688 research outputs found
HST Morphologies of z~2 Dust Obscured Galaxies I: Power-law Sources
We present high spatial resolution optical and near-infrared imaging obtained
using the ACS, WFPC2 and NICMOS cameras aboard the Hubble Space Telescope of 31
24um--bright z~2 Dust Obscured Galaxies (DOGs) identified in the Bootes Field
of the NOAO Deep Wide-Field Survey. Although this subset of DOGs have mid-IR
spectral energy distributions dominated by a power-law component suggestive of
an AGN, all but one of the galaxies are spatially extended and not dominated by
an unresolved component at rest-frame UV or optical wavelengths. The observed
V-H and I-H colors of the extended components are 0.2-3 magnitudes redder than
normal star-forming galaxies. All but 1 have axial ratios >0.3, making it
unlikely that DOGs are composed of an edge-on star-forming disk. We model the
spatially extended component of the surface brightness distributions of the
DOGs with a Sersic profile and find effective radii of 1-6 kpc. This sample of
DOGs is smaller than most sub-millimeter galaxies (SMGs), but larger than
quiescent high-redshift galaxies. Non-parametric measures (Gini and M20) of DOG
morphologies suggest that these galaxies are more dynamically relaxed than
local ULIRGs. We estimate lower limits to the stellar masses of DOGs based on
the rest-frame optical photometry and find that these range from ~10^(9-11)
M_sun. If major mergers are the progenitors of DOGs, then these observations
suggest that DOGs may represent a post-merger evolutionary stage.Comment: 23 pages, 9 figures, 6 tables, accepted to ApJ; lower limits on
stellar mass revised upwards by factor of (1+z
Centimetre continuum emission from young stellar objects in Cederblad 110
The low-mass star formation region associated with the reflection nebula
Cederblad 110 in the Chamaeleon I cloud was mapped with the Australian
Telescope Compact Array (ATCA) at 6 and 3.5cm. Altogether 11 sources were
detected, three of which are previously known low mass young stellar objects
associated with the nebula: the illuminating star IRS2 (Class III, Einstein
X-ray source CHX7), the brightest far-infrared source IRS4 (Class I), and the
weak X-ray source CHX10a (Class III). The other young stellar objects in the
region, including the Class 0 protostar candidate Cha-MMS1, were not detected.
The radio spectral index of IRS4 (alpha = 1.7 +/- 0.3) is consistent with
optically thick free-free emission arising from a dense ionized region,
probably a jet-induced shock occurring in the circumstellar material. As the
only Class I protostar with a 'thermal jet' IRS4 is the strongest candidate for
the central source of the molecular outflow found previously in the region. The
emission from IRS2 has a flat spectrum (alpha = 0.05 +/- 0.05) but shows no
sign of polarization, and therefore its origin is likely to be optically thin
free-free emission either from ionized wind or a collimated jet. The strongest
source detected in this survey is a new compact object with a steep negative
spectral index (-1.1) and a weak linear polarization (about 2 %), which
probably represents a background radio galaxy.Comment: 7 pages, 2 Postscript figures. Accepted for publication in Astronomy
& Astrophysic
MIPS J142824.0+352619: A Hyperluminous Starburst Galaxy at z=1.325
Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain
350 micron images of sources detected with the MIPS instrument on Spitzer, we
have discovered a remarkable object at z=1.325+/-0.002 with an apparent
Far-Infrared luminosity of 3.2(+/-0.7) x 10^13 Lsun. Unlike other z>1 sources
of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619
lacks any trace of AGN activity, and is likely a luminous analog of galaxies
selected locally by IRAS, or at high redshift in the submillimeter. This source
appears to be lensed by a foreground elliptical galaxy at z=1.034, although the
amplification is likely modest (~10). We argue that the contribution to the
observed optical/Near-IR emission from the foreground galaxy is small, and
hence are able to present the rest-frame UV through radio Spectral Energy
Distribution of this galaxy. Due to its unusually high luminosity, MIPS
J142824.0+352619 presents a unique chance to study a high redshift dusty
starburst galaxy in great detail.Comment: 6 pages, 3 figures, accepted for publication in Ap
Simulations and cosmological inference: A statistical model for power spectra means and covariances
We describe an approximate statistical model for the sample variance
distribution of the non-linear matter power spectrum that can be calibrated
from limited numbers of simulations. Our model retains the common assumption of
a multivariate Normal distribution for the power spectrum band powers, but
takes full account of the (parameter dependent) power spectrum covariance. The
model is calibrated using an extension of the framework in Habib et al. (2007)
