AGN proximity zone fossils and the delayed recombination of metal lines

We model the time-dependent evolution of metal-enriched intergalactic and circumgalactic gas exposed to the fluctuating radiation field from an active galactic nucleus (AGN). We consider diffuse gas densities (n(H) = 10(-5)-10(-2.5) cm(-3)) exposed to the extra-galactic background (EGB) and initially in thermal equilibrium (T similar to 10(4)-10(4.5) K). Once the proximate AGN field turns on, additional photo-ionization rapidly ionizes the Hi and metals. The enhanced AGN radiation field turns off after a typical AGN lifetime (tau(AGN) = 1-20 Myr) and the field returns to the EGB intensity, but the metals remain out of ionization equilibrium for time scales that can significantly exceed tau(AGN). We define this phase as the AGN proximity zone 'fossil' phase and show that high ionization stages (e.g. Ovi, Neviii, Mgx) are in general enhanced, while the abundances of low ions (e.g. Civ, Oiv, Mgii) are reduced. In contrast, Hi re-equilibrates rapidly (