1,032 research outputs found
Robotic mobile servicing platform for space station
The semi-autonomous inspection and servicing of the Space Station's major thermal, electrical, mechanical subsystems are critical needs for the safe and reliable operation of the station. A conceptual design is presented of a self-intelligent, small and highly mobile robotic platform. Equipped with suitable inspection sensors (cameras, ammonia detectors, etc.), this system's primary mission is to perform routine, autonomous inspection of the Station's primary subsystems. Typical tasks include detection of leaks from thermal fluid or refueling lines, as well as detection of micro-meteroid damage to the primary structure. Equipped with stereo cameras and a dexterous manipulator, simple teleoperator repairs and small On-orbit Replacement Unit (ORU) changeout can also be accomplished. More difficult robotic repairs would be left to the larger, more sophisticated Mobile Remote Manipulator System (MRMS). An ancillary function is to ferry crew members and equipment around the station. The primary design objectives were to provide a flexible, but uncomplicated robotic platform, one which caused minimal impact to the design of the Station's primary structure but could accept more advanced telerobotic technology as it evolves
Parametric tests of a traction drive retrofitted to an automotive gas turbine
The results of a test program to retrofit a high performance fixed ratio Nasvytis Multiroller Traction Drive in place of a helical gear set to a gas turbine engine are presented. Parametric tests up to a maximum engine power turbine speed of 45,500 rpm and to a power level of 11 kW were conducted. Comparisons were made to similar drives that were parametrically tested on a back-to-back test stand. The drive showed good compatibility with the gas turbine engine. Specific fuel consumption of the engine with the traction drive speed reducer installed was comparable to the original helical gearset equipped engine
Neutral Gas Properties and Ly Escape in Extreme Green Pea Galaxies
Mechanisms regulating the escape of Ly photons and ionizing radiation
remain poorly understood. To study these processes we analyze VLA 21cm
observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and HST
COS spectra of 17 GP galaxies at . All are highly ionized: J1608 has the
highest [O III] /[O II] for star-forming galaxies in
SDSS, and the 17 GPs have [O III]/[O II] . We set an upper limit on
J1608's HI mass of , near or below average compared to
similar mass dwarf galaxies. In the COS sample, eight GPs show Ly
absorption components, six of which also have Ly emission. The HI
column densities derived from Ly absorption are high, cm, well above the LyC optically thick limit. Using
low-ionization absorption lines, we measure covering fractions
(f_{\mbox{cov}}) of , and find that f_{\mbox{cov}} strongly
anti-correlates with Ly escape fraction. Low covering fractions may
facilitate Ly and LyC escape through dense neutral regions. GPs with
f_{\mbox{cov}}\sim1 all have low neutral gas velocities, while GPs with lower
f_{\mbox{cov}}=0.2-0.6 have a larger range of velocities. Conventional
mechanical feedback may help establish low f_{\mbox{cov}} in some cases,
whereas other processes may be important for GPs with low velocities. Finally,
we compare f_{\mbox{cov}} with proposed indicators of LyC escape. Ionizing
photon escape likely depends on a combination of neutral gas geometry and
kinematics, complicating the use of emission-line diagnostics for identifying
LyC emitters.Comment: 21 pages, 11 figures, accepted for publication in Ap
Neutral Gas Properties and Lyα Escape in Extreme Green Pea Galaxies
Mechanisms regulating the escape of Lyα photons and ionizing radiation remain poorly understood. To study these processes, we analyze Very Large Array 21 cm observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and Hubble Space Telescope Cosmic Origins Spectrograph (COS) spectra of 17 GP galaxies at . All are highly ionized: J1608 has the highest [O iii] λ5007/[O ii] λ3727 for star-forming galaxies in Sloan Digital Sky Survey, and the 17 GPs have [O iii]/[O ii] ≥ 6.6. We set an upper limit on J1608\u27s H i mass of , near or below average compared to similar-mass dwarf galaxies. In the COS sample, eight GPs show Lyα absorption components, six of which also have Lyα emission. The H i column densities derived from Lyα absorption are high, cm−2 = 19–21, well above the LyC optically thick limit. Using low-ionization absorption lines, we measure covering fractions () of 0.1–1 and find that strongly anticorrelates with Lyα escape fraction. Low covering fractions may facilitate Lyα and LyC escape through dense neutral regions. GPs with all have low neutral gas velocities, while GPs with lower have a larger range of velocities. Conventional mechanical feedback may help establish low in some cases, whereas other processes may be important for GPs with low velocities. Finally, we compare with proposed indicators of LyC escape. Ionizing photon escape likely depends on a combination of neutral gas geometry and kinematics, complicating the use of emission-line diagnostics for identifying LyC emitters
The nature of Lyman-break galaxies
Using semi-analytic models of galaxy formation, we investigate the properties
of galaxies and compare them with the observed population of