to train Gaussian processes for the power spectrum mean and covariance given a
set of simulation runs over a hypercube in parameter space. We demonstrate the
performance of this machinery by estimating the parameters of a power-law model
for the power spectrum. Within this framework, our calibrated sample variance
distribution is robust to errors in the estimated covariance and shows rapid
convergence of the posterior parameter constraints with the number of training
simulations.Comment: 14 pages, 3 figures, matches final version published in PR
The role of body rotation in bacterial flagellar bundling
In bacterial chemotaxis, E. coli cells drift up chemical gradients by a
series of runs and tumbles. Runs are periods of directed swimming, and tumbles
are abrupt changes in swimming direction. Near the beginning of each run, the
rotating helical flagellar filaments which propel the cell form a bundle. Using
resistive-force theory, we show that the counter-rotation of the cell body
necessary for torque balance is sufficient to wrap the filaments into a bundle,
even in the absence of the swirling flows produced by each individual filament
Cosmic Calibration: Constraints from the Matter Power Spectrum and the Cosmic Microwave Background
Several cosmological measurements have attained significant levels of
maturity and accuracy over the last decade. Continuing this trend, future
observations promise measurements of the statistics of the cosmic mass
distribution at an accuracy level of one percent out to spatial scales with
k~10 h/Mpc and even smaller, entering highly nonlinear regimes of gravitational
instability. In order to interpret these observations and extract useful
cosmological information from them, such as the equation of state of dark
energy, very costly high precision, multi-physics simulations must be
performed. We have recently implemented a new statistical framework with the
aim of obtaining accurate parameter constraints from combining observations
with a limited number of simulations. The key idea is the replacement of the
full simulator by a fast emulator with controlled error bounds. In this paper,
we provide a detailed description of the methodology and extend the framework
to include joint analysis of cosmic microwave background and large scale
structure measurements. Our framework is especially well-suited for upcoming
large scale structure probes of dark energy such as baryon acoustic
oscillations and, especially, weak lensing, where percent level accuracy on
nonlinear scales is needed.Comment: 15 pages, 14 figure
Conformal Mapping on Rough Boundaries II: Applications to bi-harmonic problems
We use a conformal mapping method introduced in a companion paper to study
the properties of bi-harmonic fields in the vicinity of rough boundaries. We
focus our analysis on two different situations where such bi-harmonic problems
are encountered: a Stokes flow near a rough wall and the stress distribution on
the rough interface of a material in uni-axial tension. We perform a complete
numerical solution of these two-dimensional problems for any univalued rough
surfaces. We present results for sinusoidal and self-affine surface whose slope
can locally reach 2.5. Beyond the numerical solution we present perturbative
solutions of these problems. We show in particular that at first order in
roughness amplitude, the surface stress of a material in uni-axial tension can
be directly obtained from the Hilbert transform of the local slope. In case of
self-affine surfaces, we show that the stress distribution presents, for large
stresses, a power law tail whose exponent continuously depends on the roughness
amplitude
Development and Testing of a Novel Green Propellant Piston Tank
Analytical Mechanics Associates (AMA), in cooperation with NASA Marshall Space Flight Center's (MSFC's) Spacecraft Propulsion Systems Branch, developed and tested a novel propellant tank design that employs an internal piston pressurized with an inert gas to expel propellant to thrusters. During the course of this activity, AMA designed, oversaw fabrication, and delivered to MSFC for testing, a piston propellant tank sized for 3U or larger CubeSats. MSFC conducted liquid expulsion testing using ethylene glycol as a referee fluid to map the tank's performance at different pressures and piston positions. Following the expulsion test campaign, the tank is planned to be integrated into a propulsion system test bed for hot fire tests with a 100mN monopropellant thruster to evaluate the tank's influence on thruster performance when operated in a flight like manner. Described in this paper is a comprehensive summary of how the tanks were designed, built, and tested. The fundamental knowledge gained through the fabrication and testing of these tanks gives evidence that the piston tank design may be scalable to meet the requirements and constraints of other small satellites
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