RAM-PRESSURE STRIPPING OF GAS FROM COMPANIONS AND ACCRETION ONTO A SPIRAL GALAXY - A GASEOUS MERGER

We simulated the behavior of interstellar gas clouds in a companion ga laxy during a gasdynamical interaction with the halo and disk of a spi ral galaxy. Due to the ram pressure, the gas clouds are stripped from the companion and accreted toward the disk of the spiral galaxy. If th e companion's orbit is retrograde with respect to the rotation of the spiral galaxy, infalling clouds hit the nuclear region. Angular moment um transfer causes disruption of the inner gaseous disk and makes a vo id of interstellar gas in the bulge. If the companion's orbit is eithe r prograde or polar, infalling clouds are accreted by the outer disk a nd form a rotating gas ring. We show that the ram-pressure stripping a nd accretion is one way from the companion to a gas-rich larger galaxy , which causes disposal of interstellar gas from the companion and eff ectively changes its galaxy type into an earlier (redder) type. The ra m-pressure process is significant during the merger of galaxies, in wh ich interstellar gas is stripped and accreted prior to the stellar bod y merger. Based on the simulation, we discuss a possible history of in terstellar gas in the M31 system, which comprises M32, NGC 205, and po ssible merged galaxies. The ram-pressure stripping explains the dispos al of mass lost from evolving stars in the dwarf elliptical companion M32. The peculiar ''face-on'' spirals of ionized gas and dark clouds o bserved in M31's bulge can be reproduced by a spiral inflow of accreti ng gas from the companions and/or from the merger galaxy.