Simulations of Collisions between Two Gas‐rich Galaxy Disks with Heating and Cooling

Particle hydrodynamics (SPH) simulations are presented of direct collisions between two model galaxies, most consisting of a rigid halo and a gas disk. Local self-gravity is also computed in the gas. The companion galaxy in these simulations is about one-third of the mass of the primary, and its disk is half the size. An adiabatic equation of state is combined with simple approximations for the effects of radiative cooling and local heating due to young star activity, which allows a continuous range of thermal phases to develop. These terms and multiple phases have not generally been included in galaxy collision simulations to date. Their effects are assessed in part by repeating runs with an isothermal equation of state and comparing the results. One model with a star plus gas disk is also included for comparison.