Probing the Circumgalactic Medium with X-Ray Absorption Lines

Abstract

The circumgalactic medium (CGM) fills the dark matter halo of a galaxy, serving as a gas reservoir between the intergalactic medium and the stellar disk. The properties of the CGM are governed by galaxy properties, and the CGM in turn may regulate the galaxy evolution. Therefore, it is important to study the CGM to understand galaxy evolution and the role of galactic feedback. The CGM is expected to be predominantly warm-hot, best probed in X-rays. In this chapter, we discuss the CGM as probed by X-ray absorption lines. We provide theoretical background and discuss technical advances enabling the absorption line spectroscopy of the CGM. Because of our special vantage point, the CGM has been best studied for our own galaxy, the Milky Way (MW). We discuss the two decades of progress in absorption line studies of the MW CGM. We discuss how to derive physical properties from the observed parameters, as well as their uncertainties, caveats, and underlying assumptions. We discuss the all-sky distribution of the O vii absorbers, their use in determining the CGM density profile and mass, and the associated caveats. The MW CGM is diffuse, extended, and massive and might account for the missing galactic baryons. There are exciting new discoveries of sub-virial, virial, and super-virial temperatures in the X-ray band, of non-solar abundance ratios of light elements, and of α −element enhancement. Understanding the multiphase CGM is going to be a major area of research in the coming decade. We outline several outstanding questions that are critical for understanding the CGM. We must understand the physical and hydrodynamic properties, sources of heating/cooling and non-thermal motions, chemical enrichment, anisotropy and mixing in the CGM, and the relation between the CGM properties and galaxy properties. We end with a discussion of future directions in observational studies of the CGM and comparisons with theoretical simulations. The ultimate goal is to understand the CGM in the context of galaxy evolution.KeywordsCircumgalactic mediumGalaxy structureX-rayX-ray absorptionGalaxy dark matter halosQuasar absorption line spectroscopyGalaxy formationGalaxy evolutionMilky Way formationGalactic and extragalactic astronomy