Disc galaxy with spiral arms is a common galaxy type in the local Universe. However, spiral galaxies are rarely observed in the high-redshift Universe and have massive giant clumps in their disc regions instead of spiral arms. Such clumpy galaxies become rare as redshift decreases. Thus, abundances of spiral and clumpy galaxies seem to indicate anti-correlation from redshift z=0 to ~2. In this talk, I will discuss the evolution of disc galaxies from the viewpoint of "spiral arm instability".
First, I will introduce our linear perturbation theory of spiral arms and analytically derive ynamical instability conditions of arms. Then, I adopt our analysis to isolated galaxy simulations and show its ability to characterize the fragmentation of spiral arms. Next, I apply the analysis to observations and cosmological simulations of proto-planetary discs. Finally, I would like to talk about the volution process of disc galaxies driven by spiral-arm instability, from clumpy to spiral galaxies, and discuss the difference from and connection to Toomre instability.
