Date of Award
Doctor of Philosophy
Physics and Astronomy
Physics and Astronomy
Galaxy collisions come in many forms, from weak gravitational effects to complete destruction. Structures formed from gravitational interactions, such as, tidal arms or ring waves are well documented in observation and understood through modeling. Though the full picture of these collisions is not yet realized for gas rich collisions. The gas disks of directly colliding galaxies exert a pressure on each other, in some cases transferring significant amounts of momentum between galaxies. The direct contact between two gas rich disks produces a splashing of the gas clouds out of each galaxy, hence the given name for these structures ‘splash bridges'. The displacement of this splash of gas from its host galaxy is dependent on the relative column densities of the colliding gas. The iconic images of interacting galaxies contain great examples of ringing disks, long tidal tails of old stars and chaotic structures of gas illuminated by newly formed stars. The environments and timescales for star formation in these splash bridges is not well understood. Neutral atomic HI bridges have been observed between interacting galaxies with little to no stellar component, suggesting a delay to star formation or none at all depending on the splash bridge environment. In the case of the Taffy galaxies - UGC 12914/15, extensive observations have revealed not only a radio continuum emitting gas, but HI gas, X-rays from hot diffuse gas and more H2 than exists in the Milky Way coexist in the bridge. The origins of the H2 and large asymmetric distribution of ISM (interstellar medium) are not clear. Recent observations of the Taffy galaxies by ALMA show a complex structure on tens of parsec scales in the CO emitting gas. The complexity beginning to be seen in splash bridges requires modeling collisional multiphase gas disks. Shocks produced during direct interaction strongly influence the ability of these clouds to produce stars. To begin to understand the fate of the ISM in interacting galaxies we developed an inelastic particle code with shocks and cooling calculated on a subgrid level to study the gas in direct collisions between galaxy disks. This allows us to track the temperature, density and dust fraction in each phase of ISM. We find that the morphologies of these splash bridges are most affected by the relative galaxy density profiles, disk inclinations and central offsets in the collision. Weaker effects on morphology are due to relative rotation sense and initial distributions of each phase of ISM. Our models show that multiple phases of ISM with densities ranging over many orders of magnitude can be removed from their host galaxies into a splash bridge, consistent with observations of known splash bridge systems. This particle code using simple initial conditions also successfully creates complex ISM structures such as long dense filaments and asymmetric ISM distributions similar to those observed in splash bridges. Splash bridges may persist over several orders of magnitude in time, from as few as tens of millions of years to one billion years. The mass of these bridges can exceed the gas mass of the Milky Way for two Taffy-like galaxies colliding with a low central offset and inclination. Gas rich galaxies that directly collide on a single pass trajectory may be a possible pathway for forming what are known as 'dark galaxies', large gas structures with no associated stellar component. Our code enables us to study the types of shocks that are occurring within splash bridges. The number distribution of high velocity shocks in cloud collisions, produced in our low inclination models, are in agreement with those observed in the Taffy Galaxies with ALMA, (Appleton et al., 2019a) When normalizing and binning the upstream velocities of the shocks over each entire run reveals that the shape of the distribution is independent of the disk-disk impact parameters. The continuation of shocks in the ISM for the entire integration time of about 120 Myr after the disk-disk collision provides strong evidence for continued turbulence in splash bridges. Binning the shocks that occur in 10 Myr time bins shows that the high velocity shocks (200-600 km s−1) from the disk-disk impact decay on a time scale of a few million years into a cascade of lower velocity shocks. It is the lower velocity shocks (< 100km s−1) that drive much of the prolonged turbulence. The shocks from the initial disk-disk collision result in temperatures and densities in the ISM comparable to observations in UGC 12914/5 of a hot >106 K and cold (< 104 K) phase. The duration of the hot phase depends on the strength of turbulence, which in turn has a dependence on the disk-disk impact parameters. We find that inclination plays the strongest role in the maintenance of turbulence in a splash bridge. Beyond approximately 30° inclination a strong temperature gradient forms in the splash bridge due to the rotations of the galaxies. The rotation causes a compression of distance between gas clouds on one side of the splash bridge and a rarefaction between them on the other. The compression leads to prolonged turbulence and therefore a supported hot phase of ISM and the rarefaction allows gas to cool to less than < 104 K temperatures. An analysis of cloud stability in splash bridges indicates most of the ISM involved in gas-rich disk-disk collisions will become gravitionally unstable from the increase in density due to shock compression and subsequent gas cooling. The inclination and offset parameters of the disk-disk collision will affect the delay to gravitational cloud collapse through the means of thermal shock heating. Low inclinations do not provide enough gas turbulence to prevent much of the impacted ISM from collapsing by 30 Myr after the disk-disk collision. For high inclination collisions, regions of the splash bridge where there is strong gas turbulence gas clouds are able to maintain stable temperatures for up to 80 Myr. The thermo-physical details of splash bridges can profoundly effect the star formation within the bridges and the subsequent reaccretion of gas onto the galaxies or merger remnant.
Yeager, Travis, "Simulating the kinematics, shocks and cooling of gas-rich direct disk-disk galaxy collisions to study the evolution and morphology of the multi-phase ISM" (2020). Graduate Theses and Dissertations. 17850.