Galactic spiral arms formed by central explosions

Abstract

We have made calculations of spiral arm formation due to central explosions in a nucleus surrounded by a disc containing most of the galactic mass with the purpose of obtaining estimates on lifetimes of arms and the requirements on the energy involved in the process. The ejected gas is taken to be a few percent, or less, of the central nucleus and is ejected with velocities of the order of 1000 km s−1. The gas, considered to be in forms of blods, moves under the gravitational force from the disc and the nucleus and the drag force by the gas in the disc. The orbits of the blobs evolve towards the circular orbits of the disc due to this drag force and the velocities in the arms will therefore, after some time, approach those of a normal rotation curve. A relatively open structure will last ≲5×108 years. Stable ring structures with longer lifetimes may be formed by some explosions. With an energy of ∼5×1057 erg in the initial gas-blod motion and a duration of the explosion of ∼107 years, the energy output in such explosions has to be >1043 erg s−1.