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Bimodality of low-redshift circumgalactic O VI in non-equilibrium EAGLE zoom simulations.

Oppenheimer, B. D. and Crain, R. A. and Schaye, J. and Rahmati, A. and Richings, A. J. and Trayford, J. W. and Tumlinson, J. and Bower, R. G. and Schaller, M. and Theuns, T. (2016) 'Bimodality of low-redshift circumgalactic O VI in non-equilibrium EAGLE zoom simulations.', Monthly notices of the Royal Astronomical Society., 460 (2). pp. 2157-2179.


We introduce a series of 20 cosmological hydrodynamical simulations of L★ (M200 = 1011.7 − 1012.3M⊙) and group-sized (M200 = 1012.7 − 1013.3M⊙) haloes run with the model used for the EAGLE project, which additionally includes a non-equilibrium ionization and cooling module that follows 136 ions. The simulations reproduce the observed correlation, revealed by COS-Halos at z ∼ 0.2, between O VI column density at impact parameters b < 150 kpc and the specific star formation rate (sSFR≡SFR/M★) of the central galaxy at z ∼ 0.2. We find that the column density of circumgalactic O VI is maximal in the haloes associated with L★ galaxies, because their virial temperatures are close to the temperature at which the ionization fraction of O VI peaks (T ∼ 105.5 K). The higher virial temperature of group haloes (>106 K) promotes oxygen to higher ionization states, suppressing the O VI column density. The observed NO VI-sSFR correlation therefore does not imply a causal link, but reflects the changing characteristic ionization state of oxygen as halo mass is increased. In spite of the mass-dependence of the oxygen ionization state, the most abundant circumgalactic oxygen ion in both L★ and group haloes is O VI; O VI accounts for only 0.1% of the oxygen in group haloes and 0.9-1.3% with L★ haloes. Nonetheless, the metals traced by O VI absorbers represent a fossil record of the feedback history of galaxies over a Hubble time; their characteristic epoch of ejection corresponds to z > 1 and much of the ejected metal mass resides beyond the virial radius of galaxies. For both L★ and group galaxies, more of the oxygen produced and released by stars resides in the circumgalactic medium (within twice the virial radius) than in the stars and ISM of the galaxy.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:03 May 2016
Date deposited:15 June 2016
Date of first online publication:05 May 2016
Date first made open access:15 June 2016

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