23,526 research outputs found
Spatial analysis of IRAS observations of nearby spirals
The unbiased survey of the infrared sky carried out by the Infrared Astronomy Satellite (IRAS) satellite has greatly accelerated advances in understanding the dust component of our own and external galaxies. However, most extragalactic studies to date have been based on the IRAS Point Source Catalog (PSC), which has two serious limitations. First, in sources where a significant fraction of the flux is extended, significant errors may result from using PSC fluxes in comparative studies, and these errors could be systematic if the tendency to be non-pointlike depends on physical properties of the galaxy. Additionally, use of PSC fluxes rules out any direct investigation of the spatial distribution of the IRAS emission from disks in external galaxies. Since work on the Galactic IRAS results has shown that very different physical processes can make varying contributions to the observed flux, it is important to look at a wide sample of galaxies with some spatial resolution to study the relative dominance of these processes under a variety of conditions. Here, researchers report on work they are doing to carry out this program for many nearby spirals, using an analysis package that was developed for this purpose. Researchers carried out analysis for a sample of 121 nearby spirals. The fraction of the flux contained in a point source varies from 0 to 1 across the sample, all of which are well resolved at their nominal optical diameters. There is no evidence that the galaxies of smaller angular size are less likely to be resolved by IRAS at this level. The program gives results which are quite repeatable from scan to scan; the fraction f (point source flux over total flux) at 60 microns has typical errors of 0.03 when different scans are combined. Approximately two-thirds of the sample have more flux in the extended than in the nuclear component. There is a tendency for earlier-type spirals to be less centrally concentrated, but this effect is slight and the degree of variation is large for all types. Barred spirals are also found across the spectrum of f, but are much more likely to have little or no nuclear emission
Finite and infinite h-plane bifurcation of waveguide with anisotropic plasma medium
H-plane bifurcation in parallel plate waveguide filled with homogeneous, anisotropic, and temperate plasm
A relationship between the integrated CO intensity and the radio continuum emission in spiral galaxies
In an effort to determine the role played by cosmic ray electrons and interstellar radiation fields on the collapse of molecular clouds, a survey was begun to investigate the relationship between the radio continuum brightness emission and the integrated CO intensity in spiral galaxies. The investigation was done on two scales; a global galaxy to galaxy comparison of integrated disk values, and a ring-averaged study over the disks of individual galaxies. For the large-scale survey, radio continuum flux densities integrated over the full disk at 1.49 GHz were taken from Condon (1987) and the total CO fluxes were taken from Verter (1985). The galaxies with values included in the two catalogs are displayed. It can be seen that a good correlation exists between the integrated CO emission and radio continuum emission
A high resolution CO map of M51
Observations of the CO (1-0) emission in two fields of M51 were taken with the Berkeley-Illinois-Maryland Array at Hat Creek, California from May 1988 to February 1989. When combined with two previously observed fields (Lo et al. 1988), a complete map of the central 5 minute x 4 minute at a resolution of 7 seconds x 10 seconds was obtained. The project is part of an ongoing high-resolution survey of the molecular, atomic, and ionized gas distributions in nearby spiral galaxies. The two recently observed fields can be compared to the results of the interferometric study of Vogel et al. (1988 - hereafter VKS). Since the shortest spacing in the current survey is shorter than that of VKS, researchers expect to see more of the extended emission. This is evident when comparing the width of the spiral arms in each survey; ours are a bit broader. While some of the peaks in this region correspond to the peaks in VKS, several of them do not. These discrepancies are probably because of the low signal to noise inherent in observations of this nature. Single-dish maps are currently being readied for inclusion with the interferometer data. These will help fill the short-spacing hole in the UV plane, and serve to recover the flux missing from the interferometer maps
Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows
Suspensions of motile cells are model systems for understanding the unique
mechanical properties of living materials which often consist of ensembles of
self-propelled particles. We present here a quantitative comparison of theory
against experiment for the rheology of such suspensions. The influence of
motility on viscosities of cell suspensions is studied using a novel
acoustically-driven microfluidic capillary-breakup extensional rheometer.
Motility increases the extensional viscosity of suspensions of algal pullers,
but decreases it in the case of bacterial or sperm pushers. A recent model
[Saintillan, Phys. Rev. E, 2010, 81:56307] for dilute active suspensions is
extended to obtain predictions for higher concentrations, after independently
obtaining parameters such as swimming speeds and diffusivities. We show that
details of body and flagellar shape can significantly determine macroscale
rheological behaviour.Comment: 12 pages, 1 appendix, 7 figures, submitted to Soft Matter - under
revie
Dilatonic Black Holes, Naked Singularities and Strings
We extend a previous calculation which treated Schwarschild black hole
horizons as quantum mechanical objects to the case of a charged, dilaton black
hole. We show that for a unique value of the dilaton parameter `a', which is
determined by the condition of unitarity of the S matrix, black holes transform
at the extremal limit into strings.Comment: 8 pages, REVTE
Heisenberg-picture approach to the exact quantum motion of a time-dependent forced harmonic oscillator
In the Heisenberg picture, the generalized invariant and exact quantum
motions are found for a time-dependent forced harmonic oscillator. We find the
eigenstate and the coherent state of the invariant and show that the
dispersions of these quantum states do not depend on the external force. Our
formalism is applied to several interesting cases.Comment: 15 pages, two eps files, to appear in Phys. Rev. A 53 (6) (1996
Mean Field Network based Graph Refinement with application to Airway Tree Extraction
We present tree extraction in 3D images as a graph refinement task, of
obtaining a subgraph from an over-complete input graph. To this end, we
formulate an approximate Bayesian inference framework on undirected graphs
using mean field approximation (MFA). Mean field networks are used for
inference based on the interpretation that iterations of MFA can be seen as
feed-forward operations in a neural network. This allows us to learn the model
parameters from training data using back-propagation algorithm. We demonstrate
usefulness of the model to extract airway trees from 3D chest CT data. We first
obtain probability images using a voxel classifier that distinguishes airways
from background and use Bayesian smoothing to model individual airway branches.
This yields us joint Gaussian density estimates of position, orientation and
scale as node features of the input graph. Performance of the method is
compared with two methods: the first uses probability images from a trained
voxel classifier with region growing, which is similar to one of the best
performing methods at EXACT'09 airway challenge, and the second method is based
on Bayesian smoothing on these probability images. Using centerline distance as
error measure the presented method shows significant improvement compared to
these two methods.Comment: 10 pages. Preprin
Larkin-Ovchinnikov-Fulde-Ferrell phase in the superconductor (TMTSF)2ClO4: Theory versus experiment
We consider a formation of the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase
in a quasi-one-dimensional (Q1D) conductor in a magnetic field, parallel to its
conducting chains, where we take into account both the paramagnetic
spin-splitting and orbital destructive effects against superconductivity. We
show that, due to a relative weakness of the orbital effects in a Q1D case, the
LOFF phase appears in (TMTSF)ClO superconductor for real values of its
Q1D band parameters. We compare our theoretical calculations with the recent
experimental data by Y. Maeno's group [S. Yonezawa et al., Phys. Rev. Lett.
\textbf{100}, 117002 (2008)] and show that there is a good qualitative and
quantitative agreement between the theory and experimental data.Comment: 4 pages, 1 figur
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