galactic winds And The Circumgalactic Medium

Thanks to new instrumentation on the Hubble Space Telescope, astronomers have recently begun to make better measurements of the gas in the regions surrounding galaxies. This gas is called the circumgalactic medium (CGM), and understanding its properties can help us understand how galaxies themselves grow and evolve. For example, studying the phases of gas in the CGM (its density and temperature distribution), can provide clues about how it got there.

One key contributor to the CGM is galactic winds - gas driven out of galaxies either by processes related to star formation or accretion onto a supermassive black hole. As theorists, we would like to model these outflows both analytically and numerically, but simulating outflows is difficult because of the range of scales involved. In a starburst-driven wind, supernova bubbles within the galactic disk on the scale of a few parsecs interact and over time drive gas out to radii of tens to hundreds of kiloparsecs. Resolving the details of the whole process would therefore require simulations with a quadrillion (that's 10^15) cells!


The CGOLS (pronounced Seagulls) Project

While we're not quite there yet, with a powerful code like Cholla, we can push the boundary of what's being simulated. With an INCITE allocation on the Titan supercomputer, I simulated galactic winds on scales of ~10 kiloparsecs with a resolution of ~5 parsecs - that's over an order of magnitude higher resolution than had ever been used to simulate a single galaxy. I called this project CGOLS (Cholla Galactic OutfLow Simulations). Currently, the CGOLS suite consists of five simulations. The first tested a wind model with a single spherical injection region 300 parsecs in radius at the center of the galaxy. The second tested the same model but allowed the wind to cool radiatively, and the third kept the same mass and energy injection rates but distributed the input in eight clusters distributed throughout the central 1.5 kiloparsecs of the disk. Later simulations increased the physical realism of the supernova feedback by allowing the mass and energy injection rates to vary with time according to stellar population synthesis models. The movie below shows a fly-through of the density field at a single point in time for a CGOLS simulation where clusters were distributed throughout the disk. Movies showing the time evolution of density and temperature for other models can be found in the Simulation Gallery.