3,879 research outputs found
Some Systematics of Galactic Globular Clusters
The global properties of all known Galactic globular clusters are examined.
The relationship between the luminosities and the metallicities of Galactic
globular clusters is found to be complex. Among luminous clusters there is a
correlation in the sense that the oldest clusters are slightly more metal
deficient than are younger clusters. However, no such clear-cut relationship is
found among the faintest globular clusters. The central concentration index C
of globular clusters is seen to be independent of metallicity. The dependence
of the half-light radii of globular clusters on their Galactocentric distances
can be approximated by the relation . Clusters with
collapsed cores are mostly situated close to the Galactic nucleus. For kpc the luminosities and the radii of clusters appear to be uncorrelated.
The Galaxy differs from the LMC and the SMC in that it appears to lack highly
flattened luminous clusters. Galactic globular clusters with ages 13.0
Gyr are all of Oosterhoff type II, whereas almost all of those with ages
13.0 Gyr have been assigned to Oosterhoff type I. Globular clusters with ages
11.5 Gyr are all located in the outer Galactic halo, have below-average
luminosities and above-average radii. On the other hand the very old globular
cluster NGC 6522 is situated close to the Galactic nucleus.Comment: PASP, in pres
First Report of Transmission of Soybean Mosaic Virus and Alfalfa Mosaic Virus by Aphis glycines in the New World
Originating text in English.Citation: Hill, J. H., Alleman, R., Hogg, D. B., Grau, C. R. (2001). First Report of Transmission of Soybean Mosaic Virus and Alfalfa Mosaic Virus by Aphis glycines in the New World. Plant Disease, 85(5), 561-561
Imaging of Ultraluminous Infrared Galaxies in the Near-UV
We present the first ground-based U' (3410 angstroms) images of Ultraluminous
Infrared Galaxies (ULIGs). Strong U' emission (median total M_U' = -20.8) is
seen in all systems and in some cases the extended tidal features (both the
smooth stellar distribution and compact star-forming features) contribute up to
60-80% of the total flux. The star-forming regions in both samples are found to
have ages based on spectral synthesis models in the range 10-100 Myrs, and most
differences in color between them can be attributed to the effects of dust
reddening. Additionally, it is found that star-formation in compact knots in
the tidal tails is most prominent in those ULIGs which have double nuclei,
suggesting that the star-formation rate in the tails peaks prior to the actual
coalescence of the galaxy nuclei and diminishes quickly thereafter. Similar to
results at other wavelengths, the observed star formation at U' can only
account for a small fraction of the known bolometric luminosity of the ULIGs.
Azimuthally averaged radial light profiles at U' are characterized by a sersic
law with index n=2, which is intermediate between an exponential disk and an
r^(-1/4) law and closely resembles the latter at large radii. The implications
of this near-ultraviolet imaging for optical/near-infrared observations of high
redshift counterparts of ULIGs are discussed.Comment: 30 pages, 4 tables, and 9 figures, 2 of which are JPEGs. To appear in
the August, 2000 edition of the Astronomical Journa
Quantum Portfolios
Quantum computation holds promise for the solution of many intractable
problems. However, since many quantum algorithms are stochastic in nature they
can only find the solution of hard problems probabilistically. Thus the
efficiency of the algorithms has to be characterized both by the expected time
to completion {\it and} the associated variance. In order to minimize both the
running time and its uncertainty, we show that portfolios of quantum algorithms
analogous to those of finance can outperform single algorithms when applied to
the NP-complete problems such as 3-SAT.Comment: revision includes additional data and corrects minor typo
Fast computation by block permanents of cumulative distribution functions of order statistics from several populations
The joint cumulative distribution function for order statistics arising from
several different populations is given in terms of the distribution function of
the populations. The computational cost of the formula in the case of two
populations is still exponential in the worst case, but it is a dramatic
improvement compared to the general formula by Bapat and Beg. In the case when
only the joint distribution function of a subset of the order statistics of
fixed size is needed, the complexity is polynomial, for the case of two
populations.Comment: 21 pages, 3 figure
Extremal Optimization at the Phase Transition of the 3-Coloring Problem
We investigate the phase transition of the 3-coloring problem on random
graphs, using the extremal optimization heuristic. 3-coloring is among the
hardest combinatorial optimization problems and is closely related to a 3-state
anti-ferromagnetic Potts model. Like many other such optimization problems, it
has been shown to exhibit a phase transition in its ground state behavior under
variation of a system parameter: the graph's mean vertex degree. This phase
transition is often associated with the instances of highest complexity. We use
extremal optimization to measure the ground state cost and the ``backbone'', an
order parameter related to ground state overlap, averaged over a large number
of instances near the transition for random graphs of size up to 512. For
graphs up to this size, benchmarks show that extremal optimization reaches
ground states and explores a sufficient number of them to give the correct
backbone value after about update steps. Finite size scaling gives
a critical mean degree value . Furthermore, the
exploration of the degenerate ground states indicates that the backbone order
parameter, measuring the constrainedness of the problem, exhibits a first-order
phase transition.Comment: RevTex4, 8 pages, 4 postscript figures, related information available
at http://www.physics.emory.edu/faculty/boettcher
Can the Copernican principle be tested by cosmic neutrino background?
The Copernican principle, stating that we do not occupy any special place in
our universe, is usually taken for granted in modern cosmology. However recent
observational data of supernova indicate that we may live in the under-dense
center of our universe, which makes the Copernican principle challenged. It
thus becomes urgent and important to test the Copernican principle via
cosmological observations. Taking into account that unlike the cosmic photons,
the cosmic neutrinos of different energies come from the different places to us
along the different worldlines, we here propose cosmic neutrino background as a
test of the Copernican principle. It is shown that from the theoretical
perspective cosmic neutrino background can allow one to determine whether the
Copernican principle is valid or not, but to implement such an observation the
larger neutrino detectors are called for.Comment: JHEP style, 10 pages, 4 figures, version to appear in JCA
Nested quantum search and NP-complete problems
A quantum algorithm is known that solves an unstructured search problem in a
number of iterations of order , where is the dimension of the
search space, whereas any classical algorithm necessarily scales as . It
is shown here that an improved quantum search algorithm can be devised that
exploits the structure of a tree search problem by nesting this standard search
algorithm. The number of iterations required to find the solution of an average
instance of a constraint satisfaction problem scales as , with
a constant depending on the nesting depth and the problem
considered. When applying a single nesting level to a problem with constraints
of size 2 such as the graph coloring problem, this constant is
estimated to be around 0.62 for average instances of maximum difficulty. This
corresponds to a square-root speedup over a classical nested search algorithm,
of which our presented algorithm is the quantum counterpart.Comment: 18 pages RevTeX, 3 Postscript figure
The overdensities of galaxy environments as a function of luminosity and color
We study the mean environments of galaxies in the Sloan Digital Sky Survey as
a function of rest-frame luminosity and color. Overdensities in galaxy number
are estimated in and spheres
centered on galaxies taken from the SDSS spectroscopic sample. We
find that, at constant color, overdensity is independent of luminosity for
galaxies with the blue colors of spirals. This suggests that, at fixed
star-formation history, spiral-galaxy mass is a very weak function of
environment. Overdensity does depend on luminosity for galaxies with the red
colors of early types; both low-luminosity and high-luminosity red galaxies are
found to be in highly overdense regions.Comment: submitted to ApJ
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