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

TL;DR

This study investigates the ionization mechanisms of OVI in the circumgalactic medium of simulated galaxies, revealing how the dominance of photoionization or collisional ionization varies with redshift, halo mass, and radius.

Contribution

It compares two different simulation suites to analyze the dependence of OVI ionization mechanisms on galaxy properties and cosmic time, highlighting the roles of photoionization and collisional ionization.

Findings

Collisional ionization dominates at high redshift.

Photoionization becomes significant near z~2.

Collisions are dominant in halo centers, photoionization at outskirts.

Abstract

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.