Invited review, submitted to Annual Reviews of Astronomy & Astrophysics, Vol 56, 2018.

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Jason Tumlinson 1, Molly S. Peeples 1, and Jessica K. Werk 2

1 Space Telescope Science Institute and Johns Hopkins University, Baltimore, Maryland
2 University of Washington, Seattle, Washington

Abstract: The gas surrounding galaxies outside their disks or ISM 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 co-evolution 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.

Keywords : gas, galaxies, galaxy evolution, cosmology

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