Description
The structure and evolution of the Intergalactic Medium (IGM) depends on its thermal history and the metagalactic ultra-violet background. QSOs play a crucial role in both of these at redshifts z < 4, and possibly as early as z < 6, as suggested by reports of large numbers of AGN discovered by JWST at z > 5 and in other recent surveys. We show how the structure of the IGM as detected in the Lyman-$\alpha$ forest is sensitive to QSO populations as well as to the lifetimes and extreme ultra-violet spectra of the QSOs. A large QSO population at z > 5 may explain anomalies in the optical depth distribution and clustering of the Lyman-$\alpha$ forest found from recent observations. Future measurements of the Baryonic Acoustic Oscillation peak at z > 3, such as by DESI, may additionally place strong constraints on QSO lifetimes and radiation beam opening angles.
