of source-pixels, this constraint appears to be somewhat
less strict. It is still reliable enough, however, for a priori estimates of
the fraction of false source detections to be robust and realistic.Comment: 17 pages, 7 figures, accepted for publication by A
A position sensitive phoswich hard X-ray detector system
A prototype position sensitive phoswich hard X-ray detector, designed for eventual astronomical usage, was tested in the laboratory. The scintillation crystal geometry was designed on the basis of a Monte Carlo simulation of the internal optics and includes a 3mm thick NaI(T1) primary X-ray detector which is actively shielded by a 20 mm thick CsI(T1) scintillation crystal. This phoswich arrangement is viewed by a number two inch photomultipliers. Measured values of the positional and spectral resolution of incident X-ray photons are compared with calculation
Fabrication process developments of a structural model for research on hydrogen- fueled hypersonic vehicles
Test structural model for hydrogen-fueled hypersonic vehicle
The quantum Casimir operators of \Uq and their eigenvalues
We show that the quantum Casimir operators of the quantum linear group
constructed in early work of Bracken, Gould and Zhang together with one extra
central element generate the entire center of \Uq. As a by product of the
proof, we obtain intriguing new formulae for eigenvalues of these quantum
Casimir operators, which are expressed in terms of the characters of a class of
finite dimensional irreducible representations of the classical general linear
algebra.Comment: 10 page
Microjansky sources at 1.4 GHz
We present a deep 1.4 GHz survey made with the Australia Telescope Compact
Array (ATCA), having a background RMS of 9 microJy near the image phase centre,
up to 25 microJy at the edge of a 50' field of view. Over 770 radio sources
brighter than 45 microJy have been catalogued in the field. The differential
source counts in the deep field provide tentative support for the growing
evidence that the microjansky radio population exhibits significantly higher
clustering than found at higher flux density cutoffs. The optical
identification rate on CCD images is approximately 50% to R=22.5, and the
optical counterparts of the faintest radio sources appear to be mainly single
galaxies close to this optical magnitude limit.Comment: 6 pages, 4 figures, accepted by ApJ Letters 4 May 199
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