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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^-3 cm^-3) exposed to the extra-galactic background (EGB) and initially in thermal equilibrium (T \sim 10^4-10^4.5 K). Once the proximate AGN field turns on, additional photo-ionisation rapidly ionises 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 ionisation equilibrium for timescales that can significantly exceed tau_AGN. We define this phase as the AGN proximity zone "fossil" phase and show that high ionisation stages (e.g. OVI, NeVIII, MgX) are in general enhanced, while the abundances of low ions are reduced. In contrast, HI re-equilibrates rapidly (<<tau_AGN) owing to its low neutral fraction at diffuse densities. We demonstrate that metal column densities of intervening gas observed in absorption in quasar sight lines are significantly affected by delayed recombination for a wide range of densities, metallicities, and AGN strengths, lifetimes, and duty cycles. We model the exceptionally strong z=0.9 NeVIII absorbers observed by Tripp et al. (2011) as arising in a possible fossil zone or near a recently turned-on AGN and we demonstrate that at low redshift even moderate strength AGN could significantly enhance the high-ion metal columns in the circumgalactic media of galaxies observed without active AGN. Fossil proximity zones may be very important during the quasar era, z \sim 2-5. AGN proximity zone fossils allow a whole new class of non-equilibrium solutions that may be applicable to a large fraction of observed metal absorbers and which could potentially change the inferred physical conditions and masses of diffuse gases.