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Session 109 - Galaxies: Collisions, Counter-Rotating Disks.
Display session, Thursday, January 18
North Banquet Hall, Convention Center
The Sa(r) galaxy NGC 4138 appears to harbor an extensive counterrotating disk which is still in the process of forming. There is a counterrotating stellar component in its disk as well as an extended counterrotating gas disk (see Jore et al., this meeting). We have attempted to model the process by which the counterrotating disk is forming in this galaxy, using sticky particle simulations. We investigate two possibilities: a merger with a gas-rich dwarf, and a continuous infall of gas in a retrograde orbit around the primary disk. A merger origin is only viable if the dwarf is extremely (\agt90%) gas-rich and contains little or no dark matter. For continuous infall, we assume that the HI gas is acquired gradually and it dissipates energy in cloud-cloud collisions, causing it to sink to the plane of the disk within a few dynamical times. Dynamical friction on gas clumps causes them to eventually spiral in towards the center of the galaxy, forming stars as they pass through spiral arms and hence giving rise to the counterrotating stellar disk. We explore the possibility that the H-alpha ring seen in the inner half of the disk is a consequence of counterrotating gas clouds colliding with corotating gas already present in the disk and forming stars in the process. The lack of star formation in our simulations precludes us from making a definitive statement about this, but our results show the cloud-cloud collision hypothesis to be viable, provided that the near-solar metallicity of the gas in the ring can be explained. Although the kinematic data allow an interpretation which does not require a dark halo, the formation of a counterrotating disk is facilitated when substantial dark matter is present.
