754 research outputs found
The context of the Local Volume: structures and motions in the nearby universe
The 6dF Galaxy Survey (6dFGS) and the 2MASS Redshift Survey (2MRS) provide
the most complete maps of the large-scale structures and motions in the nearby
universe. These maps have been used to reconstruct the density field in the
local volume, and to predict the corresponding velocity field and the dipole of
the Local Group motion.Comment: 4 pages, to appear in "Galaxies in the Local Volume", 2008, eds B.
Koribalski and H. Jerjen, Springer Astrophysics and Space Science Series
(proceedings of conference held in Sydney on 8-13 July 2007
Noncanonical quantization of gravity. II. Constraints and the physical Hilbert space
The program of quantizing the gravitational field with the help of affine
field variables is continued. For completeness, a review of the selection
criteria that singles out the affine fields, the alternative treatment of
constraints, and the choice of the initial (before imposition of the
constraints) ultralocal representation of the field operators is initially
presented. As analogous examples demonstrate, the introduction and enforcement
of the gravitational constraints will cause sufficient changes in the operator
representations so that all vestiges of the initial ultralocal field operator
representation disappear. To achieve this introduction and enforcement of the
constraints, a well characterized phase space functional integral
representation for the reproducing kernel of a suitably regularized physical
Hilbert space is developed and extensively analyzed.Comment: LaTeX, 42 pages, no figure
Scattering and delay time for 1D asymmetric potentials: the step-linear and the step-exponential cases
We analyze the quantum-mechanical behavior of a system described by a
one-dimensional asymmetric potential constituted by a step plus (i) a linear
barrier or (ii) an exponential barrier. We solve the energy eigenvalue equation
by means of the integral representation method, classifying the independent
solutions as equivalence classes of homotopic paths in the complex plane.
We discuss the structure of the bound states as function of the height U_0 of
the step and we study the propagation of a sharp-peaked wave packet reflected
by the barrier. For both the linear and the exponential barrier we provide an
explicit formula for the delay time \tau(E) as a function of the peak energy E.
We display the resonant behavior of \tau(E) at energies close to U_0. By
analyzing the asymptotic behavior for large energies of the eigenfunctions of
the continuous spectrum we also show that, as expected, \tau(E) approaches the
classical value for E -> \infty, thus diverging for the step-linear case and
vanishing for the step-exponential one.Comment: 14 pages, 10 figure
Weighing the local dark matter with RAVE red clump stars
We determine the Galactic potential in the solar neigbourhood from RAVE
observations. We select red clump stars for which accurate distances, radial
velocities, and metallicities have been measured. Combined with data from the
2MASS and UCAC catalogues, we build a sample of 4600 red clump stars within a
cylinder of 500 pc radius oriented in the direction of the South Galactic Pole,
in the range of 200 pc to 2000 pc distances. We deduce the vertical force and
the total mass density distribution up to 2 kpc away from the Galactic plane by
fitting a distribution function depending explicitly on three isolating
integrals of the motion in a separable potential locally representing the
Galactic one with four free parameters. Because of the deep extension of our
sample, we can determine nearly independently the dark matter mass density and
the baryonic disc surface mass density. We find (i) at 1kpc Kz/(2piG) = 68.5 pm
1.0 Msun/pc2, and (ii) at 2 kpc Kz/(2piG) = 96.9 pm 2.2 Msun/pc2. Assuming the
solar Galactic radius at R0 = 8.5 kpc, we deduce the local dark matter density
rhoDM (z=0) = 0.0143 pm 0.0011Msun pc3 = 0.542 pm 0.042 Gev/cm3 and the
baryonic surface mass density Sigma = 44.4 pm 4.1 Msun/pc2 . Our results are in
agreement with previously published Kz determinations up to 1 kpc, while the
extension to 2 kpc shows some evidence for an unexpectedly large amount of dark
matter. A flattening of the dark halo of order 0.8 can produce such a high
local density in combination with a circular velocity of 240 km/s . Another
explanation, allowing for a lower circular velocity, could be the presence of a
secondary dark component, a very thick disc resulting either from the deposit
of dark matter from the accretion of multiple small dwarf galaxies, or from the
presence of an effective phantom thick disc in the context of effective
galactic-scale modifications of gravity.Comment: 14 pages, 13 figures, accepted to Astronomy and Astrophysic
Chemical gradients in the Milky Way from the RAVE data. II. Giant stars
We provide new constraints on the chemo-dynamical models of the Milky Way by
measuring the radial and vertical chemical gradients for the elements Mg, Al,
Si, Ti, and Fe in the Galactic disc and the gradient variations as a function
of the distance from the Galactic plane (). We selected a sample of giant
stars from the RAVE database using the gravity criterium 1.7log g2.8. We
created a RAVE mock sample with the Galaxia code based on the Besan\c con model
and selected a corresponding mock sample to compare the model with the observed
data. We measured the radial gradients and the vertical gradients as a function
of the distance from the Galactic plane to study their variation across the
Galactic disc. The RAVE sample exhibits a negative radial gradient of
dex kpc close to the Galactic plane ( kpc)
that becomes flatter for larger . Other elements follow the same trend
although with some variations from element to element. The mock sample has
radial gradients in fair agreement with the observed data. The variation of the
gradients with shows that the Fe radial gradient of the RAVE sample has
little change in the range kpc and then flattens. The iron
vertical gradient of the RAVE sample is slightly negative close to the Galactic
plane and steepens with . The mock sample exhibits an iron vertical
gradient that is always steeper than the RAVE sample. The mock sample also
shows an excess of metal-poor stars in the [Fe/H] distributions with respect to
