2,766 research outputs found
A methodology for determining optimum microwave remote sensor parameters
There are no author-identified significant results in this report
Discovery of Multi-Phase Cold Accretion in a Massive Galaxy at z=0.7
We present detailed photo+collisional ionization models and kinematic models
of the multi-phase absorbing gas, detected within the HST/COS, HST/STIS, and
Keck/HIRES spectra of the background quasar TON 153, at 104 kpc along the
projected minor axis of a star-forming spiral galaxy (z=0.6610). Complementary
g'r'i'Ks photometry and stellar population models indicate that the host galaxy
is dominated by a 4 Gyr stellar population with slightly greater than solar
metallicity and has an estimated log(M*)=11 and a log(Mvir)=13. Photoionization
models of the low ionization absorption, (MgI, SiII, MgII and CIII) which trace
the bulk of the hydrogen, constrain the multi-component gas to be cold
(logT=3.8-5.2) and metal poor (-1.68<[X/H]<-1.64). A lagging halo model
reproduces the low ionization absorption kinematics, suggesting gas coupled to
the disk angular momentum, consistent with cold accretion mode material in
simulations. The CIV and OVI absorption is best modeled in a separate
collisionally ionized metal-poor (-2.50<[X/H]<-1.93) warm phase with logT=5.3.
Although their kinematics are consistent with a wind model, given the 2-2.5dex
difference between the galaxy stellar metallicity and the absorption
metallicity indicates the gas cannot arise from galactic winds. We discuss and
conclude that although the quasar sight-line passes along the galaxy minor axis
at projected distance of 0.3 virial radii, well inside its virial shock radius,
the combination of the relative kinematics, temperatures, and relative
metallicities indicated that the multi-phase absorbing gas arises from cold
accretion around this massive galaxy. Our results appear to contradict recent
interpretations that absorption probing the projected minor axis of a galaxy is
sampling winds.Comment: 16 pages, 11 figures, accepted for publication in MNRA
Induction of mammary neoplasms in the ACI rat by 430-keV neutrons, X-rays, and diethylstilbestrol
Morphology of the Nuclear Disk in M87
A deep, fuly sampled diffraction limited (FWHM ~ 70 mas) narrow-band image of
the central region in M87 was obtained with the Wide Filed and Planetary Camera
2 of the Hubble Space Telescope using the dithering technique. The
H-alpha+[NII] continuum subtracted image reveals a wealth of details in the
gaseous disk structure described earlier by Ford et al. (1994). The disk
morphology is dominated by a well defined three-arm spiral pattern. In
addition, the major spiral arms contain a large number of small "arclets"
covering a range of sizes (0.1-0.3 arcsec = 10-30 pc). The overall surface
brightness profile inside a radius ~1.5" (100 pc) is well represented by a
power-law I(mu) ~ mu^(-1.75), but when the central ~40 pc are excluded it can
be equally well fit by an exponential disk. The major axis position angle
remains constant at about PA_disk ~ 6 deg for the innermost ~1", implying the
disk is oriented nearly perpendicular to the synchrotron jet (PA_jet ~ 291
deg). At larger radial distances the isophotes twist, reflecting the gas
distribution in the filaments connecting to the disk outskirts. The ellipticity
within the same radial range is e = 0.2-0.4, which implies an inclination angle
of i~35 deg. The sense of rotation combined with the dust obscuration pattern
indicate that the spiral arms are trailing.Comment: 5 pages, 3 postscript figures, to appear in the Proceedings of the
M87 Workshop, Ringberg castle, Germany, 15-19 Sep 1997, also available from
http://jhufos.pha.jhu.edu/~zlatan/papers.htm
Evaluation of the soil moisture prediction accuracy of a space radar using simulation techniques
Image simulation techniques were employed to generate synthetic aperture radar images of a 17.7 km x 19.3 km test site located east of Lawrence, Kansas. The simulations were performed for a space SAR at an orbital altitude of 600 km, with the following sensor parameters: frequency = 4.75 GHz, polarization = HH, and angle of incidence range = 7 deg to 22 deg from nadir. Three sets of images were produced corresponding to three different spatial resolutions; 20 m x 20 m with 12 looks, 100 m x 100 m with 23 looks, and 1 km x 1 km with 1000 looks. Each set consisted of images for four different soil moisture distributions across the test site. Results indicate that, for the agricultural portion of the test site, the soil moisture in about 90% of the pixels can be predicted with an accuracy of = + or - 20% of field capacity. Among the three spatial resolutions, the 1 km x 1 km resolution gave the best results for most cases, however, for very dry soil conditions, the 100 m x 100 m resolution was slightly superior
