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VI/76 Simulation Atlas of Tidal Features in Galaxies (Howard+ 1993)
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Beginning of ReadMe :VI/76 Simulation Atlas of Tidal Features in Galaxies (Howard+ 1993)
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A Simulation Atlas of Tidal Features in Galaxies
Howard S., Keel W.C., Byrd G.G., Burkey J.
<Astrophys. J. 417, 502 (1993)>
=1993ApJ...417..502H
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ADC_Keywords: Morphology; Galaxies, peculiar; Models
Abstract:
The simulation survey of tidally perturbed galaxies provides images of
84 different encounters that vary orbit tilt of companion orbit,
perigalacticon distance ratio of primary galaxy mass to companion
mass, and the amount of inert matter in the primary galaxy. In total
1764 galaxy images are available in the survey.
This data set contains 84 separate simulations, each simulation is run
for 1000 time steps, producing image output of the star particles and
gas particles separately every 50 time steps. The first 21 images
represent the "gas", and the second 21 images represent the "stars".
Time steps for matching "gas" and "stars" are the same.
There are [256,256] grid points on a Cartesian coordinate system. The
renormalized densities are stored as 8-bit floating point numbers. The
data have been put into FITS. Each header specifies the parameters for
that simulation.
Introduction:
The simulation atlas provides an image dictionary of simulations that
span several encounter parameters with fine morphological resolution
and includes the effects of self-gravitation. Observers and theorists
can then search the dictionary for the parameters that best match a
particular observed morphology.
The survey uses a self-gravitating, 180,000 particle, two-component
("stars" and "gas") disk with an inert halo. The companion follows a
fully 3D self-gravitating orbit. The disk is constrained to two
dimensions. The "star" data and the "gas" data are stored as separate
images. The n-body code has a polar-coordinate grid especially well
suited to simulate disk galaxies. The grid is exponentially spaced in
the radial direction and provides increasing resolution from edge to
center. Each bin has a constant angular size as seen from the origin.
There are 24 radial and 36 azimutal bins which give an effective
resolution that matches a 720x720 Cartesian grid. The code combines
the speed of a particle mesh code with tree aspects for fine tuned
handling of the companion orbit. The code is stable against intrinsic
small perturbations.
The initial potential for the disk is a finite Mestel (1963) disk
which produces a flat rotation curve. The halo is represented by an
inert potential set to match the value of the Mestel potential in the
disk. The contribution of this potential is either equal to the
potential of one disk mass or ten times the potential of one disk
mass.
Of the 180,00 particles, 126,000 of the are "stars", each having a
velocity dispersion in the disk plane that ensures stability against
axisymmetric perturbations. The remaining 54,000 "gas" particles have
an initial velocity dispersion of 0.0. The particles are not allowed
to collide.
The initial velocity of the companion is set to match a zero energy
orbit. The initial position is far enough away to assure a clean
passage. The companion is treated as a point mass.
Each simulation runs for 1000 time steps. With 50 steps per crossing
time (one crossing time is the time a particle takes to travel a
distance of one disk radius traveling at the initial circular orbital
speed), there are 314 time steps for one galactic rotation. Assuming a
typical disk radius of 20kpc, and a disk orbital speed of 200 km/s,
one time step corresponds to about 1.5 million years.