A MERGER ORIGIN FOR X-STRUCTURES IN SO GALAXIES

Using numerical simulation, we study the response of a disk galaxy to a merger involving a low-mass satellite companion. During a prograde s atellite accretion, the disk galaxy forms a strong bar in response to the perturbation of the companion. After the accretion event is over, the bar buckles vertically due to a bending instability, sending disk material well out of the disk plane. The material forms into an X-shap ed feature when seen edge-on, morphologically similar to X structures observed in several SO/Sa galaxies. The mechanism described here unite s previously suggested accretion and bar instability hypotheses for fo rming X structures into a self-consistent scenario for merger-induced formation and evolution of SO galaxies. To complement these models, we also present observations of the peculiar SO galaxy Hickson 87a. The thick disk, isophotal warping, and strong X structure described in the merger model are all evident in Hickson 87a, suggesting this galaxy m ay be an excellent example of such merger-induced galaxy evolution.