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 centre. This is done utilizing two different suites of zoom-in hydro-cosmological simulations, VELA (6 haloes; z > 1) and NIHAO (18 haloes; to z = 0), which provide a broad theoretical basis because they use different codes and physical recipes for star formation and feedback. In all haloes 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 ∼ 2. In haloes of ∼10¹² M and above, collisions become important again at z < 0.5, while photoionization remains significant down to z = 0 for less massive haloes. In haloes with M_v > 3 × 10¹¹ M, at z ∼ 0 most of the photoionized OVI is in a warm, not cool, gas phase (T 3 × 10⁵ K). We also find that collisions are dominant in the central regions of haloes, while photoionization is more significant at the outskirts, around R_v, even in massive haloes. This too may be explained by the presence of warm gas or, in lower mass haloes, by cool gas inflows.