920 research outputs found
ISO far-infrared observations of rich galaxy clusters II. Sersic 159-03
The far-infrared emission from rich galaxy clusters is investigated. Maps
have been obtained by ISO at 60, 100, 135, and 200 microns using the PHT-C
camera. Ground based imaging and spectroscopy were also acquired. Here we
present the results for the cooling flow cluster Sersic 159-03. An infrared
source coincident with the dominant cD galaxy is found. Some off-center sources
are also present, but without any obvious counterparts.Comment: 6 pages, 4 postscript figures, accepted for publication in `Astronomy
and Astrophysics
Recurrent flow patterns as a basis for turbulence: predicting statistics from structures
A dynamical systems approach to turbulence envisions the flow as a trajectory
through a high-dimensional state space transiently visiting the neighbourhoods
of unstable simple invariant solutions (E. Hopf, Commun. Appl. Maths 1, 303,
1948). The hope has always been to turn this appealing picture into a
predictive framework where the statistics of the flow follows from a weighted
sum of the statistics of each simple invariant solution. Two outstanding
obstacles have prevented this goal from being achieved: (1) paucity of known
solutions and (2) the lack of a rational theory for predicting the required
weights. Here we describe a method to substantially solve these problems, and
thereby provide the first compelling evidence that the PDFs of a fully
developed turbulent flow can be reconstructed with a set of unstable periodic
orbits. Our new method for finding solutions uses automatic differentiation,
with high-quality guesses constructed by minimising a trajectory-dependent loss
function. We use this approach to find hundreds of new solutions in turbulent,
two-dimensional Kolmogorov flow. Robust statistical predictions are then
computed by learning weights after converting a turbulent trajectory into a
Markov chain for which the states are individual solutions, and the nearest
solution to a given snapshot is determined using a deep convolutional
autoencoder. To our knowledge, this is the first time the PDFs of a
spatio-temporally-chaotic system have been successfully reproduced with a set
of simple invariant states, and provides a fascinating connection between
self-sustaining dynamical processes and the more well-known statistical
properties of turbulence
Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275
Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been
combined with HST WFPC2 photometry to explore the nature and origin of these
objects and discriminate between merger and cooling flow scenarios for globular
cluster formation. The objects we have studied are not HII regions, but rather
star clusters, yet their integrated spectral properties do not resemble young
or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The
clusters' Balmer absorption appears to be too strong to be consistent with any
of the standard Bruzual & Charlot evolutionary models at any metallicity. If
these models are adopted, an IMF which is skewed to high masses provides a
better fit to the data. A truncated IMF with a mass range of 2-3 Mo reproduces
the observed Balmer equivalent widths and colors at about 450 Myr. Formation in
a continuous cooling flow appears to be ruled out since the age of the clusters
is much larger than the cooling time, the spatial scale of the clusters is much
smaller than the cooling flow radius, and the deduced star formation rate in
the cooling flow favors a steep rather than a flat IMF. A merger would have to
produce clusters only in the central few kpc, presumably from gas in the
merging galaxies which was channeled rapidly to the center. Widespread shocks
in merging galaxies cannot have produced these clusters. If these objects are
confirmed to have a relatively flat, or truncated, IMF it is unclear whether or
not they will evolve into objects we would regard as bona fide globular
clusters.Comment: 30 pages (AAS two column style, including 9 tables and 7 figures) to
appear in the AJ (August issue), also available at
http://www.ucolick.org/~mkissler/Sages/sages.html (with a full resolution
Fig.1) Revised Version: previous posted version was an uncorrect ealier
iteration, parts of the text, tables and figures changed. The overall
conclusions remain unchange
Ultraviolet Imaging Observations of the cD Galaxy in Abell 1795: Further Evidence for Massive Star Formation in a Cooling Flow
We present images from the Ultraviolet Imaging Telescope of the Abell 1795
cluster of galaxies. We compare the cD galaxy morphology and photometry of
these data with those from existing archival and published data. The addition
of a far--UV color helps us to construct and test star formation model
scenarios for the sources of UV emission. Models of star formation with rates
in the range \sim5-20M_{\sun}yr indicate that the best fitting models
are those with continuous star formation or a recent ( Myr old) burst
superimposed on an old population. The presence of dust in the galaxy,
dramatically revealed by HST images complicates the interpretation of UV data.
