Proximity zone fossils (PZFs) are ionization signatures around recently active galactic nuclei (AGN) where metal species in the circumgalactic medium remain overionized 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 − 3× 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 supermassive 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.

We study the origin of the stellar α-element-to-iron abundance ratio, [α/Fe] * , of present-day central galaxies, using cosmological, hydrodynamical simulations from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project. For galaxies with stellar masses of M * > 10 10.5 M , [α/Fe] * increases with increasing galaxy stellar mass and age. These trends are in good agreement with observations of early-type galaxies, and are consistent with a 'downsizing' galaxy formation scenario: more massive galaxies have formed the bulk of their stars earlier and more rapidly, hence from an interstellar medium that was mostly α-enriched by massive stars. In the absence of feedback from active galactic nuclei (AGN), however, [α/Fe] * in M * > 10 10.5 M galaxies is roughly constant with stellar mass and decreases with mean stellar age, extending the trends found for lower-mass galaxies in both simulations with and without AGN. We conclude that AGN feedback can account for the α-enhancement of massive galaxies, as it suppresses their star formation, quenching more massive galaxies at earlier times, thereby preventing the iron from longer-lived intermediatemass stars (supernova Type Ia) from being incorporated into younger stars.

We use cosmological, hydrodynamical simulations from the EAGLE and OWLS projects to assess the significance of recycled stellar ejecta as fuel for star formation. The fractional contributions of stellar mass loss to the cosmic star formation rate (SFR) and stellar mass densities increase with time, reaching 35% and 19%, respectively, at z = 0. The importance of recycling increases steeply with galaxy stellar mass for M * < 10 10.5 M , and decreases mildly at higher mass. This trend arises from the mass dependence of feedback associated with star formation and AGN, which preferentially suppresses star formation fuelled by recycling. Recycling is more important for satellites than centrals and its contribution decreases with galactocentric radius. The relative contribution of AGB stars increases with time and towards galaxy centers. This is a consequence of the more gradual release of AGB ejecta compared to that of massive stars, and the preferential removal of the latter by star formationdriven outflows and by lock up in stellar remnants. Recycling-fuelled star formation exhibits a tight, positive correlation with galaxy metallicity, with a secondary dependence on the relative abundance of alpha elements (which are predominantly synthesized in massive stars), that is insensitive to the subgrid models for feedback. Hence, our conclusions are directly relevant for the origin of the mass-metallicity relation and metallicity gradients. Applying the relation between recycling and metallicity to the observed mass-metallicity relation yields our best estimate of the mass-dependent contribution of recycling. For centrals with a mass similar to that of the Milky Way, we infer the contributions of recycled stellar ejecta to the SFR and stellar mass to be 35% and 20%, respectively.

We study the effect of a fluctuating active galactic nucleus (AGN) on the abundance of circumgalactic OVI in galaxies selected from the EAGLE simulations. We follow the time-variable OVI abundance in post-processing around four galaxies -two at z = 0.1 with stellar masses of M * ∼ 10 10 M and M * ∼ 10 11 M , and two at z = 3 with similar stellar masses -out to impact parameters of twice their virial radii, implementing a fluctuating central source of ionizing radiation. Due to delayed recombination, the AGN leave significant 'AGN proximity zone fossils' around all four galaxies, where OVI and other metal ions are out of ionization equilibrium for several megayears after the AGN fade. The column density of OVI is typically enhanced by ≈ 0.3 − 1.0 dex at impact parameters within 0.3R vir , and by ≈ 0.06 − 0.2 dex at 2R vir , thereby also enhancing the covering fraction of OVI above a given column density threshold. The fossil effect tends to increase with increasing AGN luminosity, and towards shorter AGN lifetimes and larger AGN duty cycle fractions. In the limit of short AGN lifetimes, the effect converges to that of a continuous AGN with a luminosity of ( f duty /100%) times the AGN luminosity. We also find significant fossil effects for other metal ions, where low-ionization state ions are decreased (SiIV, CIV at z = 3) and high-ionization state ions are increased (CIV at z = 0.1, NeVIII, MgX). Using observationally motivated AGN parameters, we predict AGN proximity zone fossils to be ubiquitous around M * ∼ 10 10−11 M galaxies, and to affect observations of metals in the circumgalactic medium at both low and high redshifts.

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