Metals in the circumgalactic medium are out of ionization equilibrium due to fluctuating active galactic nuclei

Abstract

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.

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

“…Segers et al (2017) ran the first cosmological hydro simulations of PZFs using individual EAGLE haloes, finding high ions are significantly enhanced for a variety of parameters. Here, we argue that PZFs significantly enhance O vi observed by COS-Halos around normal star-forming galaxies (Tumlinson et al 2011) using the first dynamic simulations including PZFs.…”

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.…”

A comparison of observed and simulated absorption from H i, C iv, and Si ivaroundz ≈ 2 star-forming galaxies suggests redshift–space distortions are due to inflows

“…According to their equation (8), at a transverse distance (TD) of 118 pkpc (corresponding to the median impact parameter of galaxies in the two innermost bins), a high escape fraction of 10 per cent would result in a reduction of H I pixel optical depths by a factor of two. While the higher ionization energy metal absorption lines are likely not impacted by the stellar radiation because it drops steeply in intensity at wavelengths below 912 Å, fluctuating AGN could be an issue if the time between outbursts is smaller than the metal ion recombination time-scales (Oppenheimer & Schaye 2013;Segers et al 2017). However, the inclusion of non-equilibrium ionization is beyond the scope of this work.…”