13,029 research outputs found
The Accuracy of Morphological Decomposition of Active Galactic Nucleus Host Galaxies
In order to assess the accuracy with which we can determine the morphologies
of AGN host galaxies, we have simulated more than 50,000 ACS images of galaxies
with z < 1.25, using image and noise properties appropriate for the GOODS
survey. We test the effect of central point-source brightness on host galaxy
parameter recovery with a set of simulated AGN host galaxies made by adding
point sources to the centers of normal galaxies. We extend this analysis and
also quantify the recovery of intrinsic morphological parameters of AGN host
galaxies with a set of fully simulated inactive and AGN host galaxies.
We can reliably separate good from poor fit results using a combination of
reasonable error cuts, in the regime where L_{host}:L_{PS} > 1:4. We give
quantitative estimates of parameter errors as a function of
host-to-point-source ratio. In general, we separate host and point-source
magnitudes reliably at all redshifts; point sources are well recovered more
than 90% of the time, although spurious detection of central point sources can
be as high as 25% for bulge-dominated sources. We find a general correlation
between Sersic index and intrinsic bulge-to-total ratio, such that a host
galaxy with Sersic n < 1.5 generally has at least 80% of its light from a disk
component. Likewise, "bulge-dominated" galaxies with n > 4 typically derive at
least 70% of their total host galaxy light from a bulge, but this number can be
as low as 55%. Single-component Sersic fits to an AGN host galaxy are
statistically very reliable to z < 1.25 (for ACS survey data like ours). In
contrast, two-component fits involving separate bulge and disk components tend
to over-estimate the bulge fraction by ~10%, with uncertainty of order 50%.Comment: 45 pages, 20 figures, submitted to ApJ ; Accepted Version --
additions to introduction and conclusions; title changed, was "Simulations of
AGN Host Galaxy Morphologies
Polarimetric variations of binary stars. II. Numerical simulations for circular and eccentric binaries in Mie scattering envelopes
We present numerical simulations of the periodic polarimetric variations
produced by a binary star placed at the center of an empty spherical cavity
inside a circumbinary ellipsoidal and optically thin envelope made of dust
grains. Mie single-scattering is considered along with pre- and post-scattering
extinction factors which produce a time-varying optical depth and affect the
morphology of the periodic variations. We are interested in the effects that
various parameters will have on the average polarization, the amplitude of the
polarimetric variations, and the morphology of the variability. We show that
the absolute amplitudes of the variations are smaller for Mie scattering than
for Thomson scattering. Among the four grain types that we have studied, the
highest polarizations are produced by grains with sizes in the range 0.1-0.2
micron. In general, the variations are seen twice per orbit. In some cases,
because spherical dust grains have an asymmetric scattering function, the
polarimetric curves produced also show variations seen once per orbit.
Circumstellar disks produce polarimetric variations of greater amplitude than
circumbinary envelopes.
Another goal of these simulations is to see if the 1978 BME (Brown, McLean, &
Emslie, ApJ, 68, 415) formalism, which uses a Fourier analysis of the
polarimetric variations to find the orbital inclination for Thomson-scattering
envelopes, can still be used for Mie scattering. We find that this is the case,
if the amplitude of the variations is sufficient and the true inclinations is
i_true > 45 deg. For eccentric orbits, the first-order coefficients of the
Fourier fit, instead of second-order ones, can be used to find almost all
inclinations.Comment: 23 pages, 5 figures, to be published in Astronomical Journa
A 14-day ground-based hypokinesia study in nonhuman primates: A compilation of results
A 14 day ground based hypokinesia study with rhesus monkeys was conducted to determine if a spaceflight of similar duration might affect bone remodeling and calcium homeostatis. The monkeys were placed in total body casts and sacrificed either immediately upon decasting or 14 days after decasting. Changes in vertebral strength were noted and further deterioration of bone strength continued during the recovery phase. Resorption in the vertebrae increased dramatically while formation decreased. Cortical bone formation was impaired in the long bones. The immobilized animals showed a progressive decrease in total serum calcium which rebounded upon remobilization. Most mandibular parameters remained unchanged during casting except for retardation of osteon birth or maturation rate and density distribution of matrix and mineral moieties
Single top or bottom production associated with a scalar in \gamma p collision as a probe of topcolor-assisted technicolor
In the framework of the topcolor-assisted technicolor (TC2) models, we study