However, we find that the broad--band UV/optical colors of this cD galaxy can
be reasonably matched by models using a Galactic form for the extinction law
with . We also briefly discuss other objects in the large UIT
field of view.Comment: To appear in the Astrophysical Journal. 14 AAS preprint style pages
plus 7 figure
Using the stated preference method for the calculation of social discount rate
The aim of this paper is to build the stated preference method into the social discount rate methodology. The first part of the paper presents the results of a survey about stated time preferences through pair-choice decision situations for various topics and time horizons. It is assumed that stated time preferences differ from calculated time preferences and that the extent of stated rates depends on the time period, and on how much respondents are financially and emotionally involved in the transactions. A significant question remains: how can the gap between the calculation and the results of surveys be resolved, and how can the real time preferences of individuals be interpreted using a social time preference rate. The second part of the paper estimates the social time preference rate for Hungary using the results of the survey, while paying special attention to the pure time preference component. The results suggest that the current method of calculation of the pure time preference rate does not reflect the real attitudes of individuals towards future generations
The Spectrum of Integrated Millimeter Flux of the Magellanic Clouds and 30-Doradus from TopHat and DIRBE Data
We present measurements of the integrated flux relative to the local
background of the Large and Small Magellanic Clouds and the region 30-Doradus
(the Tarantula Nebula) in the LMC in four frequency bands centered at 245, 400,
460, and 630 GHz, based on observations made with the TopHat telescope. We
combine these observations with the corresponding measurements for the DIRBE
bands 8, 9, and 10 to cover the frequency range 245 - 3000 GHz (100 - 1220
micrometers) for these objects. We present spectra for all three objects and
fit these spectra to a single-component greybody emission model and report
best-fit dust temperatures, optical depths, and emissivity power-law indices,
and we compare these results with other measurements in these regions and
elsewhere. Using published dust grain opacities, we estimate the mass of the
measured dust component in the three regions.Comment: 41 pages, 4 figures. Accepted for publication in Astrophysical
Journa
Magnetism, Spin-Orbit Coupling, and Superconducting Pairing in UGe
A consistent picture on the mean-field level of the magnetic properties and
electronic structure of the superconducting itinerant ferromagnet UGe is
shown to require inclusion of correlation effects beyond the local density
approximation (LDA). The "LDA+U" approach reproduces both the magnitude of the
observed moment, composed of strongly opposing spin and orbital parts, and the
magnetocrystalline anisotropy. The largest Fermi surface sheet is comprised
primarily of spin majority states with orbital projection =0,
suggesting a much simpler picture of the pairing than is possible for general
strong spin-orbit coupled materials. This occurrence, and the
quasi-two-dimensional geometry of the Fermi surface, support the likelihood of
magnetically mediated p-wave triplet pairing.Comment: accepted for publication in Phys. Rev. Lett; URL for better quality
image of Fig.3 (2MB) at http://yammer.ucdavis.edu/public/UGe2/fig3.ep
Microquasars as sources of positron annihilation radiation
We consider the production of positrons in microquasars, i.e. X-ray binary
systems that exhibit jets frequently, but not continuously. We estimate the
production rate of positrons in microquasars, both by simple energy
considerations and in the framework of various proposed models. We then
evaluate the collective emissivity of the annihilation radiation produced by
Galactic microquasars and we find that it might constitute a substantial
contribution to the annihilation flux measured by INTEGRAL/SPI. We also discuss
the possible spatial distribution of Galactic microquasars, on the basis of the
(scarce) available data and the resulting morphology of the flux received on
Earth. Finally, we consider nearby 'misaligned' microquasars, with jets
occasionally hitting the atmosphere of the companion star; these would
represent interesting point sources, for which we determine the annihilation
flux and the corresponding light curve, as well as the line's spectral profile.
We discuss the possibility of detection of such point sources by future
instruments.Comment: 13 pages, 7 figures, accepted in A&
WFPC2 Observations of the Cooling Flow Elliptical in Abell 1795
We present WFPC2 images of the core of the cooling flow cD galaxy in Abell
1795. An irregular, asymmetric dust lane extends 7 \h75 kpc in projection to
the north-northwest. The dust shares the morphology observed in the H
and excess UV emission. We see both diffuse and knotty blue emission around the
dust lane, especially at the ends. The dust and emission features lie on the
edge of the radio lobes, suggesting star formation induced by the radio source
or the deflection of the radio jets off of pre-existing dust and gas. We
measure an apparent R significantly less than 3.1, implying that the
extinction law is not Galactic in the dust lane, or the presence of line
emission which is proportional to the extinction. The dust mass is at least
2 M\solar\ and is more likely to be 6.5 M\solar.Comment: 14 pages, LaTeX, Figure 4 included, Postscript Figs. 1-3 available at
ftp://astro.nmsu.edu/pub/JASON/A1795/, accepted for publication in ApJ
Letter
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