Galaxy Collision as an Origin of SO Galaxies

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In order to understand the origin of SO galaxies, we have comprehensively summarized observational properties of SO galaxies as well as elliptical and spiral galaxies. This comparative study reveals the following important observational properties of SO galaxies: (a) Morphological properties of SO galaxies shows a number of unusual features such as ripples, dust lanes, polar rings, and so on. Further, disks of SO galaxies are kinematically hot, being suggested by the absence of spiral arms. There are many of lines of evidence for large star formation events about a few Gyr ago. These properties are considered to be related to second events such as galaxy-galaxy collisions. (b) The luminosities and the masses of SO galaxies are not intermediate between those of elliptical and spiral galaxies. Therefore, it is unlikely that SO galaxies do not correspond to the transition class between elliptical and spiral galaxies.Taking account of the above observational results, we propose the following senario for the formation and evolution of SO galaxies. (1) Second events (mergers and galaxy-galaxy collisions) occurred 10^<10> years ago built most of elliptical and bulges of SO galaxies. These second events are considered to a triggering mechanism of nuclear starburst activity. (2) Second events occurred in about 10^9 years ago made various fine structures which are observed in nearby galaxies. These events selectively affected on structures of cool-gas poor stellar systems (i.e., ellipticals and SO galaxies). In some cases, the second events triggered bar instability in disk galaxies. These bar structures would be considered to be destroyed and be evolved to puff up the bulges. Namely, evolutionary processes in disk galaxies should go from late-type to early-type disk galaxies.