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!

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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 am currently simulating galactic winds on scales of ~10 kiloparsecs with a resolution of ~5 parsecs - that's over an order of magnitude higher resolution than has ever been used to simulate a single galaxy. I call this project CGOLS (Cholla Galactic OutfLow Simulations). Currently, the CGOLS suite consists of three simulations - one testing a wind model with a single central injection region 300 parsecs in radius, one testing the same model but allowing the wind to cool radiatively, and one where the supernova energy is injected in eight clusters distributed throughout the central 1.5 kiloparsecs of the disk. Density and temperature projections of the clustered feedback simulation can be seen in the movies below.

This simulation shows a disk galaxy undergoing a nuclear starburst. The mass and energy injected by supernovae at the galaxy's center force gas out of the disk and into the surrounding halo. The simulation was performed with the Cholla astrophysics code, and has a resolution of ~17 billion cells.