Flickering AGN can explain the strong circumgalactic O viobserved by COS-Halos

Abstract

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.

Kinematics of the O vi Circumgalactic Medium: Halo Mass Dependence and Outflow Signatures

“…For less massive galaxies, the virial temperature would be too cool for strong O VI presence, whereas the virial temperature would be too high for group environments as a large fraction of O VI is ionized out to higher ionization species. Nelson et al (2018) found similar trends in the O VI column density with halo mass, but attributed them to black hole feedback (also see Oppenheimer et al 2018). Several other works have also shown this trend in both observations (Bielby et al 2019;Zahedy et al 2019) and with gaseous halo models (Qu & Bregman 2018).…”

Galactic Gas Flows from Halo to Disk: Tomography and Kinematics at the Milky Way’s Disk–Halo Interface

“…Peeples et al 2018;Hummels et al 2018). Incorporating AGN feedback has improved predictions of high-ion abundances, but simulations still struggle to reproduce low-ion abundances, even for hydrogen (Oppenheimer et al 2018). If low-ion CGM clouds do form on scales < 500 pc as our analysis and that of others suggests, then the resolution of CGM gas in simulations must increase by orders of magnitude in order to reflect the hydrodynamics on relevant scales (Marinacci et al 2010;Crighton et al 2015;McCourt et al 2018;Liang & Remming 2018;Sparre et al 2019).…”

Detection of low-metallicity warm plasma in a galaxy overdensity environment at z ∼ 0.2

“…There could be actively star-forming galaxies or relatively faint AGNs in close proximity to the absorber, below the magnitude limit of SDSS spectroscopic survey. Also, as Segers et al (2017) and Oppenheimer et al (2017) have proposed, galaxies with past AGN activity in time scales shorter than the recombination time scales of high ions (t 10 6 yrs) can have enhanced O vi ionization fractions in their haloes. We cannot dismiss such a scenario.…”