Smoothed particle hydrodynamics simulations of counterrotating disk formation in spiral galaxies

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.