Flickering AGN can explain the strong circumgalactic OVI observed by COS-Halos

Oppenheimer, Benjamin D.; Segers, Marijke; Schaye, Joop; Richings, Alexander J.; Crain, Robert A.

Abstract

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 offdue 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 ~ 0.2 Universe. Previous simulations typically underestimated the OVI content of galactic haloes, but we show that plausible PZF models increase OVI 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 ≳ 10^43.6erg s-1, duty cycle fractions ≲ 10 per cent, 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 time-scale comparable to the recombination time of a high metal ion, which for circumgalactic OVI is ≈ 10^7 yr. HI, by contrast, returns to equilibrium much more rapidly due to its low neutral fraction and does not show a significant PZF effect. OVI 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.

Citation

Oppenheimer, B. D., Segers, M., Schaye, J., Richings, A. J., & Crain, R. A. (2018). Flickering AGN can explain the strong circumgalactic OVI observed by COS-Halos. Monthly notices of the Royal Astronomical Society, 474(4), 4740-4755. https://doi.org/10.1093/mnras/stx2967

• Journal Article Type: Article
• Acceptance Date: Nov 15, 2017
• Online Publication Date: Nov 17, 2017
• Publication Date: Mar 1, 2018
• Deposit Date: Jan 11, 2023
• Publicly Available Date: Jan 19, 2023
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 474
• Issue: 4
• Pages: 4740-4755
• DOI: https://doi.org/10.1093/mnras/stx2967
• Public URL: https://hull-repository.worktribe.com/output/4132538
• Publisher URL: https://academic.oup.com/mnras/article/474/4/4740/4638546