Hydrodynamical simulations of interstellar gas : from star-forming clouds to spiral arms

This thesis presents numerical investigations of interstellar gas under various conditions. Discrete, pressure-bounded gas ‘cloudlets’ are modelled with Smoothed Particle Hydrodynamics (SPH) which is shown to reproduce accurately their properties and the outcomes of collisions between them. A simple model of a molecular cloud is investigated, consisting of a spherical ensemble of such cloudlets. This model dissipates its kinetic energy on a timescale controlled by the collision timescale. Distributed star formation does not result, and hierarchical mergers of cloudlets do not play a significant part, due to the high Mach number of collisions. A model gravitational potential for a spiral galaxy is introduced, and the interaction of large groups of cloudlets with this potential is investigated. Density and velocity structure are induced by the passage through a spiral arm. Comparisons to observed molecular clouds fail to reproduce the correct size-linewidth relation. The response of a uniform gaseous galactic disc to the spiral potential is investigated using two-dimensional SPH, a two-dimensional Eulerian code and a semi-analytical approach. Good agreement in the predicted location of spiral shocks is seen between all methods. Instability of spiral arms in open spirals is found, and possible formation mechanism of spur-like structure are discussed. The location of spiral shocks relative to the peaks of the stellar surface density is proposed as a new way to constrain the corotation radius in spiral galaxies. The potential form an N-body simulation of a galaxy is introduced and the response of the gaseous disc shows complex and rapidly changing structure. Observational implications for spatial galaxies are suggested.EThOS - Electronic Theses Online ServiceGBUnited Kingdo