Lyman-break galaxies (LBGs). In addition to the usual quiescent mode of star
formation, we introduce a physical model for starbursts triggered by
galaxy-galaxy interactions. We find that with the merger rate that arises
naturally in the CDM-based merging hierarchy, a significant fraction of bright
galaxies identified at high redshift (z \ga 2) are likely to be low-mass,
bursting satellite galaxies. The abundance of LBGs as a function of redshift
and the luminosity function of LBGs both appear to be in better agreement with
the data when the starburst mode is included, especially when effects of dust
extinction are considered. The objects that we identify as LBGs have observable
properties including low velocity dispersions that are in good agreement with
the available data. In this ``Bursting Satellite'' scenario, quiescent star
formation at z\ga2 is relatively inefficient and most of the observed LBGs
are starbursts triggered by satellite mergers within massive halos. In
high-resolution N-body simulations, we find that the most massive dark matter
halos cluster at redshift much as the LBGs are observed to do. This
is true for both the CHDM model and low- \LCDM and OCDM
models, all of which have fluctuation power spectra consistent with the
distribution of low-redshift galaxies. The Bursting Satellite scenario can
resolve the apparent paradox of LBGs that cluster like massive dark matter
halos but have narrow linewidths and small stellar masses.Comment: 10 pages, latex, including 2 figures, to appear in the Proceedings of
the 3rd International Symposium on Sources and Detection of Dark Matter in
the Universe (DM98), Feb. 1998, ed. D. Clin
Optical Morphologies of Millijansky Radio Galaxies Observed by HST and in the VLA FIRST Survey
We report on a statistical study of the 51 radio galaxies at the millijansky
flux level from the Faint Images of the Radio Sky at Twenty centimeters,
including their optical morphologies and structure obtained with the Hubble
Space Telescope. Our optical imaging is significantly deeper (~2 mag) than
previous studies with the superior angular resolution of space-based imaging.
We that find 8/51 (16%) of the radio sources have no optically identifiable
counterpart to AB~24 mag. For the remaining 43 sources, only 25 are
sufficiently resolved in the HST images to reliably assign a visual
classification: 15 (60%) are elliptical galaxies, 2 (8%) are late-type spiral
galaxies, 1 (4%) is an S0, 3 (12%) are point-like objects (quasars), and 4
(16%) are merger systems. We find a similar distribution of optical types with
measurements of the Sersic index. The optical magnitude distribution of these
galaxies peaks at I~20.7+-0.5 AB mag, which is ~3 mag brighter than the depth
of our typical HST field and is thus not due to the WFPC2 detection limit. This
supports the luminosity-dependent density evolutionary model, where the
majority of faint radio galaxies typically have L*-optical luminosities and a
median redshift of z~0.8 with a relatively abrupt redshift cut-off at z>~2. We
discuss our results in the context of the evolution of elliptical galaxies and
active galactic nuclei.Comment: 20 pages, 8 figures, 51 galaxy images, and 5 tables. Uses
emulateapj.cls and natbib.sty. Accepted to ApJS. High resolution images are
available upon reques
Deep 1.1 mm-wavelength imaging of the GOODS-South field by AzTEC/ASTE -- II. Redshift distribution and nature of the submillimetre galaxy population
We report the results of the counterpart identification and a detailed
analysis of the physical properties of the 48 sources discovered in our deep
1.1mm wavelength imaging survey of the GOODS-South field using the AzTEC
instrument on the Atacama Submillimeter Telescope Experiment (ASTE). One or
more robust or tentative counterpart candidate is found for 27 and 14 AzTEC
sources, respectively, by employing deep radio continuum, Spitzer MIPS & IRAC,
and LABOCA 870 micron data. Five of the sources (10%) have two robust
counterparts each, supporting the idea that these galaxies are strongly
clustered and/or heavily confused. Photometric redshifts and star formation
rates (SFRs) are derived by analyzing UV-to-optical and IR-to-radio SEDs. The
median redshift of z~2.6 is similar to other earlier estimates, but we show
that 80% of the AzTEC-GOODS sources are at z>2, with a significant high
redshift tail (20% at z>3.3). Rest-frame UV and optical properties of AzTEC
sources are extremely diverse, spanning 10 magnitude in the i- and K-band
photometry with median values of i=25.3 and K=22.6 and a broad range of red
colour (i-K=0-6). These AzTEC sources are some of the most luminous galaxies in
the rest-frame optical bands at z>2, with inferred stellar masses of (1-30) x
10^{10} solar masses and UV-derived star formation rates of SFR(UV) > 10-1000
solar masses per year. The IR-derived SFR, 200-2000 solar masses per year, is
independent of redshift or stellar mass. The resulting specific star formation
rates, SSFR = 1-100 per Gyr, are 10-100 times higher than similar mass galaxies
at z=0, and they extend the previously observed rapid rise in the SSFR with
redshift to z=2-5. These galaxies have a SFR high enough to have built up their
entire stellar mass within their Hubble time. We find only marginal evidence
for an AGN contribution to the near-IR and mid-IR SEDs. (abridged)Comment: 31 pages including 14 figures, accepted for publication in the MNRAS.
