74,394 research outputs found
Flare in the Galactic stellar outer disc detected in SDSS-SEGUE data
Aims. We explore the outer Galactic disc up to a Galactocentric distance of
30 kpc to derive its parameters and measure the magnitude of its flare.
Methods. We obtained the 3D density of stars of type F8V-G5V with a colour
selection from extinction-corrected photometric data of the Sloan Digital Sky
Survey - Sloan Extension for Galactic Understanding and Exploration
(SDSS-SEGUE) over 1,400 deg^2 in off-plane low Galactic latitude regions and
fitted it to a model of flared thin+thick disc.
Results. The best-fit parameters are a thin-disc scale length of 2.0 kpc, a
thin-disc scale height at solar Galactocentric distance of 0.24 kpc, a
thick-disc scale length of 2.5 kpc, and a thick-disc scale height at solar
Galactocentric distance of 0.71 kpc. We derive a flaring in both discs that
causes the scale height of the average disc to be multiplied with respect to
the solar neighbourhood value by a factor of 3.3^{+2.2}_{-1.6} at R=15 kpc and
by a factor of 12^{+20}_{-7} at R=25 kpc.
Conclusions. The flare is quite prominent at large R and its presence
explains the apparent depletion of in-plane stars that are often confused with
a cut-off at R>15 kpc. Indeed, our Galactic disc does not present a truncation
or abrupt fall-off there, but the stars are spread in off-plane regions, even
at z of several kpc for R>20 kpc. Moreover, the smoothness of the observed
stellar distribution also suggests that there is a continuous structure and not
a combination of a Galactic disc plus some other substructure or extragalactic
component: the hypothesis to interpret the Monoceros ring in terms of a tidal
stream of a putative accreted dwarf galaxy is not only unnecessary because the
observed flare explains the overdensity in the Monoceros ring observed in SDSS
fields, but it appears to be inappropriate.Comment: Accepted for publication in A&
Complete Zeldovich approximation
We have developed a generalization of the Zeldovich approximation (ZA) that
is exact in a wide variety of situations, including plannar, spherical and
cilyndrical symmetries. We have shown that this generalization, that we call
complete Zeldovich approximation (CZA), is exact to second order at an
arbitrary point within any field. For gaussian fields, the third order error
have been obtained and shown to be very small. For statistical purposes, the
CZA leads to results exact to the third order.Comment: 11 pages+1 figure, accepted in ApJ Letter
Tests for the asymptotic behaviour of the gamma^* --> gamma pi^0 form factor
The gamma^* --> gamma pi^0 transition measured at different photon
virtualities already provides us with a clean test for the behaviour of the
pi^0 gamma^* gamma off-shell axial anomaly at large time-like squared momenta.
It also allows reliable predictions for the branching ratio of heavy quarkonium
decays into omega pi^0.Comment: Latex, 7 pages + 1 .ps figure, comments on the Brodsky-Lepage limit
and 5 new references adde
Hopfield Networks in Relevance and Redundancy Feature Selection Applied to Classification of Biomedical High-Resolution Micro-CT Images
We study filter–based feature selection methods for classification of biomedical images. For feature selection, we use two filters — a relevance filter which measures usefulness of individual features for target prediction, and a redundancy filter, which measures similarity between features. As selection method that combines relevance and redundancy we try out a Hopfield network. We experimentally compare selection methods, running unitary redundancy and relevance filters, against a greedy algorithm with redundancy thresholds [9], the min-redundancy max-relevance integration [8,23,36], and our Hopfield network selection. We conclude that on the whole, Hopfield selection was one of the most successful methods, outperforming min-redundancy max-relevance when\ud
more features are selected
New Precision Electroweak Tests of SU(5) x U(1) Supergravity
We explore the one-loop electroweak radiative corrections in supergravity via explicit calculation of vacuum-polarization and
vertex-correction contributions to the and
parameters. Experimentally, these parameters are obtained from a global fit to
the set of observables , and . We
include -dependent effects, which induce a large systematic negative shift
on for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). The
(non-oblique) supersymmetric vertex corrections to \Zbb, which define the
parameter, show a significant positive shift for light chargino
masses, which for can be nearly compensated by a negative
shift from the charged Higgs contribution. We conclude that at the 90\%CL, for
m_t\lsim160\GeV the present experimental values of and
do not constrain in any way supergravity in
both no-scale and dilaton scenarios. On the other hand, for m_t\gsim160\GeV
the constraints on the parameter space become increasingly stricter. We
demonstrate this trend with a study of the m_t=170\GeV case, where only a
small region of parameter space, with \tan\beta\gsim4, remains allowed and
corresponds to light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). Thus
supergravity combined with high-precision LEP data would
suggest the presence of light charginos if the top quark is not detected at the
Tevatron.Comment: LaTeX, 11 Pages+4 Figures(not included), the figures available upon
request as an uuencoded file(0.4MB) or 4 PS files from [email protected],
CERN-TH.7078/93, CTP-TAMU-68/93, ACT-24/9
Active elastic dimers: Cells moving on rigid tracks
Experiments suggest that the migration of some cells in the three-dimensional
extra cellular matrix bears strong resemblance to one-dimensional cell
migration. Motivated by this observation, we construct and study a minimal
one-dimensional model cell made of two beads and an active spring moving along
a rigid track. The active spring models the stress fibers with their
myosin-driven contractility and alpha-actinin-driven extendability, while the
friction coefficients of the two beads describe the catch/slip bond behavior of
the integrins in focal adhesions. In the absence of active noise, net motion
arises from an interplay between active contractility (and passive
extendability) of the stress fibers and an asymmetry between the front and back
of the cell due to catch bond behavior of integrins at the front of the cell
and slip bond behavior of integrins at the back. We obtain reasonable cell
speeds with independently estimated parameters. We also study the effects of
hysteresis in the active spring, due to catch bond behavior and the dynamics of
cross-linking, and the addition of active noise on the motion of the cell. Our
model highlights the role of alpha-actinin in three-dimensional cell motility
and does not require Arp2/3 actin filament nucleation for net motion.Comment: 13 pages, 9 figure
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