2005ApJ...625L...5W

Quasars represent a brief phase in the life cycle of most massive galaxies, but the evolutionary connection between central black holes and their host galaxies remains unclear. While quasars are active and shining brighter than the Compton-limit luminosity, their radiation heats the surrounding medium to the Compton temperature, forming Compton spheres extending to the Strömgren radius of Fe²⁶⁺/He²⁺. After the quasars shut off, their ``afterglow'' can be detected through three signatures: (1) an extended X-ray envelope, with a characteristic temperature of ∼3x10⁷ K, (2) Lyα and Lyβ lines and the K edge of Fe²⁶⁺, and (3) nebulosity from hydrogen and helium recombination emission lines. We discuss the possibility of detecting these signatures using Chandra, the planned X-Ray Evolving Universe Spectroscopy mission, and ground-based optical telescopes. The luminosity and size of quasar afterglows can be used to constrain the lifetime of quasars.