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 galaxy during a gas-dynamical interaction with the halo and disk of a spiral galaxy. By ram pressure, the gas clouds are stripped from the companion, and accreted toward the disk of the spiral galaxy. If the companion’s orbit is retrograde with respect to the rotation of the spiral galaxy, infalling clouds hit the nuclear region. Angular momentum transfer causes disruption of the inner gaseous disk, and makes a void of interstellar gas in the bulge. If the companion’s orbit is either prograde or polar, infalling clouds are accreted by the outer disk, and form a rotating gas ring. We show that the ram-pressure stripping-and-accretion is one way from the companion to a gas-rich larger galaxy, which causes disposal of interstellar gas from the companion and effectively changes its galaxy type into earlier (redder). The ram-pressure process is significant durig merger of galaxies, in which interstellar gas is stripped and accreted prior to the stellar-body merger. Based on the simulation, we discuss a possible history of interstellar gas in M31 system, which comprises M32, NGC 205 and possible merged galaxies. The ram-pressure stripping explains the disposal of mass lost from evolving stars in the dwarf elliptical companion M32. The peculiar “face-on” spirals of ionized gas and dark clouds observed in M31’s bulge can be reproduced by a spiral inflow of accreting gas from the companions and/or from the merger galaxy. Subject headings: accretion – dwarf ellipticals – galaxies: individual (M31, M32, NGC 205) – interstellar matter – merger – ram-pressure.