Image synthesis for SAR system, calibration and processor design
The Point Scattering Method of simulating radar imagery rigorously models all aspects of the imaging radar phenomena. Its computational algorithms operate on a symbolic representation of the terrain test site to calculate such parameters as range, angle of incidence, resolution cell size, etc. Empirical backscatter data and elevation data are utilized to model the terrain. Additionally, the important geometrical/propagation effects such as shadow, foreshortening, layover, and local angle of incidence are rigorously treated. Applications of radar image simulation to a proposed calibrated SAR system are highlighted: soil moisture detection and vegetation discrimination
Using Astrometry to Deblend Microlensing Events
We discuss the prospect of deblending microlensing events by observing
astrometric shifts of the lensed stars. Since microlensing searches are
generally performed in very crowded fields, it is expected that stars will be
confusion limited rather than limited by photon statistics. By performing
simulations of events in crowded fields, we find that if we assume a dark lens
and that the lensed star obeys a power law luminosity function, , over half the simulated events show a measurable astrometric
shift. Our simulations included 20000 stars in a Nyquist
sampled CCD frame. For , we found that 58% of the events were
significantly blended , and of those, 73% had a
large astrometric shift . Likewise, for , we found
that 85% of the events were significantly blended, and that 85% of those had
large shifts. Moreover, the shift is weakly correlated to the degree of
blending, suggesting that it may be possible not only to detect the existence
of a blend, but also to deblend events statistically using shift information.Comment: 24 pages, 7 postscript Figure
Optical identification of the companion to PSR J1911-5958A, the pulsar binary in the outskirts of NGC 6752
We report on the identification of the optical counterpart of the binary
millisecond pulsar PSR J1911-5958A, located in the outskirts of the globular
cluster NGC 6752. At the position of the pulsar we find an object with V=22.08,
B-V=0.38, U-B=-0.49. The object is blue with respect to the cluster main
sequence by 0.8 magnitudes in B-V. We argue that the object is the white dwarf
companion of the pulsar. Comparison with white dwarf cooling models shows that
this magnitude and colors are consistent with a low-mass white dwarf at the
distance of NGC 6752. If associated with NGC 6752, the white dwarf is
relatively young, <2 Gyr, which sets constraints on the formation of the binary
and its ejection from the core of the globular cluster.Comment: Accepted for publication in A&A Letters (September 1st, 2003
Development of a New, Precise Near-infrared Doppler Wavelength Reference: A Fiber Fabry-Perot Interferometer
We present the ongoing development of a commercially available Micron Optics
fiber-Fabry Perot Interferometer as a precise, stable, easy to use, and
economic spectrograph reference with the goal of achieving <1 m/s long term
stability. Fiber Fabry-Perot interferometers (FFP) create interference patterns
by combining light traversing different delay paths. The interference creates a
rich spectrum of narrow emission lines, ideal for use as a precise Doppler
reference. This fully photonic reference could easily be installed in existing
NIR spectrographs, turning high resolution fiber-fed spectrographs into precise
Doppler velocimeters. First light results on the Sloan Digital Sky Survey III
(SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE)
spectrograph and several tests of major support instruments are also presented.
These instruments include a SuperK Photonics fiber supercontinuum laser source
and precise temperature controller. A high resolution spectrum obtained using
the NIST 2-m Fourier transform spectrometer (FTS) is also presented. We find
our current temperature control precision of the FFP to be 0.15 mK,
corresponding to a theoretical velocity stability of 35 cm/s due to temperature
variations of the interferometer cavity.Comment: 16 pages, 11 figures. To appear in the proceedings of the SPIE 2012
Astronomical Instrumentation and Telescopes conferenc
Simulation of orbital radar images
Many of the questions that arise concerning the operating parameters
for spaceborne synthetic aperture imaging radar systems can
be addressed in a cost-effective manner by using simulation techniques.
This can include use of airborne images, Seasat images, and computer
simulation. The first computer simulation of spaceborne radar imagery
has been analyzed for system definition studies. Analysis of the
simulation indicates that incidence angles as small as 30° are useful
for general terrain geomorphologic analysis
- …