Sinking Satellite Disk Galaxies. I. Shell Formation Preceded by Cessation of Star Formation

Abstract

Detailed numerical simulations have been carried out on the sinking of a gas-rich disk galaxy into a large elliptial (spherical) galaxy. Both elliptical and spiral galaxies have been modeled as self-gravitating particle systems. The interstellar gas component in the disk has been modeled as a system of inelastic cloud particles dissipating kinetic energy in mutual collisions. Star formation processes and gas consumption owing to star formation are included in the numerical code. Sinking on a radial or slightly retrograde orbit has been found to produce regular shell structures (i.e., without loops or filaments) made from the disk material. In this shell formation, no significant segregation of the stellar and gaseous components occurs. Global distribution of the gas clouds is similar to that of the stars, though the latter make clearer shells. Star formation is turned off well before the shells develop, because gas clouds are widely scattered and the gas density is much decreased. We propose that the poststarburst nuclei often observed in shell galaxies are not necessarily the result of preceding starbursts, as is widely believed, but could be the result of this drastic truncation of star formation activity.