FORMATION OF MASSIVE COUNTERROTATING DISKS IN SPIRAL GALAXIES

We present results of numerical simulations of the formation of a mass ive counterrotating gas disk in a spiral galaxy. Using a hierarchical tree gravity solver combined with a sticky particle gas-dissipation sc heme for our simulations, we have investigated three mechanisms: episo dic gas infall, continuous gas infall, and a merger with a gas-rich dw arf galaxy. We find that both episodic and continuous gas infall work reasonably well and are able to produce a substantial counterrotating gas disk without upsetting the stability of the existing disk drastica lly, but it is very important for the gas to be well dispersed in phas e space and not form concentrated clumps prior to its absorption by th e disk galaxy. The initial angular momentum of the gas also plays a cr ucial role in determining the scale length of the counterrotating disk that is formed and the time if 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 pr oduce an extensive disk with a scale length similar to that of the ori ginal, prograde disk. A gas-rich dwarf merger does not appear to be a viable mechanism to produce a massive counterrotating disk because onl y a very small dwarf galaxy can produce a counterrotating disk without increasing the thickness of the existing disk by an order of magnitud e, and the timescale for this process is prohibitively long because it makes it very unlikely that several such mergers can accumulate a mas sive counterrotating disk over a Hubble time.