Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 474, Issue 4, Pages 4740-4755Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx2967
Keywords
galaxies: formation; intergalactic medium; quasars: absorption lines; galaxies: Seyfert; cosmology: theory
Categories
Funding
- NASA ATP [NNX16AB31G]
- European Research Council under the European Union's Seventh Framework Programme/ERC [278594-GasAroundGalaxies]
- Netherlands Organisation for Scientific Research (NWO) through VICI [639.043.409]
- BIS National E-infrastructure capital grant [ST/K00042X/1]
- STFC [ST/H008519/1, ST/K00087X/1]
- STFCDiRAC [ST/K003267/1]
- Durham University
- Northwestern University
- STFC [ST/I00162X/1, ST/H008519/1, ST/R000832/1] Funding Source: UKRI
- Science and Technology Facilities Council [ST/I00162X/1, ST/H008519/1, ST/R000832/1] Funding Source: researchfish
- NASA [907925, NNX16AB31G] Funding Source: Federal RePORTER
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Proximity zone fossils (PZFs) are ionization signatures around recently active galactic nuclei (AGNs) where metal species in the circumgalactic medium remain overionized after the AGNs have shut off due to their long recombination time scales. We explore cosmological zoom hydrodynamic simulations, using the EAGLE (Evolution and Assembly of GaLaxies and their Environments) model paired with a non-equilibrium ionization and cooling module including time-variable AGN radiation to model PZFs around star-forming disc galaxies in the z similar to 0.2 Universe. Previous simulations typically underestimated the O VI content of galactic haloes, but we show that plausible PZF models increase O VI column densities by 2 - 3 x to achieve the levels observed around COS-Halos star-forming galaxies out to 150 kpc. Models with AGN bolometric luminosities greater than or similar to 10(43.6)erg s(-1), duty cycle fractions less than or similar to 10 per cent, and AGN lifetimes less than or similar to 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 time-scale comparable to the recombination time of a high metal ion, which for circumgalactic O VI is approximate to 10(7) yr. 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 photoionization, 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.
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