Flickering AGN can explain the strong circumgalactic O VI observed by
  COS-Halos
  Proximity zone fossils (PZFs) are ionization signatures around recently
active galactic nuclei (AGN) where metal species in the circumgalactic medium
remain over-ionized after the AGN has shut-off due to their long recombination
timescales. We explore cosmological zoom hydrodynamic simulations using the
EAGLE model paired with a non-equilibrium ionization and cooling module
including time-variable AGN radiation to model PZFs around star-forming, disk
galaxies in the z~0.2 Universe. Previous simulations typically under-estimated
the O VI content of galactic haloes, but we show that plausible PZF models
increase O VI column densities by 2-3x to achieve the levels observed around
COS-Halos star-forming galaxies out to 150 kpc. Models with AGN bolometric
luminosities >~10^43.6 erg s^-1, duty cycle fractions <~10%, and AGN lifetimes
<~10^6 yr are the most promising, because their super-massive black holes grow
at the cosmologically expected rate and they mostly appear as inactive AGN,
consistent with COS-Halos. The central requirement is that the typical
star-forming galaxy hosted an active AGN within a timescale comparable to the
recombination time of a high metal ion, which for circumgalactic O VI is 10^7
years. H I, by contrast, returns to equilibrium much more rapidly due to its
low neutral fraction and does not show a significant PZF effect. O VI
absorption features originating from PZFs appear narrow, indicating
photo-ionization, and are often well-aligned with lower metal ion species. PZFs
are highly likely to affect the physical interpretation of circumgalactic high
ionization metal lines if, as expected, normal galaxies host flickering AGN.
