HI Absorption in the Intergalactic Medium

We review studies of the intergalactic medium (IGM) via HI absorption with emphasis on the decades of previous research with quasar absorption line studies. The chapter begins with a historical perspective and then offers a pedagogical description of the quantum mechanics underlying the Lyman series and Lyman continuum opacity of the HI atom. We describe the manifestation of these opacities in absorption spectroscopy and the challenges related to normalization of quasar emission. Standard measurement techniques (equivalent width, line-profile fitting) are introduced. We then introduce the \(N_\mathrm{HI}\) frequency distribution \(f(N_\mathrm{HI})\) and efforts to constrain this distribution across cosmic time with emphasis on the optically thin Ly\(\alpha \) forest. A discussion of optically thick gas (Lyman limit systems and damped Ly\(\alpha \) systems) and its relation to the mean free path is presented. Online presentations and Python notebooks supplement this chapter with examples, a review of modern work, and thoughts on future progress. See https://github.com/profxj/SaasFee2016 for additional resources.