How does dust end up in the circumgalactic medium (CGM)? Although dust originates in the interstellar medium through condensation in stellar winds and supernovae, it is frequently observed in galaxy CGMs. Observations of redenning in the halos of numerous galaxies suggest that as much as half of the cosmic dust mass may exist outside of galaxies (Ménard et al. 2010). Dust grains of all sizes (including very small grains) are observed outside of galaxies, raising questions about the mechanism that transports dust to these regions. Galactic winds are a common feature of star-forming galaxies and are thought to be a source of cool gas observed in the CGM. This phenomenon could explain the large amounts of dust we see outside of galaxies, but since winds are composed of extremely hot ( \( \sim10^6-10^7~\text{K} \) ) gas, they might destroy dust on relatively short timescales. Can dust survive in galactic winds? The multi-phase nature of galactic outflows may enable efficient shielding of dust in cool clouds as they move out of galaxies. If cool clouds do carry dust out of galaxies, how does the grain size evolution evolve during this process? What cloud and wind properties does dust survival depend on? What effect do dusty outflows have on star formation, and galaxy evolution as a whole?
Watching the evolution of dusty winds unfold in simulations can help us understand these questions. A number of recent numerical hydrodynamics simulations have reproduced the multi-phase outflows observed in galaxies. Above, we see a side-by-side comparison of one of these simulations (made by the Cholla hydrodynamics code) and a nearby, edge-on galaxy, NGC 4217. Both show filamentary structures outflowing from the disk of the galaxy. In particular, the outflow structure of NGC 4217 is traced by dust extinction. My project has been to add a dust model to Cholla in order to create simulations of galaxies like the one shown above (which only includes gas) with dusty galactic outflows. By directly modeling dust evolution in galactic winds, we can determine whether wind conditions are detrimental to dust, if cloud shielding can enable long-term survival, and more.
In the process of building up to galaxy-scale simulations of dusty outflows, I ran simulations of individual dusty clouds in hot galactic winds. An example of this simulation setup is shown above. The aim of this work was to understand whether clouds in outflows can efficiently shield dust as they are accelerated by galactic winds and, if so, what factors dust survival depend on. The results of this work were published in Richie et al. (2024). We found that dust survival is most sensitive to cloud dynamics, and also dust grain size. This work also showed that, due to the rapid decline of wind density and temperature with distance from the galaxy, larger dust grains can survive completely exposed to the wind. More details on this paper can be found in this post. Moving forward, we plan to expand this dust model to include a full treatment of dust evolution so that we can create realistic simulations of entire galaxies with dust.