The gas surrounding galaxies outside their disks or interstellar medium and inside their virial radii is known as the circumgalactic medium (CGM). In recent years this component of galaxies has assumed an important role in our understanding of galaxy evolution owing to rapid advances in observational access to this diffuse, nearly invisible material. Observations and simulations of this component of galaxies suggest that it is a multiphase medium characterized by rich dynamics and complex ionization states. The CGM is a source for a galaxy's star-forming fuel, the venue for galactic feedback and recycling, and perhaps the key regulator of the galactic gas supply. We review our evolving knowledge of the CGM with emphasis on its mass, dynamical state, and coevolution with galaxies. Observations from all redshifts and from across the electromagnetic spectrum indicate that CGM gas has a key role in galaxy evolution. We summarize the state of this field and pose unanswered questions for future research.

中文翻译：

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环绕星系介质

围绕星系盘或星际介质外和维力半径内的气体被称为环星系介质（CGM）。近年来，由于对这种弥散的、几乎不可见的物质的观测取得了快速进展，星系的这一组成部分在我们对星系演化的理解中发挥了重要作用。对该星系组成部分的观察和模拟表明，它是一种多相介质，具有丰富的动力学和复杂的电离状态。CGM 是银河系恒星形成燃料的来源，是银河系反馈和回收的场所，也许是银河系气体供应的关键调节器。我们回顾了我们对 CGM 不断发展的知识，重点是它的质量、动力学状态以及与星系的共同演化。来自所有红移和整个电磁光谱的观察表明，CGM 气体在星系演化中起着关键作用。我们总结了该领域的现状，并为未来的研究提出了未解决的问题。