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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 byphotoionization versus collisional ionization, as a function of halo mass,redshift, and distance from the galaxy halo center. This is done utilizing twodifferent suites of zoom-in hydro-cosmological simulations, VELA (6 halos;$z>1$) and NIHAO (18 halos; to $z=0$), which provide a broad theoretical basisbecause they use different codes and physical recipes for star formation andfeedback. In all halos studied in this work, we find that collisionalionization by thermal electrons dominates at high redshift, whilephotoionization of cool or warm gas by the metagalactic radiation takes overnear $z\sim2$. In halos of $\sim 10^{12}M_{\odot}$ and above, collisions becomeimportant again at $z<0.5$, while photoionization remains significant down to$z=0$ for less massive halos. In halos with $M_{\textrmv}>3\times10^{11}~M_{\odot}$, at $z\sim 0$ most of the photoionized OVI is in awarm, not cool, gas phase ($T\lesssim 3\times 10^5$~K). We also find thatcollisions are dominant in the central regions of halos, while photoionizationis more significant at the outskirts, around $R_{\textrm v}$, even in massivehalos. This too may be explained by the presence of warm gas or, in lower masshalos, by cool gas inflows.

CGM properties in VELA and NIHAO simulations; the OVI ionization mechanism: dependence on redshift, halo mass and radius