How Do Mass and Momentum Couple in Multiphase Galactic Outflows?
Starburst galaxies are frequently observed to have outflows of gas that span many orders of magnitude in density and temperature. Cool, dense clouds are interspersed with warm, ionized gas, and million-degree plasma. My thesis work focused on understanding how these different phases interact as gas is expelled from galaxies.
At only ~3.5 Mpc distance, M82 is one of the closest starburst galaxies in the Universe. As such, it is also the best observed, and many stunning composite images (such as this one) have been created. Here, yellow-green optical light highlights the stellar disk, while red shows the extended H-alpha filaments in the galactic wind, and blue-purple regions represent the hot X-ray plasma. (Image credit: NASA)
Cloud-Wind Simulations
To quantify how mass and momentum transfer from one gas phase to another, I ran a series of idealized simulations containing a single, cool cloud getting shredded by a hot, fast outflow. Unlike previous work in this area, I started with clouds that had turbulent density distributions, rather than just constant density spheres. In addition, I used boxes that were large enough to track the cloud material across a large volume, while maintaining high enough resolution to capture the hydrodynamic instabilities responsible for mixing different phases of gas.
This simulation shows the interaction of a cool, dense cloud of gas with a hot, fast outflow. The simulation was performed with the Cholla astrophysics code on the Titan supercomputer, and has a resolution of ~1.2 billion cells.
momentum transfer to the densest gas is inefficient
There were many results from this study, but one of the most important is the fact that the transfer of momentum from the hot, low density wind to the denser clumps of entrained gas is inefficient. In order to be significantly accelerated, dense gas must either span a large area, or first get shredded and rarified. I found that turbulent clouds are destroyed more quickly in these sorts of interactions than constant density spheres, even when they contain more mass initially.