Dynamical Evolution and Activities in Gas-Rich Galaxies

通过数值模拟研究了富气体星系的动态演化和活动，包括星际气体和恒星形成过程。首先，研究了星系、大麦哲伦星云和小麦哲伦星云之间的动力学相互作用。其次，模拟了大型椭圆星系和小型盘状星系之间的合并。这个研究项目的后半部分集中在星际气体的引力如何影响孤立星系的动态演化。研究发现，演化早期存在的气体量对非轴对称结构特别是棒状结构的发生有很大影响。初始气体量本身是由圆盘形成的时间尺度决定的。这一结果导致了一幅统一的图景，即早期星系的条形是由星系间的相遇形成的，而晚期星系的条形则是由星系盘的引力不稳定性自发形成的。对原星系的坍缩和随后盘状星系的形成也进行了模拟。该研究提出了一种新的演化模型，即早期盘中含有的丰富星际气体由于引力不稳定而形成大质量团块，这些团块是演化的主要驱动力。该模型解释了年轻星系特有的形态和光度特征，并为类星体等活动的出现与宿主星系的动态演化之间的因果关系提供了新的图景。

Dynamical evolution and activities of gas-rich galazies have been investigated by numerical simulations which include interstellar gas and star formation process. First, the dynamical interactions between the Galaxy, Large and Small Magellanic Clouds have been examined as a case study. Second, mergers between large elliptical galaxies and small disk galaxies have been simulated. The second half of this research project has been focused on how the gravity of the interstellar gas affects the dynamical evolution of isolated galaxies. It was found that the amount of the gas existent in the early evolutionary phase strongly affects the occurrence of non-axisymmetric structures, especially bars. The initial gas amount itself is determined by the timescale of disk formation. This result leads to a unified picture that the bars of early-type galaxies have been created by galaxy-galaxy encounters, whereas the late-type galaxies have developed their bars spontaneously by the gravitational instability of the disk. Simulations have been performed also on the collapse of protogalaxies and subsequent formation of disk galaxies. This research resulted into a new evolution model, in which the copious interstellar gas contained in the early disk forms massive clumps due to gravitational instability, and these clumps act as the major driving force of the evolution. This model explains peculiar morphology and photometric characteristics of young galaxies, and provides a new picture on the causal relationship between the emergence of activities such as quasars and the dynamical evolution of host galaxies.