A higher quality Figure 1 is also included as Figure1.jp
The relationship between magnetic and electrophysiological responses to complex tactile stimuli
<p>Abstract</p> <p>Background</p> <p>Magnetoencephalography (MEG) has become an increasingly popular technique for non-invasively characterizing neuromagnetic field changes in the brain at a high temporal resolution. To examine the reliability of the MEG signal, we compared magnetic and electrophysiological responses to complex natural stimuli from the same animals. We examined changes in neuromagnetic fields, local field potentials (LFP) and multi-unit activity (MUA) in macaque monkey primary somatosensory cortex that were induced by varying the rate of mechanical stimulation. Stimuli were applied to the fingertips with three inter-stimulus intervals (ISIs): 0.33s, 1s and 2s.</p> <p>Results</p> <p>Signal intensity was inversely related to the rate of stimulation, but to different degrees for each measurement method. The decrease in response at higher stimulation rates was significantly greater for MUA than LFP and MEG data, while no significant difference was observed between LFP and MEG recordings. Furthermore, response latency was the shortest for MUA and the longest for MEG data.</p> <p>Conclusion</p> <p>The MEG signal is an accurate representation of electrophysiological responses to complex natural stimuli. Further, the intensity and latency of the MEG signal were better correlated with the LFP than MUA data suggesting that the MEG signal reflects primarily synaptic currents rather than spiking activity. These differences in latency could be attributed to differences in the extent of spatial summation and/or differential laminar sensitivity.</p
Measuring the Halo Mass of z=3 Damped Ly-alpha Absorbers from the Absorber-Galaxy Cross-correlation
[Abridged] We test the reliability of a method to measure the mean halo mass
of Damped Ly-alpha absorbers (DLAs). The method is based on measuring the ratio
of the cross-correlation between DLAs and galaxies to the auto-correlation of
the galaxies themselves (), which is (in linear theory)
the ratio of their bias factor. This is shown to be true irrespective of the
galaxy redshift distribution, provided that one uses the same galaxies for the
two correlation functions. The method is applicable to all redshifts. Here, we
focus on z=3 DLAs and we demonstrate that the method robustly constrains the
mean DLA halo mass using smoothed particle hydrodynamics (SPH) cosmological
simulations. If we use the bias formalism of Mo & White with the DLA and galaxy
mass distributions of these simulations, we predict a bias ratio of 0.771.
Direct measurement from the simulations of st yields a
ratio of 0.73+/-0.08, in excellent agreement with that prediction.
Equivalently, inverting the measured correlation ratio to infer a mean DLA halo
mass yields (log. averaging, in solar units) =11.13+/-013, in
excellent agreement with the true value in the simulations: 11.16. The cross-
correlation method thus appears to yield a robust estimate of the average host
halo mass even though the DLAs and the galaxies occupy a broad mass spectrum of
halos, and massive halos contain multiple galaxies with DLAs. We show that the
inferred mean DLA halo mass is independent of the galaxy sub-sample used, i.e.
the cross-correlation technique is also reliable. Our results imply that the
cross-correlation length between DLAs and LBGs is predicted to be, at most,
2.85 Mpc. Future observations will soon distinguish models in which DLAs are in
low mass halos from those in which DLAs are in massive halos.Comment: 15 pages, 7 figures, to be published in ApJ 2005 July 20th (Full
resolution of Fig.2 at
http://www.mpe.mpg.de/~nbouche/papers/Xcorr/f2-orig.eps); minor changes to
match the published tex
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