Galaxy Collision as an Origin of SO Galaxies

为了更好地理解SO星系的起源，我们对SO星系以及椭圆星系和旋涡星系的观测特性进行了全面的总结。这些比较研究揭示了SO星系的以下重要观测性质：（a）SO星系的形态特征显示出一些不寻常的特征，如波纹、尘埃带、极环等，此外，SO星系的盘在运动学上是热的，因为没有旋臂。关于几个Gyr前的大星星形成事件有许多证据。这些性质被认为与第二次事件有关，如星系-星系碰撞。(b)SO星系的光度和质量并不介于椭圆星系和旋涡星系之间。因此，SO星系不可能不对应于椭圆星系和旋涡星系之间的过渡类。考虑到上述观测结果，我们对SO星系的形成和演化提出了以下几点看法。(1)10年前发生的第二次事件（合并和星系-星系碰撞）<10>造就了SO星系的大部分椭圆和凸起。这些第二次事件被认为是核星暴活动的触发机制。(2)第二次事件发生在大约10^9年前，形成了在附近星系中观察到的各种精细结构。这些事件选择性地影响缺乏冷气体的恒星系统的结构（即，椭圆星系和SO星系）。在某些情况下，第二次事件触发了盘星系中的棒不稳定性。这些条坝结构被认为是被破坏的，并被演化为膨胀的凸起。也就是说，盘星系的演化过程应该是从晚型盘星系向早型盘星系演化。

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.