Bimodality of low-redshift circumgalactic O VI in non-equilibrium EAGLE
  zoom simulations
  We introduce a series of 20 cosmological hydrodynamical simulations of Lstar
(M_200 =10^11.7 - 10^12.3 Msol) and group-sized (M_200 = 10^12.7 - 10^13.3
Msol) 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/Mstar) 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 Lstar galaxies, because their virial temperatures are close to
the temperature at which the ionization fraction of O VI peaks (T~10^5.5 K).
The higher virial temperature of group haloes (> 10^6 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 Lstar and group haloes is O VII; O
VI accounts for only 0.1% of the oxygen in group haloes and 0.9-1.3% with Lstar
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 Lstar 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.
