Ar. Thakar et Bs. Ryden, Smoothed particle hydrodynamics simulations of counterrotating disk formation in spiral galaxies, ASTROPHYS J, 506(1), 1998, pp. 93-115
We present the results of smoothed particle hydrodynamics (SPH) simulations
of the formation of a massive counterrotating disk in a spiral galaxy. The
current study revisits and extends (with SPH) previous work carried out wi
th sticky particle gas dynamics, in which adiabatic gas infall and a retrog
rade gas-rich dwarf merger were tested as the two most likely processes for
producing such a counterrotating disk. We report on experiments with a col
d primary similar to our Galaxy, as well as a hot, compact primary modeled
after NGC 4138. We have also conducted numerical experiments with varying a
mounts of prograde gas in the primary disk and an alternative infall model
(a spherical shell with retrograde angular momentum). The structure of the
resulting counterrotating disks is dramatically different with SPH. The dis
ks we produce are considerably thinner than the primary disks and those pro
duced with sticky particles. The timescales for counterrotating disk format
ion are shorter with SPH, because the gas loses kinetic energy and angular
momentum more rapidly. Spiral structure is evident in most of the disks, bu
t an exponential radial profile is not a natural by-product of these proces
ses. The infalling gas shells that we tested produce counterrotating bulges
and rings rather than disks. The presence of a considerable amount of pree
xisting prograde gas in the primary causes, at least in the absence of star
formation, a rapid inflow of gas to the center and a subsequent hole in th
e counterrotating disk. For a normal counterrotating disk to form, there mu
st be either little or no preexisting prograde gas in the primary, or its d
issipative influence must be offset by significant star formation activity.
The latter scenario, along with the associated feedback to the interstella
r medium, may be necessary to produce a counterrotating disk similar in sca
le length and scale height to the primary disk. In general, our SPH experim
ents yield stronger evidence to suggest that the accretion of massive count
errotating disks drives the evolution of the host galaxies toward earlier (
S0/Sa) Hubble types.