the productions of a single top or bottom quark associated with a scalar in
\gamma-p collision, which proceed via the subprocesses c\gamma -> t\pi_t^0,
c\gamma -> t h_t^0 and c\gamma -> b\pi^+_t mediated by the anomalous top or
bottom coupling tc\pi_t^0, tch_t^0 and bc\pi_t^+. These productions, while
extremely suppressed in the Standard Model, are found to be significantly
enhanced in the large part of the TC2 parameter space, especially the
production via c\gamma -> b\pi^+ can have a cross section of 100 fb, which may
be accessible and allow for a test of the TC2 models.Comment: 13 pages, 4 figures, comments and references adde
Photoluminescence Detected Doublet Structure in the Integer and Fractional Quantum Hall Regime
We present here the results of polarized magneto-photoluminescence
measurements on a high mobility single-heterojunction. The presence of a
doublet structure over a large magnetic field range (2>nu>1/6) is interpreted
as possible evidence for the existence of a magneto-roton minima of the charged
density waves. This is understood as an indication of strong electronic
correlation even in the case of the IQHE limit.Comment: submitted to Solid State Communication
Modern and ancient hiatuses in the pelagic caps of Pacific guyots and seamounts and internal tides
Incidences of non-deposition or erosion at the modern seabed and hiatuses within the pelagic caps of guyots and seamounts are evaluated along with paleo-temperature and
physiographic information to speculate on the character of Late Cenozoic internal tidal waves in the upper Pacific Ocean. Drill core and seismic reflection data are used to
classify sediment at the drill sites as having been either accumulating or eroding/nondepositing in the recent geological past. When those classified sites are compared against predictions of a numerical model of the modern internal tidal wave field (Simmons, 2008), the sites accumulating particles over the past few million years are found to lie away from beams of the modeled internal tide, while those that have not been accumulating are in internal tide beams. Given the correspondence to the modern internal wave field, we examine whether internal tides can explain ancient hiatuses at the drill sites. For example, Late Cenozoic pelagic caps on guyots among the Marshall Islands contain two hiatuses of broadly similar age, but the dates of the first pelagic sediments deposited following each hiatus do not correlate between guyots, suggesting that they originate not from ocean chemical changes but from physical processes, such as erosion by internal tidal waves. We investigate how changing conditions such as ocean
temperature and basin physiography may have affected internal tides through the Cenozoic. Allowing for subsequent rotation or uplift by plate tectonics, ancient
submarine ridges among the Solomon, Bonin and Marianas Island chains may have been responsible for some sediment hiatuses at these distant guyot sites
Valley filtering and spatial maps of coupling between silicon donors and quantum dots
Exchange coupling is a key ingredient for spin-based quantum technologies
since it can be used to entangle spin qubits and create logical spin qubits.
However, the influence of the electronic valley degree of freedom in silicon on
exchange interactions is presently the subject of important open questions.
Here we investigate the influence of valleys on exchange in a coupled
donor/quantum dot system, a basic building block of recently proposed schemes
for robust quantum information processing. Using a scanning tunneling
microscope tip to position the quantum dot with sub-nm precision, we find a
near monotonic exchange characteristic where lattice-aperiodic modulations
associated with valley degrees of freedom comprise less than 2~\% of exchange.
From this we conclude that intravalley tunneling processes that preserve the
donor's and valley index are filtered out of the interaction
with the valley quantum dot, and that the and
intervalley processes where the electron valley index changes are weak.
Complemented by tight-binding calculations of exchange versus donor depth, the
demonstrated electrostatic tunability of donor/QD exchange can be used to
compensate the remaining intravalley oscillations to realise uniform
interactions in an array of highly coherent donor spins.Comment: 6 pages, 4 figures, 6 pages Supplemental Materia
Giant microwave photoresistance of two-dimensional electron gas
We measure microwave frequency (4-40 GHz) photoresistance at low magnetic
field B, in high mobility 2D electron gas samples, excited by signals applied
to a transmission line fabricated on the sample surface. Oscillatory
photoresistance vs B is observed. For excitation at the cyclotron resonance
frequency, we find an unprecedented, giant relative photoresistance (\Delta
R)/R of up to 250 percent. The photoresistance is apparently proportional to
the square root of applied power, and disappears as the temperature is
increased.Comment: 4 pages, 3 figure
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