The generation of a multi-phase medium in "Splash" bridge systems: Towards an understanding of star formation suppression in turbulent galaxy systems

Abstract

Cloud-cloud collisions in splash bridges produced in gas-rich disk galaxy collisions offer a brief but interesting environment to study the effects of shocks and turbulence on star formation rates in the diffuse IGM, far from the significant feedback effects of massive star formation and AGN. Expanding on our earlier work, we describe simulated collisions between counter-rotating disk galaxies of relatively similar mass, focusing on the thermal and kinematic effects of relative inclination and disk offset at the closest approach. This includes essential heating and cooling signatures, which go some way towards explaining the luminous power in H$_2$ and [CII] emission in the Taffy bridge, as well as providing a partial explanation of the turbulent nature of the recently observed compact CO-emitting clouds observed in Taffy by ALMA. The models show counter-rotating disk collisions result in swirling, shearing kinematics for the gas in much of the post-collision bridge. Gas with little specific angular momentum due to collisions between counter-rotating streams accumulates near the center of mass. The disturbances and mixing in the bridge drive continuing cloud collisions, differential shock heating, and cooling throughout. A wide range of relative gas phases and line-of-sight velocity distributions are found in the bridges, depending sensitively on initial disk orientations and the resulting variety of cloud collision histories. Most cloud collisions can occur promptly or persist for quite a long duration. Cold and hot phases can largely overlap throughout the bridge or can be separated into different parts of the bridge.