Patterns in the outer parts of galactic discs

This paper describes test‐particle simulations of the response of the outer parts of Galactic discs to barring and spiral structure. Simulations are conducted for cold Mestel discs and warm quasi‐exponential discs with completely flat rotation curves, subjected to pure quadrupoles and logarithmic spirals. Even though the starting velocity distributions are smooth, the end‐points of the bar simulations show bimodality and multipeaked structures at locations near the outer Lindblad resonance (OLR), although spirality can make this smoother. The growth of a bar may cause the disc isophotes to become boxy at the OLR, as stars accumulate particularly along the minor axis. The growth of a bar is also accompanied by substantial heating of the disc stars near the OLR. For the growth of a 10 10 ‐M ⊙ bar, the radial velocity dispersion is typically quadrupled for initially cold discs (initial σ u ∼ 10  km  s −1 ), and typically doubled for discs with final σ u ∼ 45  km  s −1 . Simulations performed of the growth and dissolution of bars give very similar results, demonstrating that the heat once given to disc stars is very difficult to remove.