Dynamical effects of minor and major galaxy merging on halo ISM in disk galaxies I. Tidal truncation of gas replenishment

We numerically investigate how minor and major galaxy merging between two late-type disks affects mass distribution of interstellar medium (ISM) surrounding the haloes of disk galaxies. The purpose of this study is to explore dynamical effects of galaxy merging on gas replenishment from galactic haloes, which is generally considered to play vital roles in various aspects of disk galaxy evolution (in particular, star formation histories of disk galaxies). Main results obtained in this study are the following three. (1) Galaxy merging greatly changes initial mass distribution of halo ISM of disk galaxies in such a way that final mean ISM density of the halo in a galaxy merger is considerably smaller compared with initial one. The essential reason for this is that a large amount of halo ISM is tidally stripped away from galaxies owing to strong tidal interaction between two disks during galaxy merging. This result implies that star formation history of a disk galaxy, which is generally considered to depend on infall rate of the halo ISM and thus on time evolution of mass distribution of the halo ISM, can be largely determined by the past merger events of the disk. (2) The mass-ratio of two precursor disks in a merger (m2) can determine total amount of halo ISM stripped away from the galaxy merger and thus can control final mass distribution of halo ISM in the merger remnant. For example, in merger models with the cut off radius of halo ISM four times larger than initial disk radius, the fractional mass of halo ISM that is tidally stripped during galaxy merging is 0.52 for a merger with m2 = 0.3 and 0.73 for that with m2 = 1.0. Final mean halo ISM density is accordingly smaller for a galaxy merger with larger m2. Considering that the mass-ratio (m2) can also determine structure and kinematics of the merger remnant, this result implies that star formation histories of galaxies, which can be determined by mass distribution of halo ISM, can correlate with galactic structure and kinematics. (3) Total amount of halo ISM tidally stripped away from galaxies during galaxy merging does not depend so strongly on initial mass distribution and kinematics of halo ISM in merger precursor disks. Final mass distribution of halo ISM in a merger remnant accordingly does not depend so strongly on the initial Send offprint requests to: K. Bekki mass distribution and kinematics of halo ISM of the merger precursor disks. These three results suggest that star formation history of a disk galaxy can be determined largely by the nature of the past merger events of the galaxy, principally because galaxy merging can greatly affect mass distribution of halo ISM and thus gas replenishment from the halo in the disk. The present numerical results furthermore stress that for deeper understanding of disk galaxy evolution, not only dynamical evolution of disk ISM but also that of halo ISM should be more extensively investigated.