4,137 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
Dark Energy as a Born-Infeld Gauge Interaction Violating the Equivalence Principle
We investigate the possibility that dark energy does not couple to
gravitation in the same way than ordinary matter, yielding a violation of the
weak and strong equivalence principles on cosmological scales. We build a
transient mechanism in which gravitation is pushed away from general relativity
by a Born-Infeld gauge interaction acting as an "Abnormally Weighting" (dark)
Energy. This mechanism accounts for the Hubble diagram of far-away supernovae
by cosmic acceleration and time variation of the gravitational constant while
accounting naturally for the present tests on general relativity.Comment: 5 pages, 3 figures, sequel of Phys. Rev. D 73 023520 (2006), to
appear in Physical Review Letter
Supergiant Shells and Molecular Cloud Formation in the LMC
We investigate the influence of large-scale stellar feedback on the formation
of molecular clouds in the Large Magellanic Cloud (LMC). Examining the
relationship between HI and 12CO(J=1-0) in supergiant shells (SGSs), we find
that the molecular fraction in the total volume occupied by SGSs is not
enhanced with respect to the rest of the LMC disk. However, the majority of
objects (~70% by mass) are more molecular than their local surroundings,
implying that the presence of a supergiant shell does on average have a
positive effect on the molecular gas fraction. Averaged over the full SGS
sample, our results suggest that ~12-25% of the molecular mass in supergiant
shell systems was formed as a direct result of the stellar feedback that
created the shells. This corresponds to ~4-11% of the total molecular mass of
the galaxy. These figures are an approximate lower limit to the total
contribution of stellar feedback to molecular cloud formation in the LMC, and
constitute one of the first quantitative measurements of feedback-triggered
molecular cloud formation in a galactic system.Comment: 14 pages, 6 figures. Accepted for publication in Ap
An exploratory study of the hard X-ray variability properties of PG quasars with RXTE
We have monitored with the RXTE PCA the variability pattern of the 2-20 keV
flux in four PG quasars (QSOs) from the Laor et al. (1994) sample. Six
observations of each target at regular intervals of 1 day were performed. The
sample comprises objects with extreme values of Balmer line width (and hence
soft X-ray steepness) and spans about one order of magnitude in luminosity. The
most robust result is that the variability amplitude decreases as energy
increases. Several options for a possible ultimate driver of the soft and hard
X-ray variability, such as the influx rate of Comptonizing relativistic
particles, instabilities in the accretion flow or the number of X-ray active
sites, are consistent with our results.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
The Supershell-Molecular Cloud Connection in the Milky Way and Beyond
The role of large-scale stellar feedback in the formation of molecular clouds
has been investigated observationally by examining the relationship between HI
and 12CO(J=1-0) in supershells. Detailed parsec-resolution case studies of two
Milky Way supershells demonstrate an enhanced level of molecularisation over
both objects, and hence provide the first quantitative observational evidence
of increased molecular cloud production in volumes of space affected by
supershell activity. Recent results on supergiant shells in the LMC suggest
that while they do indeed help to organise the ISM into over-dense structures,
their global contribution to molecular cloud formation is of the order of only
~10%.Comment: Proceedings of IAUS 292 - Molecular Gas, Dust, and Star Formation in
Galaxies, eds. T. Wong & J. Ott. 4 pages, 3 figure
Dressing Technique for Intermediate Hierarchies
A generalized AKNS systems introduced and discussed recently in \cite{dGHM}
are considered. It was shown that the dressing technique both in matrix
pseudo-differential operators and formal series with respect to the spectral
parameter can be developed for these hierarchies.Comment: 16 pages, LaTeX Report/no: DFTUZ/94/2
Courant-like brackets and loop spaces
We study the algebra of local functionals equipped with a Poisson bracket. We
discuss the underlying algebraic structures related to a version of the
Courant-Dorfman algebra. As a main illustration, we consider the functionals
over the cotangent bundle of the superloop space over a smooth manifold. We
present a number of examples of the Courant-like brackets arising from this
analysis.Comment: 20 pages, the version published in JHE
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