significantly increase, and the disk remains dynamically cold. Owing to this low degree of disk heating, the short-lived spiral arms can recurrently develop over many rotational periods. The resultant structure of the spiral arms in the $N$-body simulations is consistent with some observational nature of spiral galaxies. We conclude that the formation and structure of spiral arms in isolated disk galaxies can be reasonably understood by non-linear interactions between a spiral arm and its constituent stars.'
author:
- 'Junichi <span style="font-variant:small-caps;">Baba</span>, Takayuki R. <span style="font-variant:small-caps;">Saitoh</span>, and Keiichi <span style="font-variant:small-caps;">Wada</span>'
bibliography:
- 'ms.bib'
title: 'Dynamics of Non-Steady Spiral Arms in Disk Galaxies'
---

Introduction
============

The spiral arms of disk galaxies are the most prominent structures, and the arms are formed due to gravitationally driven variations in the surface density in the stellar disk [@RixZaritsky1995; @Grosbol+2004; @Zibetti+2009; @Elmegreen+2011]. It is known that the spiral arms of disk galaxies can be excited by tidal interactions with nearby companion galaxies [e.g., @Oh+2008; @Dobbs+2010; @Struck+2011], as well as by the central stellar bar [e.g., @SellwoodSparke1988]. In addition, spiral arms can also be self-induced and maintained without these external gravitational perturbations, and further, they can propagate stationary density waves in globally stable disks, as hypothesized 