1,854 research outputs found
SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation
The intrinsic anisotropy delta and flattening epsilon of simulated merger
remnants is compared with elliptical galaxies that have been observed by the
SAURON collaboration, and that were analysed using axisymmetric Schwarzschild
models. Collisionless binary mergers of stellar disks and disk mergers with an
additional isothermal gas component, neglecting star formation cannot reproduce
the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit
of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is
however found for merger simulations of disks with gas fractions >= 20%,
including star formation and stellar energy feedback. Massive black hole
feedback does not strongly affect this result. Subsequent dry merging of merger
remnants however does not generate the slowly-rotating SAURON ellipticals which
are characterized by low ellipticities epsilon < 0.25 and low anisotropies.
This indicates that at least some ellipticals on the red galaxy sequence did
not form by binary mergers of disks or early-type galaxies. We show that
stellar spheroids resulting from multiple, hierarchical mergers of
star-bursting subunits in a cosmological context are in excellent agreement
with the low ellipticities and anisotropies of the slowly rotating SAURON
ellipticals and their observed trend of delta with epsilon. The numerical
simulations indicate that the SAURON relation might be a result of strong
violent relaxation and phase mixing of multiple, kinematically cold stellar
subunits with the angular momentum of the system determining its location on
the relation.Comment: 13 pages, 3 figures, submitted to Ap
New results on SIDIS SSA from Jefferson Lab
We present studies of single-spin and double-spin asymmetries in
semi-inclusive electroproduction of pions using the CEBAF 6 GeV polarized
electron beam. Kinematic dependences of single and double spin asymmetries have
been measured in a wide kinematic range at CLAS with a polarized NH target.
Significant target-spin and asymmetries have been
observed. The hypothesis of factorization has been tested with -dependence
of the double spin asymmetry.Comment: 4 pages, 6 figure
QCD radiative and power corrections and Generalized GDH sum rules
We extend the earlier suggested QCD-motivated model for the -dependence
of the generalized Gerasimov-Drell-Hearn (GDH) sum rule which assumes the
smooth dependence of the structure function , while the sharp dependence
is due to the contribution and is described by the elastic part of the
Burkhardt-Cottingham sum rule. The model successfully predicts the low crossing
point for the proton GDH integral, but is at variance with the recent very
accurate JLAB data. We show that, at this level of accuracy, one should include
the previously neglected radiative and power QCD corrections, as boundary
values for the model. We stress that the GDH integral, when measured with such
a high accuracy achieved by the recent JLAB data, is very sensitive to QCD
power corrections. We estimate the value of these power corrections from the
JLAB data at . The inclusion of all QCD corrections leads
to a good description of proton, neutron and deuteron data at all .Comment: 10 pages, 4 figures (to be published in Physical Review D
Global Nonradial Instabilities of Dynamically Collapsing Gas Spheres
Self-similar solutions provide good descriptions for the gravitational
collapse of spherical clouds or stars when the gas obeys a polytropic equation
of state, (with ). We study the behaviors of
nonradial perturbations in the similarity solutions of Larson, Penston and
Yahil, which describe the evolution of the collapsing cloud prior to core
formation. Our global stability analysis reveals the existence of unstable
bar-modes () when . In particular, for the collapse of
isothermal spheres, which applies to the early stages of star formation, the
density perturbation relative to the background, , increases as ,
where denotes the epoch of core formation, and is the cloud
central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal
shape (prolate bar or oblate disk, depending on initial conditions) as the
collapse proceeds. In the context of Type II supernovae, core collapse is
described by the equation of state, and our analysis
indicates that there is no growing mode (with density perturbation) in the
collapsing core before the proto-neutron star forms, although nonradial
perturbations can grow during the subsequent accretion of the outer core and
envelope onto the neutron star. We also carry out a global stability analysis
for the self-similar expansion-wave solution found by Shu, which describes the
post-collapse accretion (``inside-out'' collapse) of isothermal gas onto a
protostar. We show that this solution is unstable to perturbations of all
's, although the growth rates are unknown.Comment: 28 pages including 7 ps figures; Minor changes in the discussion; To
be published in ApJ (V.540, Sept.10, 2000 issue
The Structure of Dark Matter Haloes in Dwarf Galaxies
Recent observations indicate that dark matter haloes have flat central
density profiles. Cosmological simulations with non-baryonic dark matter
predict however self similar haloes with central density cusps. This
contradiction has lead to the conclusion that dark matter must be baryonic.
Here it is shown that the dark matter haloes of dwarf spiral galaxies represent
a one parameter family with self similar density profiles. The observed global
halo parameters are coupled with each other through simple scaling relations
which can be explained by the standard cold dark matter model if one assumes
that all the haloes formed from density fluctuations with the same primordial
amplitude. We find that the finite central halo densities correlate with the
other global parameters. This result rules out scenarios where the flat halo
cores formed subsequently through violent dynamical processes in the baryonic
component. These cores instead provide important information on the origin and
nature of dark matter in dwarf galaxies.Comment: uuencoded Z-compressed postscript file, 10 pages, 3 figures included,
to appear in ApJ Letter
A Fast Algorithm for Solving the Poisson Equation on a Nested Grid
We present a numerical method for solving the Poisson equation on a nested
grid. The nested grid consists of uniform grids having different grid spacing
and is designed to cover the space closer to the center with a finer grid. Thus
our numerical method is suitable for computing the gravity of a centrally
condensed object. It consists of two parts: the difference scheme for the
Poisson equation on the nested grid and the multi-grid iteration algorithm. It
has three advantages: accuracy, fast convergence, and scalability. First it
computes the gravitational potential of a close binary accurately up to the
quadraple moment, even when the binary is resolved only in the fine grids.
Second residual decreases by a factor of 300 or more by each iteration. We
confirmed experimentally that the iteration converges always to the exact
solution of the difference equation. Third the computation load of the
iteration is proportional to the total number of the cells in the nested grid.
Thus our method gives a good solution at the minimum expense when the nested
grid is large. The difference scheme is applicable also to the adaptive mesh
refinement in which cells of different sizes are used to cover a domain of
computation.Comment: 22 pages 21 figures. To appear in Ap
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