441 research outputs found
Constraining dark matter halo profiles and galaxy formation models using spiral arm morphology. II. Dark and stellar mass concentrations for 13 nearby face-on galaxies
We investigate the use of spiral arm pitch angles as a probe of disk galaxy
mass profiles. We confirm our previous result that spiral arm pitch angles (P)
are well correlated with the rate of shear (S) in disk galaxy rotation curves.
We use this correlation to argue that imaging data alone can provide a powerful
probe of galactic mass distributions out to large look-back times. We then use
a sample of 13 galaxies, with Spitzer 3.6-m imaging data and observed
H rotation curves, to demonstrate how an inferred shear rate coupled
with a bulge-disk decomposition model and a Tully-Fisher-derived velocity
normalization can be used to place constraints on a galaxy's baryon fraction
and dark matter halo profile. Finally we show that there appears to be a trend
(albeit a weak correlation) between spiral arm pitch angle and halo
concentration. We discuss implications for the suggested link between
supermassive black hole (SMBH) mass and dark halo concentration, using pitch
angle as a proxy for SMBH mass.Comment: 14 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Operating Characteristics of the Multiple Critical Venturi System and Secondary Calibration Nozzles Used for Weight-Flow Measurements in the Langley 16-Foot Transonic Tunnel
An investigation has been conducted in the Langley 16 Foot Transonic Tunnel to determine the weight flow measurement characteristics of a multiple critical Venturi system and the nozzle discharge coefficient characteristics of a series of convergent calibration nozzles. The effects on model discharge coefficient of nozzle throat area, model choke plate open area, nozzle pressure ratio, jet total temperature, and number and combination of operating Venturis were investigated. Tests were conducted at static conditions (tunnel wind off) at nozzle pressure ratios from 1.3 to 7.0
Collective resonances in plasmonic crystals: Size matters
Periodic arrays of metallic nanoparticles may sustain Surface Lattice
Resonances (SLRs), which are collective resonances associated with the
diffractive coupling of Localized Surface Plasmon Resonances (LSPRs). By
investigating a series of arrays with varying number of particles, we traced
the evolution of SLRs to its origins. Polarization resolved extinction spectra
of arrays formed by a few nanoparticles were measured, and found to be in very
good agreement with calculations based on a coupled dipole model. Finite size
effects on the optical properties of the arrays are observed, and our results
provide insight into the characteristic length scales for collective plasmonic
effects: for arrays smaller than 5 x 5 particles, the Q-factors of SLRs are
lower than those of LSPRs; for arrays larger than 20 x 20 particles, the
Q-factors of SLRs saturate at a much larger value than those of LSPRs; in
between, the Q-factors of SLRs are an increasing function of the number of
particles in the array.Comment: 4 figure
Counts-in-Cylinders in the Sloan Digital Sky Survey with Comparisons to N-body Simulations
Environmental statistics provide a necessary means of comparing the
properties of galaxies in different environments and a vital test of models of
galaxy formation within the prevailing, hierarchical cosmological model. We
explore counts-in-cylinders, a common statistic defined as the number of
companions of a particular galaxy found within a given projected radius and
redshift interval. Galaxy distributions with the same two-point correlation
functions do not necessarily have the same companion count distributions. We
use this statistic to examine the environments of galaxies in the Sloan Digital
Sky Survey, Data Release 4. We also make preliminary comparisons to four models
for the spatial distributions of galaxies, based on N-body simulations, and
data from SDSS DR4 to study the utility of the counts-in-cylinders statistic.
There is a very large scatter between the number of companions a galaxy has and
the mass of its parent dark matter halo and the halo occupation, limiting the
utility of this statistic for certain kinds of environmental studies. We also
show that prevalent, empirical models of galaxy clustering that match observed
two- and three-point clustering statistics well fail to reproduce some aspects
of the observed distribution of counts-in-cylinders on 1, 3 and 6-Mpc/h scales.
All models that we explore underpredict the fraction of galaxies with few or no
companions in 3 and 6-Mpc/h cylinders. Roughly 7% of galaxies in the real
universe are significantly more isolated within a 6 Mpc/h cylinder than the
galaxies in any of the models we use. Simple, phenomenological models that map
galaxies to dark matter halos fail to reproduce high-order clustering
statistics in low-density environments.Comment: 17 pages, 10 figures. Accepted, Ap
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Design Optimization of a Structurally Flexible Wave Energy Converter with a Direct Search Algorithm
Several wave energy converter designs have been recently proposed that are made of flexible materials. Flexible devices are able to generate electricity by being stretched, and are intended to simplify deployment and maintenance concerns over existing wave energy devices. Due to the relative infancy of flexible wave energy converters, however, little is known about how to optimally design one for its deployment location. We chose here to optimize a long flexible tube, a seemingly straightforward design nonetheless having a large design space including geometry, submergence, material, and mooring considerations. A brute force search of all these factors would take engineers an impractical amount of time to completely explore, so we used a direct search optimization algorithm to explore potential design concept changes and exploit those that result in an overall improvement. We focused on the geometry and submergence of the flexible device with fixed mooring and material parameters, and successfully used the device’s overall power performance as our metric for improvement. We intend our optimization work to be a case study in using design automation and optimization methods to improve other flexible wave energy devices
Small-Scale Structure in the SDSS and LCDM: Isolated L* Galaxies with Bright Satellites
We use a volume-limited spectroscopic sample of isolated galaxies in the
Sloan Digital Sky Survey (SDSS) to investigate the frequency and radial
distribution of luminous (M_r <~ -18.3) satellites like the Large Magellanic
Cloud (LMC) around ~L* Milky Way analogs and compare our results
object-by-object to LCDM predictions based on abundance matching in
simulations. We show that 12% of Milky Way-like galaxies host an LMC-like
satellite within 75 kpc (projected), and 42 % within 250 kpc (projected). This
implies ~10% have a satellite within the distance of the LMC, and ~40% of L*
galaxies host a bright satellite within the virialized extent of their dark
matter halos. Remarkably, the simulation reproduces the observed frequency,
radial dependence, velocity distribution, and luminosity function of observed
secondaries exceptionally well, suggesting that LCDM provides an accurate
reproduction of the observed Universe to galaxies as faint as L~10^9 Lsun on
~50 kpc scales. When stacked, the observed projected pairwise velocity
dispersion of these satellites is sigma~160 km/s, in agreement with
abundance-matching expectations for their host halo masses. Finally, bright
satellites around L* primaries are significantly redder than typical galaxies
in their luminosity range, indicating that environmental quenching is operating
within galaxy-size dark matter halos that typically contain only a single
bright satellite. This redness trend is in stark contrast to the Milky Way's
LMC, which is unusually blue even for a field galaxy. We suggest that the LMC's
discrepant color might be further evidence that it is undergoing a triggered
star-formation event upon first infall.Comment: 14 pages, 11 figures; accepted to Ap
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