NGC-4138 - A CASE-STUDY IN COUNTERROTATING DISK FORMATION

The Sa(r) galaxy NGC 4138 has been recently found to contain an extens ive counterrotating disk that appears to be still forming. Up to one-t hird of the stars in the disk system may be on retrograde orbits. A co unterrotating ring of H II regions, along with extended counterrotatin g H I gas, suggests that the retrograde material has been recently acq uired in the gas phase and is still trickling in. Using numerical simu lations, we have attempted to model the process by which the counterro tating mass has been accreted by this galaxy. We investigate two possi bilities: continuous retrograde infall of gas and a retrograde merger with a gas-rich dwarf galaxy. Both processes are successful in produci ng a counterrotating disk of the observed mass and dimensions without heating up the primary disk significantly. Contrary to our experience with a fiducial cold, thin primary disk, the gas-rich merger works wel l for the massive, compact primary disk of NGC 4138 even though the ma ss of the dwarf galaxy is a significant fraction of the mass of the pr imary disk. Although we have restricted ourselves mainly to coplanar i nfall and mergers, we report on one inclined infall simulation as well . We also explore the possibility that the Ha ring seen in the inner h alf of the disk is a consequence of counterrotating gas clouds collidi ng with corotating gas already present in the disk and forming stars i n the process.