A Numerical Study of Counterrotating Disks in Spiral Galaxies

The serendipitous discovery in 1992 of a second spiral disk rotating in a sense opposite to the rest of the galaxy, followed by subsequent discoveries of other such counterrotating disks, poses a formidable challenge to theorists. Current galaxy formation theories do not permit such a disk to be formed with the rest of the galaxy, and it appears to have been built up from external matter after the galaxy was formed. The aim of this thesis was to determine how a counterrotating disk can form in a spiral galaxy without destroying the preexisting disk. The most promising theory invokes the adiabatic infall, on a retrograde orbit, of intergalactic gas that dissipates energy and precesses to the plane of the primary disk. Using numerical simulations that incorporate both stars and gas, I have tested two competing theories for the formation of counterrotating disks: adiabatic gas infall and a retrograde gas-rich dwarf merger. A tree code was used for the gravity solver. Sticky particle gas dynamics were used to study the general characteristics of the formation process, and the structure of the counterrotating disk formed was investigated with smoothed particle hydrodynamics. I show that under restricted conditions, dissipational infall of gas can indeed produce a counterrotating disk without excessive damage to the preexisting disk. A cold, thin primary can survive secular gas infall and the production of a massive counterrotating disk if the infalling gas is well-dispersed in phase-space. A minor merger with a gas-rich dwarf galaxy can also yield a counterrotating disk if the primary galaxy's disk is massive and compact to begin with. Although the origin of counterrotating disks is an intriguing question in its own right, it has profound implications for the evolution and interaction histories of spiral galaxie.