3,349 research outputs found
Spectral energy distributions of submm/radio bright gamma-ray burst host galaxies
We present optical to radio spectral energy distribution fitting of the host
galaxies of four long gamma-ray bursts: 980703, 000210, 000418 and 010222,
which were detected at submillimetre and/or radio wavelengths. We find that
only very young starburst galaxy models are consistent with the data having
both blue optical colors and a pronounced submm emission. For each host we are
able to construct a model consistent with the short- and long-wavelength parts
of the spectra. We find galaxy ages ranging from 0.09 to 2.0 Gyrs and star
formation rates ranging from 138 to 380 MSun/yr.Comment: 6 pages, 1 figure, Proceedings of the Conference "The Multicoloured
Landscape of Compact Objects and their Explosive Origins" (Cefalu, Sicily,
2006 June 11-24). Eds. L. Burderi et al. (New York: AIP), in press, for SED
templates, see http://archive.dark-cosmology.dk
Detection of a spiral lens galaxy and optical variability in the gravitational lens system B1600+434
The gravitationally lensed quasar B1600+434 (z=1.61, mV=21.6) has been
observed at the 2.56m Nordic Optical Telescope (NOT). In this Letter we report
the discovery of an edge-on late-type galaxy located between the two lensed
components (separation 1\farcs4), close to the fainter image. The galaxy
photometry indicates that its redshift is approximately 0.4. We detect a large
colour difference between the two images due to significant obscuration of the
faint image. The estimated amount of absorption as a function of colour
indicates that the extinction may be due to dust in the lensing galaxy. We also
present evidence of flux variability in B1600+434 with a detected change of
0.25mag in one year. The theoretically expected time delay is of the order of
one month and so the system may be an interesting object for determining the
Hubble constant.Comment: 4 pages, 3 postscript figures, accepted for publication in A&A
Letter
Shaping the dust mass - star-formation rate relation
There is a remarkably tight relation between the observationally inferred
dust masses and star-formation rates (SFRs) of SDSS galaxies, Mdust
SFR (Da Cunha et al. 2010). Here we extend the Mdust-SFR relation to
the high end and show that it bends over at very large SFRs (i.e., dust masses
are lower than predicted for a given SFR). We identify several distinct
evolutionary processes in the diagram: (1) A star-bursting phase in which dust
builds up rapidly at early times. The maximum attainable dust mass in this
process is the cause of the bend-over of the relation. A high dust-formation
efficiency, a bottom-light initial mass function, and negligible supernova
shock dust destruction are required to produce sufficiently high dust masses.
(2) A quiescent star-forming phase in which the subsequent parallel decline in
dust mass and SFR gives rise to the Mdust-SFR relation, through astration and
dust destruction. The dust-to-gas ratio is approximately constant along the
relation. We show that the power-law slope of the Mdust-SFR relation is
inversely proportional to the global Schmidt-Kennicutt law exponent (i.e.,
) in simple chemical evolution models. (3) A quenching phase which
causes star formation to drop while the dust mass stays roughly constant or
drops proportionally. Combined with merging, these processes, as well as the
range in total baryonic mass, give rise to a complex population of the diagram
which adds significant scatter to the original Mdust-SFR relation. (4) At very
high redshifts, a population of galaxies located significantly below the local
relation is predicted.Comment: 5 pages, 1 figure, ApJL, in pres
Generalized contour deformation method in momentum space: two-body spectral structures and scattering amplitudes
A generalized contour deformation method (GCDM) which combines complex
rotation and translation in momentum space, is discussed. GCDM gives accurate
results for bound, virtual (antibound), resonant and scattering states starting
with a realistic nucleon-nucleon interaction. It provides a basis for full
off-shell -matrix calculations both for real and complex input energies.
Results for both spectral structures and scattering amplitudes compare
perfectly well with exact values for the separable Yamaguchi potential.
Accurate calculation of virtual states in the Malfliet-Tjon and the realistic
CD-Bonn nucleon-nucleon interactions are presented.
GCDM is also a promising method for the computation of in-medium properties
such as the resummation of particle-particle and particle-hole diagrams in
infinite nuclear matter. Implications for in-medium scattering are discussed.Comment: 15 pages, revte
Ballerina - Pirouettes in Search of Gamma Bursts
The cosmological origin of gamma ray bursts has now been established with
reasonable certainty. Many more bursts will need to be studied to establish the
typical distance scale, and to map out the large diversity in properties which
have been indicated by the first handful of events. We are proposing Ballerina,
a small satellite to provide accurate positions and new data on the gamma-ray
bursts. We anticipate a detection rate an order of magnitude larger than
obtained from Beppo-SAX.Comment: A&AS in press, proceedings of the Workshop "Gamma Ray Bursts in the
Afterglow Era" in Rome, November 199
Unveiling the Secrets of Gamma Ray Bursts
Gamma Ray Bursts (GRBs) are unpredictable and brief flashes of gamma rays
that occur about once a day in random locations in the sky. Since gamma rays do
not penetrate the Earth's atmosphere, they are detected by satellites, which
automatically trigger ground-based telescopes for follow-up observations at
longer wavelengths. In this introduction to Gamma Ray Bursts we review how
building a multi-wavelength picture of these events has revealed that they are
the most energetic explosions since the Big Bang and are connected with stellar
deaths in other galaxies. However, in spite of exceptional observational and
theoretical progress in the last 15 years, recent observations raise many
questions which challenge our understanding of these elusive phenomena. Gamma
Ray Bursts therefore remain one of the hottest topics in modern astrophysics.Comment: 20 pages, 11 figures, review article, draft version, final version
will appear in Contemporary Physic
Simulated Dark-Matter Halos as a Test of Nonextensive Statistical Mechanics
In the framework of nonextensive statistical mechanics, the equilibrium
structures of astrophysical self-gravitating systems are stellar polytropes,
parameterized by the polytropic index n. By careful comparison to the
structures of simulated dark-matter halos we find that the density profiles, as
well as other fundamental properties, of stellar polytropes are inconsistent
with simulations for any value of n. This result suggests the need to
reconsider the applicability of nonextensive statistical mechanics (in its
simplest form) to equilibrium self-gravitating systems.Comment: Accepted for publication in Physical Review
Stryker Osteonics: Prosthetic Knee Joint
We examine, within a simple bearing model of a knee joint that only consideres pure sliding, the effect of the presence of a small vertical hole in the load area on the fluid film properties. The calculations indicate that fluid is entrapped in such a hole, which, for constant load, causes a smaller minimal film separation of the two surfaces.
This will lower the horizontal friction, but may also bring about surface contact in high load situations
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