60 research outputs found
Dynamical modelling of the elliptical galaxy NGC 2974
In this paper we analyse the relations between a previously described oblate
Jaffe model for an ellipsoidal galaxy and the observed quantities for NGC 2974,
and obtain the length and velocity scales for a relevant elliptical galaxy
model. We then derive the finite total mass of the model from these scales, and
finally find a good fit of an isotropic oblate Jaffe model by using the
Gauss-Hermite fit parameters and the observed ellipticity of the galaxy NGC
2974. The model is also used to predict the total luminous mass of NGC 2974,
assuming that the influence of dark matter in this galaxy on the image,
ellipticity and Gauss-Hermite fit parameters of this galaxy is negligible
within the central region, of radius Comment: 7 figure
Anisotropic distribution functions for spherical galaxies
A method is presented for finding anisotropic distribution functions for
stellar systems with known, spherically symmetric, densities, which depends
only on the two classical integrals of the energy and the magnitude of the
angular momentum. It requires the density to be expressed as a sum of products
of functions of the potential and of the radial coordinate. The solution
corresponding to this type of density is in turn a sum of products of functions
of the energy and of the magnitude of the angular momentum. The products of the
density and its radial and transverse velocity dispersions can be also
expressed as a sum of products of functions of the potential and of the radial
coordinate. Several examples are given, including some of new anisotropic
distribution functions. This device can be extended further to the related
problem of finding two-integral distribution functions for axisymmetric
galaxies.Comment: 5 figure
Exact General Relativistic Perfect Fluid Disks with Halos
Using the well-known ``displace, cut and reflect'' method used to generate
disks from given solutions of Einstein field equations, we construct static
disks made of perfect fluid based on vacuum Schwarzschild's solution in
isotropic coordinates. The same method is applied to different exactsolutions
to the Einstein'sequations that represent static spheres of perfect fluids. We
construct several models of disks with axially symmetric perfect fluid halos.
All disks have some common features: surface energy density and pressures
decrease monotonically and rapidly with radius. As the ``cut'' parameter
decreases, the disks become more relativistic, with surface energy density and
pressure more concentrated near the center. Also regions of unstable circular
orbits are more likely to appear for high relativistic disks. Parameters can be
chosen so that the sound velocity in the fluid and the tangential velocity of
test particles in circular motion are less then the velocity of light. This
tangential velocity first increases with radius and reaches a maximum.Comment: 22 pages, 25 eps.figs, RevTex. Phys. Rev. D to appea
Exact General Relativistic Thick Disks
A method to construct exact general relativistic thick disks that is a simple
generalization of the ``displace, cut and reflect'' method commonly used in
Newtonian, as well as, in Einstein theory of gravitation is presented. This
generalization consists in the addition of a new step in the above mentioned
method. The new method can be pictured as a ``displace, cut, {\it fill} and
reflect'' method. In the Newtonian case, the method is illustrated in some
detail with the Kuzmin-Toomre disk. We obtain a thick disk with acceptable
physical properties. In the relativistic case two solutions of the Weyl
equations, the Weyl gamma metric (also known as Zipoy-Voorhees metric) and the
Chazy-Curzon metric are used to construct thick disks. Also the Schwarzschild
metric in isotropic coordinates is employed to construct another family of
thick disks. In all the considered cases we have non trivial ranges of the
involved parameter that yield thick disks in which all the energy conditions
are satisfied.Comment: 11 pages, RevTex, 9 eps figs. Accepted for publication in PR
Self-Consistent Gravitational Chaos
The motion of stars in the gravitational potential of a triaxial galaxy is
generically chaotic. However, the timescale over which the chaos manifests
itself in the orbital motion is a strong function of the degree of central
concentration of the galaxy. Here, chaotic diffusion rates are presented for
orbits in triaxial models with a range of central density slopes and nuclear
black-hole masses. Typical diffusion times are found to be less than a galaxy
lifetime in triaxial models where the density increases more rapidly than 1/r
at the center, or which contain black holes with masses that exceed roughly
0.1% of the galaxy mass. When the mass of a central black hole exceeds roughly
0.02 times the mass of the galaxy, there is a transition to global
stochasticity and the galaxy evolves to an axisymmetric shape in little more
than a crossing time. This rapid evolution may provide a negative feedback
mechanism that limits the mass of nuclear black holes to a few percent of the
stellar mass of a galaxy.Comment: 15 Tex pages, 7 Postscript figures. To appear in the Twelfth Annual
Florida Workshop in Nonlinear Astronomy and Physics: Long Range Correlations
in Astrophysical and Other Systems, eds. J. R. Buchler, J. Dufty and H.
Kandru
Non-thermal leptogenesis with almost degenerate superheavy neutrinos
We present a model with minimal assumptions for non-thermal leptogenesis with
almost degenerate superheavy right-handed neutrinos in a supersymmetric set up.
In this scenario a gauge singlet inflaton is directly coupled to the
right-handed (s)neutrinos with a mass heavier than the inflaton mass. This
helps avoiding potential problems which can naturally arise otherwise. The
inflaton decay to the Standard Model leptons and Higgs, via off-shell
right-handed (s)neutrinos, reheats the Universe. The same channel is also
responsible for generating the lepton asymmetry, thus requiring no stage of
preheating in order to excite superheavy (s)neutrinos. The suppressed decay
rate of the inflaton naturally leads to a sufficiently low reheat temperature,
which in addition, prevents any wash out of the yielded asymmetry. We will
particularly elaborate on important differences from leptogenesis with on-shell
(s)neutrinos. It is shown that for nearly degenerate neutrinos a successful
leptogenesis can be accommodated for a variety of inflationary models with a
rather wide ranging inflationary scale.Comment: 10 revtex pages, 2 figure (uses axodraw). The derivation of the
asymmetry parameter for the general case and one figure added. Final version
to appear in Phys. Rev.
Observational Constraints on the Modified Gravity Model (MOG) Proposed by Moffat: Using the Magellanic System
A simple model for the dynamics of the Magellanic Stream (MS), in the
framework of modified gravity models is investigated. We assume that the galaxy
is made up of baryonic matter out of context of dark matter scenario. The model
we used here is named Modified Gravity (MOG) proposed by Moffat (2005). In
order to examine the compatibility of the overall properties of the MS under
the MOG theory, the observational radial velocity profile of the MS is compared
with the numerical results using the fit method. In order to obtain
the best model parameters, a maximum likelihood analysis is performed. We also
compare the results of this model with the Cold Dark Matter (CDM) halo model
and the other alternative gravity model that proposed by Bekenstein (2004), so
called TeVeS. We show that by selecting the appropriate values for the free
parameters, the MOG theory seems to be plausible to explain the dynamics of the
MS as well as the CDM and the TeVeS models.Comment: 14 pages, 3 Figures, accepted in Int. J. Theor. Phy
Big Bang Baryogenesis
An overview of baryogenesis in the early Universe is presented. The standard
big bang model including big bang nucleosynthesis and inflation is breifly
reviewed. Three basic models for baryogenesis will be developed: The
``standard" out-of-equilibrium decay model; the decay of scalar consensates
along flat directions in supersymmetric models; and lepto-baryogenesis, which
is the conversion of a lepton asymmetry into a baryon asymmetry via
non-perturbative electroweak interactions.Comment: 36 pages, LaTeX, UMN-TH-1249, Lectures given at the 33rd
International Winter School on Nuclear and Particle Physics, ``Matter Under
Extreme Conditions", Feb. 27 - March 5 1994, Schladming Austri
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