Spurred by rich, multiwavelength observations and enabled by new simulations, ranging from cosmological to subparsec scales, the past decade has seen major theoretical progress in our understanding of the circumgalactic medium (CGM). We review key physical processes in the CGM. Our conclusions include the following: ▪The properties of the CGM depend on a competition between gravity-driven infall and gas cooling. When cooling is slow relative to free fall, the gas is hot (roughly virial temperature), whereas the gas is cold (T ∼ 104 K) when cooling is rapid.▪Gas inflows and outflows play crucial roles, as does the cosmological environment. Large-scale structure collimates cold streams and provides angular momentum. Satellite galaxies contribute to the CGM through winds and gas stripping.▪In multiphase gas, the hot and cold phases continuously exchange mass, energy, and momentum. The interaction between turbulent mixing and radiative cooling is critical. A broad spectrum of cold gas structures, going down to subparsec scales, arises from fragmentation, coagulation, and condensation onto gas clouds.▪Magnetic fields, thermal conduction, and cosmic rays can substantially modify how the cold and hot phases interact, although microphysical uncertainties are presently large. Key open questions for future work include the mutual interplay between small-scale structure and large-scale dynamics, and how the CGM affects the evolution of galaxies.

中文翻译：

---

银河系外介质中的关键物理过程


在丰富的多波长观测的推动下，在从宇宙学到亚秒差距尺度的新模拟的支持下，过去十年我们在理解银河系环介质 （CGM） 方面取得了重大的理论进展。我们回顾了 CGM 中的关键物理过程。我们的结论包括以下内容：▪CGM 的特性取决于重力驱动的下降和气体冷却之间的竞争。当冷却相对于自由落体缓慢时，气体是热的（大致是维里温度），而当冷却迅速时，气体是冷的（T ∼ 104 K）。 ▪大型结构准直冷流并提供角动量。卫星星系通过风和气体剥离为 CGM 做出贡献。▪在多相气体中，热相和冷相不断交换质量、能量和动量。湍流混合和辐射冷却之间的相互作用至关重要。▪磁场、热传导和宇宙射线可以极大地改变冷相和热相的相互作用方式，尽管目前的微观物理不确定性很大。未来工作的关键开放问题包括小尺度结构和大尺度动力学之间的相互作用，以及 CGM 如何影响星系的演化。