1,809 research outputs found
HI Emission and Absorption in the Southern Galactic Plane Survey
We present preliminary results from the Southern Galactic Plane Survey (SGPS)
Test Region and Parkes data. As part of the pilot project for the Southern
Galactic Plane Survey, observations of a Test Region (325.5 deg < l < 333.5
deg; -0.5 deg < b < 3.5 deg) were completed in December 1998. Single dish
observations of the full survey region (253 deg < l < 358 deg; |b| <1 deg) with
the Parkes Radio Telescope were completed in March 2000. We present a sample of
SGPS HI data with particular attention to the smallest and largest scale
structures seen in absorption and emission, respectively. On the large scale,
we detect many prominent HI shells. On the small scale, we note extremely
compact, cold clouds seen in HI self-absorption. We explore how these two
classes of objects probe opposite ends of the HI spatial power spectrum.Comment: 9 pages, 3 embedded postscript & 4 jpeg figures. Presented at the
Astronomical Society of Australia, Hobart, Tasmania, July 4-7 2000. To appear
in PASA Vol. 18(1
Soliton solutions of the Kadomtsev-Petviashvili II equation
We study a general class of line-soliton solutions of the
Kadomtsev-Petviashvili II (KPII) equation by investigating the Wronskian form
of its tau-function. We show that, in addition to previously known line-soliton
solutions, this class also contains a large variety of new multi-soliton
solutions, many of which exhibit nontrivial spatial interaction patterns. We
also show that, in general, such solutions consist of unequal numbers of
incoming and outgoing line solitons. From the asymptotic analysis of the
tau-function, we explicitly characterize the incoming and outgoing
line-solitons of this class of solutions. We illustrate these results by
discussing several examples.Comment: 28 pages, 4 figure
The constrained modified KP hierarchy and the generalized Miura transformations
In this letter, we consider the second Hamiltonian structure of the
constrained modified KP hierarchy. After mapping the Lax operator to a pure
differential operator the second structure becomes the sum of the second and
the third Gelfand-Dickey brackets defined by this differential operator. We
simplify this Hamiltonian structure by factorizing the Lax operator into linear
terms.Comment: 8 pages, latex, no figure
A High Galactic Latitude HI 21cm-line Absorption Survey using the GMRT: I. Observations and Spectra
We have used the Giant Meterwave Radio Telescope (GMRT) to measure the
Galactic HI 21-cm line absorption towards 102 extragalactic radio continuum
sources, located at high (|b| >15deg.) Galactic latitudes. The Declination
coverage of the present survey is Decl. ~ -45deg.. With a mean rms optical
depth of ~0.003, this is the most sensitive Galactic HI 21-cm line absorption
survey to date. To supplement the absorption data, we have extracted the HI
21-cm line emission profiles towards these 102 lines of sight from the Leiden
Dwingeloo Survey of Galactic neutral hydrogen. We have carried out a Gaussian
fitting analysis to identify the discrete absorption and emission components in
these profiles. In this paper, we present the spectra and the components. A
subsequent paper will discuss the interpretation of these results.Comment: 46 pages, Accepted for publication in Journal of Astrophysics &
Astronom
The HI and Ionized Gas Disk of the Seyfert Galaxy NGC 1144 = Arp 118: A Violently Interacting Galaxy with Peculiar Kinematics
We present observations of the distribution and kinematics of neutral and
ionized gas in NGC 1144, a galaxy that forms part of the Arp 118 system.
