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abstract
Extensive counterrotating disks seen in spirals/S0s in recent years pose a challenge to theorists. Adiabatic, dissipational infall of ambient gas in a retrograde orbit around the disk is thought to be the most likely mechanism for the formation of such disks. We present results of sticky particle simulations of this process. We find that gas which is constrained to move in a retrograde orbit around the disk and loses kinetic energy in cloud-cloud collisions settles into the plane of the disk and forms a counterrotating gas disk within a fraction of a Hubble time. The mass of the infalling gas is critical to the stability of the disk, and the gas must fall in a little at a time to avoid disruption of the disk. Dissipationless infall is unable to produce counterrotating disks because the material does not settle into the disk plane within a Hubble time. The initial angular momentum of the infalling gas determines the time-scale of the counterrotating disk formation for the most part.
