The origin and evolution of cold gaseous structures in galaxies and galactic outflows

Abstract

This thesis examines the formation and evolution of cold gaseous structures in galaxies and galactic outflows in two distinct scenarios. Previous analytic estimates of the viscous time-scale due to cloud-cloud collisions in Milky Way-like discs have produced values on the order of t[subscript v] ~ 1000 Gyr, and hence it has been concluded that cloud-cloud collisions are not important to the dynamical evolution of these galaxies. However, these estimates had not been tested with full three dimensional hydrodynamic simulations, which we perform using the smoothed particle hydrodynamics code HYDRA-OMP, making improvements to its parallelism to do so. These simulations produce a viscous time-scale of t[subscript v] ~ 10 Gyr, suggesting that while the effective viscosity is weak, it is not entirely insignificant. The discrepancy between the analytic and the numerical results is traced to an error in the analytic calculation. Observations have revealed cold gas with large velocity dispersions (FWHM ~ 300 km/s) within the hot outflows of Ultra-luminous Infrared Galaxies (ULIRGs). This gas may trace its origin to the Rayleigh-Taylor (RT) fragmentation of a super-bubble wall. We model this scenario at two scales to attempt to recreate this effect in three-dimensional hydrodynamic simulations using FLASH. Although the models are not well-converged with respect to resolution, we are able to produce cold gas in outflows with large velocity dispersions (FWHM ~ 200-300 km/s). Our small-scale models indeed produce this cold gas through RT fragmentation of the super-bubble wall, but our large-scale models produce this cold gas by hot bubbles fragmenting the disc's gas into cold clumps which are then accelerated by thermal pressure, or by cooling within the outflow. We also make use of a sub-grid turbulence model. After several significant errors in a code supplied by a collaborator were corrected, this model produces simulations that are better converged, at the cost of smoothing away the cold gas.