NGC 4138: A Case Study in Counterrotating Disk Formation

The Sa(r) galaxy NGC 4138 has been recently found to contain an extensivecounterrotating disk which appears to be still forming. Up to a third of thestars in the disk system may be on retrograde orbits. A counterrotating ring ofH II regions, along with extended counterrotating H I gas, suggests that theretrograde material has been recently acquired in the gas phase and is stilltrickling in. Using numerical simulations, we have attempted to model theprocess by which the counterrotating mass has been accreted by this galaxy. Weinvestigate two possibilities: continuous retrograde infall of gas, and aretrograde merger with a gas-rich dwarf galaxy. Both processes are successfulin producing a counterrotating disk of the observed mass and dimensions withoutheating up the primary significantly. Contrary to our experience with afiducial cold, thin primary disk, the gas-rich merger works well for themassive, compact primary disk of NGC 4138 even though the mass of the dwarfgalaxy is a significant fraction of the mass of the primary disk. Although wehave restricted ourselves mainly to coplanar infall and mergers, we report onone inclined infall simulation as well. We also explore the possibility thatthe H-alpha ring seen in the inner half of the disk is a consequence ofcounterrotating gas clouds colliding with corotating gas already present in thedisk and forming stars in the process.