Production of Cool Gas in Thermally-Driven Outflows

TL;DR

This study uses high-resolution galaxy simulations to show that rapid radiative cooling can produce fast-moving cool gas in galactic outflows, explaining observations without needing to accelerate cool gas directly.

Contribution

It demonstrates that cooling flows in simulated outflows can generate multiphase, fast-moving cool gas, offering a new explanation for observed galactic halo phenomena.

Findings

Cooling flows can produce $v \, \sim \, 1000$ km/s cool gas.

Reasonable mass-loading rates ($\sim 0.5$ of SFR) can generate cool outflows.

Multiphase outflows can arise from interactions and cloud destruction near the galaxy disk.

Abstract

Galactic outflows commonly contain multiphase gas, and its physical origin requires explanation. Using the CGOLS (Cholla Galactic OutfLow Simulations) suite of high-resolution isolated galaxy models, we demonstrate the viability of rapid radiative cooling as a source of fast-moving ($v \sim 1000$ km/s), cool ($10^4$ K) gas observed in absorption line studies of outflows around some star-forming galaxies. By varying the mass-loading and geometry of the simulated winds, we identify a region of parameter space that leads to cool gas in outflows. In particular, when using an analytically-motivated central feedback model, we find that cooling flows can be produced with reasonable mass-loading rates ($\dot{M}_{wind} / \dot{M}_{SFR} \sim 0.5$), provided the star formation rate surface density is high. When a more realistic clustered feedback model is applied, destruction of high density clouds near the disk and interactions between different outflow regions indicate that lower mass-loading rates of the hot gas within the feedback region may still produce multiphase outflows. These results suggest an origin for fast-moving cool gas in outflows that does not rely on directly accelerating cool gas from the interstellar medium. These cooling flows may additionally provide an explanation for the multiphase gas ubiquitously observed in the halos of star-forming galaxies at low redshift.