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Session 77 - Structures of Galaxies.
Display session, Friday, January 09
We present the results of Smoothed Particle Hydrodynamics (SPH) simulations of the formation of a massive counterrotating disk in a spiral galaxy. In previous studies, using sticky particle gas dynamics instead of SPH, we tested adiabatic gas infall and a retrograde gas-rich dwarf merger as the two most likely processes for producing such a counterrotating disk. We report here on SPH simulations involving a cold primary similar to our Galaxy, as well as a hot, compact primary modeled after NGC 4138. We have also conducted numerical experiments with varying amounts 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 disks we produce are considerably thinner than the primary disks and those produced with sticky particles. The time-scales for counterrotating disk formation are shorter with SPH because the gas loses kinetic energy and angular momentum more rapidly. Spiral structure is evident in most of the disks, but an exponential radial profile is not a natural byproduct of these processes. The presence of a considerable amount of preexisting 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 the counterrotating disk. These results may point to the importance of other processes such as star formation and energy feedback in producing a counterrotating disk similar in scale length and scale height to the primary disk.
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Program listing for Friday