Ionized gas is present over a huge spread in velocity (1100 km/s) in the disk
of NGC 1144, but HI emission is detected over only 1/3 of this velocity range,
in an area that corresponds to the NW half of the disk. In the nuclear region
of NGC 1144, a jump in velocity in the ionized gas component of 600 km/s is
observed. Faint, narrow HI absorption lines are also detected against radio
sources in the SE part of the disk of NGC 1144, which includes regions of
massive star formation and a Seyfert nucleus. The peculiar HI distribution,
which is concentrated in the NW disk, seems to be the inverse of the molecular
distribution which is concentrated in the SE disk. Although this may partly be
the result of the destruction of HI clouds in the SE disk, there is
circumstantial evidence that the entire HI emission spectrum of NGC 1144 is
affected by a deep nuclear absorption line covering a range of 600 km/s, and is
likely blueshifted with respect to the nucleus. In this picture, a high
column-density HI stream is associated with the nuclear ionized gas velocity
discontinuity, and the absorption effectively masks any HI emission that would
be present in the SE disk of NGC 1144.Comment: manuscript, arp118.ps: 28 pages; 1 Table: arp118.tab1.ps; 16 Figures:
arp118.fig1-16.ps; Accepted to Ap
Mechanisms of Addition and Substitution Reactions of Furfural and Derivatives
In a complete series of substitution reactions (such as halogenation, nitration, mercuration, etc.,) of furfural and its derivatives, it appears that substitution is preceded by addition. For example, in bromination, two atoms of bromine first add to give a relatively unstable intermediate which then loses hydrogen bromide. These preliminary additions may be 1, 2- or 1, 4- or 1, 6- additions depending upon reactants and upon experimental conditions. Accompanying some of these transformations are ring fission and ring closure, and rearrangements involving unsaturated linkages. Typical illustrations of these changes were presented
Tests of Bayesian Model Selection Techniques for Gravitational Wave Astronomy
The analysis of gravitational wave data involves many model selection
problems. The most important example is the detection problem of selecting
between the data being consistent with instrument noise alone, or instrument
noise and a gravitational wave signal. The analysis of data from ground based
gravitational wave detectors is mostly conducted using classical statistics,
and methods such as the Neyman-Pearson criteria are used for model selection.
Future space based detectors, such as the \emph{Laser Interferometer Space
Antenna} (LISA), are expected to produced rich data streams containing the
signals from many millions of sources. Determining the number of sources that
are resolvable, and the most appropriate description of each source poses a
challenging model selection problem that may best be addressed in a Bayesian
framework. An important class of LISA sources are the millions of low-mass
binary systems within our own galaxy, tens of thousands of which will be
detectable. Not only are the number of sources unknown, but so are the number
of parameters required to model the waveforms. For example, a significant
subset of the resolvable galactic binaries will exhibit orbital frequency
evolution, while a smaller number will have measurable eccentricity. In the
Bayesian approach to model selection one needs to compute the Bayes factor
between competing models. Here we explore various methods for computing Bayes
factors in the context of determining which galactic binaries have measurable
frequency evolution. The methods explored include a Reverse Jump Markov Chain
Monte Carlo (RJMCMC) algorithm, Savage-Dickie density ratios, the Schwarz-Bayes
Information Criterion (BIC), and the Laplace approximation to the model
evidence. We find good agreement between all of the approaches.Comment: 11 pages, 6 figure
On the B\"acklund Transformation for the Moyal Korteweg-de Vries Hierarchy
We study the B\"acklund symmetry for the Moyal Korteweg-de Vries (KdV)
hierarchy based on the Kuperschmidt-Wilson Theorem associated with second
Gelfand-Dickey structure with respect to the Moyal bracket, which generalizes
the result of Adler for the ordinary KdV.Comment: 9 pages, Revte
Detection of Cold Atomic Clouds in the Magellanic Bridge
We report a detection of cold atomic hydrogen in the Magellanic Bridge using
21-cm absorption spectroscopy toward the radio source B0312-770. With a column
density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10
and a maximum 21-cm emission brightness temperature of 1.4 K, this line of
sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm
absorption and emission spectroscopy toward 7 other low column density
sightlines on the periphery of the LMC and SMC reveal absorption toward one
additional background radio source behind the SMC with tau=0.03. The data have
typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and
sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold
atomic phase which is probably accompanied by molecular condensations in the
tenuous interstellar medium of the Bridge region. Young OB stars observed in
the Magellanic Bridge could form "in situ" from these cold condensations rather
than migrate from regions of active star formation in the main body of the SMC.
The existence of cold condensations and star formation in the Magellanic Bridge
might be understood as a small scale version of the mechanism that produces
star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the
Astronomical Journa
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