Mon. Not. R. Astron. Soc., 434, 1063-1078 (2013/September-2)
AGN proximity zone fossils and the delayed recombination of metal lines.
OPPENHEIMER B.D. and SCHAYE J.
Abstract (from CDS):
We model the time-dependent evolution of metal-enriched intergalactic and circumgalactic gas exposed to the fluctuating radiation field from an active galactic nucleus (AGN). We consider diffuse gas densities (nH = 10–5-10–2.5/cm3) exposed to the extra-galactic background (EGB) and initially in thermal equilibrium (T ∼ 104-104.5K). Once the proximate AGN field turns on, additional photo-ionization rapidly ionizes the Hi and metals. The enhanced AGN radiation field turns off after a typical AGN lifetime (τAGN = 1-20Myr) and the field returns to the EGB intensity, but the metals remain out of ionization equilibrium for time scales that can significantly exceed τAGN. We define this phase as the AGN proximity zone `fossil' phase and show that high ionization stages (e.g. Ovi, Neviii, Mgx) are in general enhanced, while the abundances of low ions (e.g. Civ, Oiv, Mgii) are reduced. In contrast, Hi re-equilibrates rapidly (≪τAGN) owing to its low neutral fraction at diffuse densities. We demonstrate that metal column densities of intervening gas observed in absorption in quasar sight lines are significantly affected by delayed recombination for a wide range of densities, metallicities, AGN strengths, AGN lifetimes and AGN duty cycles. As an example, we show that a fossil zone model can simultaneously reproduce the observed Neviii, Mgii, Hi and other metal columns of the z = 0.927 PG1206+259 absorption system observed by Tripp et al. using a single, T ∼ 104K phase model. At low redshift even moderate-strength AGN that are off for 90 percent of the time could significantly enhance the high-ion metal columns in the circum-galactic media of galaxies observed without active AGN. Fossil proximity zones may be particularly important during the quasar era, z ∼ 2-5. Indeed, we demonstrate that at these redshifts a large fraction of the metal-enriched intergalactic medium may consist of out-of-equilibrium fossil zones. AGN proximity zone fossils allow a whole new class of non-equilibrium solutions that may be applicable to a large fraction of observed metal absorbers and which could potentially change the inferred physical conditions and masses of diffuse gases.
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society (2013)
atomic processes - plasmas - galaxies: formation - intergalactic medium - quasars: absorption lines - cosmology: theory
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