18,436 research outputs found
The role of terminators and occlusion cues in motion integration and segmentation: a neural network model
The perceptual interaction of terminators and occlusion cues with the functional processes of motion integration and segmentation is examined using a computational model. Inte-gration is necessary to overcome noise and the inherent ambiguity in locally measured motion direction (the aperture problem). Segmentation is required to detect the presence of motion discontinuities and to prevent spurious integration of motion signals between objects with different trajectories. Terminators are used for motion disambiguation, while occlusion cues are used to suppress motion noise at points where objects intersect. The model illustrates how competitive and cooperative interactions among cells carrying out these functions can account for a number of perceptual effects, including the chopsticks illusion and the occluded diamond illusion. Possible links to the neurophysiology of the middle temporal visual area (MT) are suggested
Cosmological Simulations with TreeSPH
We describe numerical methods for incorporating gas dynamics into
cosmological simulations and present illustrative applications to the cold dark
matter (CDM) scenario. Our evolution code, a version of TreeSPH (Hernquist \&
Katz 1989) generalized to handle comoving coordinates and periodic boundary
conditions, combines smoothed--particle hydrodynamics (SPH) with the
hierarchical tree method for computing gravitational forces. The Lagrangian
hydrodynamics approach and individual time steps for gas particles give the
algorithm a large dynamic range, which is essential for studies of galaxy
formation in a cosmological context. The code incorporates radiative cooling
for an optically thin, primordial composition gas in ionization equilibrium
with a user-specified ultraviolet background. We adopt a phenomenological
prescription for star formation that gradually turns cold, dense,
Jeans-unstable gas into collisionless stars, returning supernova feedback
energy to the surrounding medium. In CDM simulations, some of the baryons that
fall into dark matter potential wells dissipate their acquired thermal energy
and condense into clumps with roughly galactic masses. The resulting galaxy
population is insensitive to assumptions about star formation; we obtain
similar baryonic mass functions and galaxy correlation functions from
simulations with star formation and from simulations without star formation in
which we identify galaxies directly from the cold, dense gas.Comment: compressed postscript, 38 pages including 6 out of 7 embedded
figures. Submitted to ApJ Supplements. Version with all 7 figures available
from ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprint
Photoionization, Numerical Resolution, and Galaxy Formation
Using cosmological simulations that incorporate gas dynamics and
gravitational forces, we investigate the influence of photoionization by a UV
radiation background on the formation of galaxies. In our highest resolution
simulations, we find that photoionization has essentially no effect on the
baryonic mass function of galaxies at , down to our resolution limit of
5e9 M_\sun. We do, however, find a strong interplay between the mass
resolution of a simulation and the microphysics included in the computation of
heating and cooling rates. At low resolution, a photoionizing background can
appear to suppress the formation of even relatively massive galaxies. However,
when the same initial conditions are evolved with a factor of eight better mass
resolution, this effect disappears. Our results demonstrate the need for care
in interpreting the results of cosmological simulations that incorporate
hydrodynamics and radiation physics. For example, we conclude that a simulation
with limited resolution may yield more realistic results if it ignores some
relevant physical processes, such as photoionization. At higher resolution, the
simulated population of massive galaxies is insensitive to the treatment of
photoionization and star formation, but it does depend significantly on the
amplitude of the initial density fluctuations. By , an cold
dark matter model normalized to produce the observed masses of present-day
clusters has already formed galaxies with baryon masses exceeding 1e11
M_\sun.Comment: 25 pages, w/ embedded figures. Submitted to ApJ. Also available at
http://www-astronomy.mps.ohio-state.edu/~dhw/Docs/preprints.htm
Simulating Cosmic Structure Formation
We describe cosmological simulation techniques and their application to
studies of cosmic structure formation, with particular attention to recent
hydrodynamic simulations of structure in the high redshift universe.
Collisionless N-body simulations with Gaussian initial conditions produce a
pattern of sheets, filaments, tunnels, and voids that resembles the observed
large scale galaxy distribution. Simulations that incorporate gas dynamics and
dissipation form dense clumps of cold gas with sizes and masses similar to the
luminous parts of galaxies. Models based on inflation and cold dark matter
predict a healthy population of high redshift galaxies, including systems with
star formation rates of 20 M_{\sun}/year at z=6. At z~3, most of the baryons in
these models reside in the low density intergalactic medium, which produces
fluctuating Lyman-alpha absorption in the spectra of background quasars. The
physical description of this ``Lyman-alpha forest'' is particularly simple if
the absorption spectrum is viewed as a 1-dimensional map of a continuous medium
instead of a collection of lines. The combination of superb observational data
and robust numerical predictions makes the Lyman-alpha forest a promising tool
for testing cosmological models.Comment: Latex w/ paspconf.sty, 25 pages, 8 ps figs. To appear in Origins,
eds. J. M. Shull, C. E. Woodward, & H. Thronson (ASP Conference Series
Unconditional stability of semi-implicit discretizations of singular flows
A popular and efficient discretization of evolutions involving the singular
-Laplace operator is based on a factorization of the differential operator
into a linear part which is treated implicitly and a regularized singular
factor which is treated explicitly. It is shown that an unconditional energy
stability property for this semi-implicit time stepping strategy holds. Related
error estimates depend critically on a required regularization parameter.
Numerical experiments reveal reduced experimental convergence rates for smaller
regularization parameters and thereby confirm that this dependence cannot be
avoided in general.Comment: 21 pages, 8 figure
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