Models of multicomponent splash bridges in face-on galaxy disc collisions

Abstract

We use an inelastic particle code with shocks and cooling calculated on a subgrid level to study the gas in direct collisions between galaxy discs. The interstellar media (ISM) of the discs are modeled with continuous thermal phases. The models produce many unique structures, collectively called splash bridges. They range from central bridge discs to swirled sheets, which resemble those observed in interacting galaxies. These morphologies are sensitive to the rotation, relative mass, disc offsets and the gas structure in the discs. In the case of the Taffy galaxies - NGC 12914/15, extensive observations have revealed radio continuum emitting gas, HI gas, hot X-rays from hot diffuse gas and more $H_2$ than exists in the Milky Way coexisting in the bridge. The origins of the $H_2$ and large asymmetric distribution of ISM are not clear. We show that for small disc impact parameters, multiple phases of ISM with densities over many orders of magnitude can be removed from their host galaxies into a Taffy-like bridge. The orientation of the discs initial overlap can have a great effect on the distributions of each phase of ISM. In some cases, the models also predict the creation of a possible `dark galaxy,' a large flat region of dense ISM far from the stellar disc potential of either galaxy.