the observed data. These discrepancies can be reduced by decreasing the number
of thick disc stars and increasing their average metallicity in the Besan\c con
model.Comment: 13 pages, 9 figures, 5 tables, A&A accepte
Kinematic modelling of the Milky Way using the RAVE and GCS stellar surveys
We investigate the kinematic parameters of the Milky Way disc using the RAVE
and GCS stellar surveys. We do this by fitting a kinematic model to the data
taking the selection function of the data into account. For stars in the GCS we
use all phase-space coordinates, but for RAVE stars we use only . Using MCMC technique, we investigate the full posterior distributions
of the parameters given the data. We investigate the `age-velocity dispersion'
relation for the three kinematic components
(), the radial dependence of the velocity
dispersions, the Solar peculiar motion (), the
circular speed at the Sun and the fall of mean azimuthal motion with
height above the mid-plane. We confirm that the Besan\c{c}on-style Gaussian
model accurately fits the GCS data, but fails to match the details of the more
spatially extended RAVE survey. In particular, the Shu distribution function
(DF) handles non-circular orbits more accurately and provides a better fit to
the kinematic data. The Gaussian distribution function not only fits the data
poorly but systematically underestimates the fall of velocity dispersion with
radius. We find that correlations exist between a number of parameters, which
highlights the importance of doing joint fits. The large size of the RAVE
survey, allows us to get precise values for most parameters. However, large
systematic uncertainties remain, especially in and . We
find that, for an extended sample of stars, is underestimated by as
much as if the vertical dependence of the mean azimuthal motion is
neglected. Using a simple model for vertical dependence of kinematics, we find
that it is possible to match the Sgr A* proper motion without any need for
being larger than that estimated locally by surveys like GCS.Comment: 27 pages, 13 figures, accepted for publication in Ap
The RAVE survey: the Galactic escape speed and the mass of the Milky Way
We construct new estimates on the Galactic escape speed at various
Galactocentric radii using the latest data release of the Radial Velocity
Experiment (RAVE DR4). Compared to previous studies we have a database larger
by a factor of 10 as well as reliable distance estimates for almost all stars.
Our analysis is based on the statistical analysis of a rigorously selected
sample of 90 high-velocity halo stars from RAVE and a previously published data
set. We calibrate and extensively test our method using a suite of cosmological
simulations of the formation of Milky Way-sized galaxies. Our best estimate of
the local Galactic escape speed, which we define as the minimum speed required
to reach three virial radii , is km/s (90%
confidence) with an additional 5% systematic uncertainty, where is
the Galactocentric radius encompassing a mean over-density of 340 times the
critical density for closure in the Universe. From the escape speed we further
derive estimates of the mass of the Galaxy using a simple mass model with two
options for the mass profile of the dark matter halo: an unaltered and an
adiabatically contracted Navarro, Frenk & White (NFW) sphere. If we fix the
local circular velocity the latter profile yields a significantly higher mass
than the un-contracted halo, but if we instead use the statistics on halo
concentration parameters in large cosmological simulations as a constraint we
find very similar masses for both models. Our best estimate for , the
mass interior to (dark matter and baryons), is M (corresponding to M). This estimate is in good agreement with recently published
independent mass estimates based on the kinematics of more distant halo stars
and the satellite galaxy Leo I.Comment: 16 pages, 15 figures; accepted for publication in Astronomy &
Astrophysic
Polarization phenomena in open charm photoproduction processes
We analyze polarization effects in associative photoproduction of
pseudoscalar () charmed mesons in exclusive processes , , . Circularly polarized photons
induce nonzero polarization of the -hyperon with - and -components
(in the reaction plane) and non vanishing asymmetries and for polarized nucleon target. These polarization observables can be
predicted in model-independent way for exclusive -production processes
in collinear kinematics. The T-even -polarization and asymmetries for
non-collinear kinematics can be calculated in framework of an effective
Lagrangian approach. The depolarization coefficients , characterizing
the dependence of the -polarization on the nucleon polarization are also
calculated.Comment: 36 pages 13 figure
Scattering in flatland: Efficient representations via wave atoms
This paper presents a numerical compression strategy for the boundary
integral equation of acoustic scattering in two dimensions. These equations
have oscillatory kernels that we represent in a basis of wave atoms, and
compress by thresholding the small coefficients to zero. This phenomenon was
perhaps first observed in 1993 by Bradie, Coifman, and Grossman, in the context
of local Fourier bases \cite{BCG}. Their results have since then been extended
in various ways. The purpose of this paper is to bridge a theoretical gap and
prove that a well-chosen fixed expansion, the nonstandard wave atom form,
provides a compression of the acoustic single and double layer potentials with
wave number as -by- matrices with
nonnegligible entries, with a constant that depends on the relative
accuracy \eps in an acceptable way. The argument assumes smooth, separated,
and not necessarily convex scatterers in two dimensions. The essential features
of wave atoms that enable to write this result as a theorem is a sharp
time-frequency localization that wavelet packets do not obey, and a parabolic
scaling wavelength (essential diameter). Numerical experiments
support the estimate and show that this wave atom representation may be of
interest for applications where the same scattering problem needs to be solved
for many boundary conditions, for example, the computation of radar cross
sections.Comment: 39 page
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