CGM properties in VELA and NIHAO simulations; the OVI ionization
  mechanism: dependence on redshift, halo mass and radius
  We study the components of cool and warm/hot gas in the circumgalactic medium
(CGM) of simulated galaxies and address the relative production of OVI by
photoionization versus collisional ionization, as a function of halo mass,
redshift, and distance from the galaxy halo center. This is done utilizing two
different suites of zoom-in hydro-cosmological simulations, VELA (6 halos;
$z>1$) and NIHAO (18 halos; to $z=0$), which provide a broad theoretical basis
because they use different codes and physical recipes for star formation and
feedback. In all halos studied in this work, we find that collisional
ionization by thermal electrons dominates at high redshift, while
photoionization of cool or warm gas by the metagalactic radiation takes over
near $z\sim2$. In halos of $\sim 10^{12}M_{\odot}$ and above, collisions become
important again at $z<0.5$, while photoionization remains significant down to
$z=0$ for less massive halos. In halos with $M_{\textrm
v}>3\times10^{11}~M_{\odot}$, at $z\sim 0$ most of the photoionized OVI is in a
warm, not cool, gas phase ($T\lesssim 3\times 10^5$~K). We also find that
collisions are dominant in the central regions of halos, while photoionization
is more significant at the outskirts, around $R_{\textrm v}$, even in massive
halos. This too may be explained by the presence of warm gas or, in lower mass
halos, by cool gas inflows.
