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Abstract
We present results of numerical simulations of the formation of a massive counterrotating
gas disk in a spiral galaxy. Using a hierarchical tree gravity solver combined with
a sticky-particle gas dissipation scheme for our simulations, we have investigated
three mechanisms: episodic and continuous gas infall, and a merger with a gas-rich
dwarf galaxy. We find that both episodic and continuous gas infall work reasonably
well and are able to produce a substantial gas counterrotating disk without upsetting
the stability of the existing disk drastically, but it is very important for the gas
to be well-dispersed in phase-space and not form concentrated clumps prior to its
absorption by the disk galaxy. The initial angular momentum of the gas also plays
a crucial role in determining the scale length of the counterrotating disk formed
and the time it takes to form. The rate of infall, i.e. the mass of gas falling in
per unit time, has to be small enough to preclude excessive heating of the preexisting
disk. It is much easier in general to produce a smaller counterrotating disk than
it is to produce an extensive disk whose scale length is similar to that of the original
prograde disk.
A gas-rich dwarf merger does not appear to be a viable mechanism to produce a massive
counterrotating disk, because only a very small dwarf galaxy can produce a counterrotating
disk without increasing the thickness of the existing disk by an order of magnitude,
and the time-scale for this process is prohibitively long because it makes it very
unlikely that several such mergers can accumulate a massive counterrotating disk over
a Hubble time.
Comments Accepted by ApJ, 22 pages, uuencoded compressed Postscript. 18
Figures (compressed Postscript) available from anonymous ftp at
ftp://bessel.mps.ohio-state.edu/pub/thakar/cr1/figs.ps.Z A complete
(text+figs) compressed PostScript preprint is also available at
ftp://bessel.mps.ohio-state.edu/pub/thakar/cr1/pp.ps.gz