Galaxies are surrounded by low-density and highly ionized clouds of gas, called the Circumgalactic Medium (CGM). Observations of this medium are highly sensitive to ionization levels, which in turn require us to understand the dominant ionization mechanisms in order to interpret them. The two possible forms of ionization are photoionization (PI), which is density dependent and generally assumed to dominate in cooler gas, and collisional ionization (CI), which is temperature dependent and assumed to dominate in hotter gas. Using the definitions of PI-dominated and CI-dominated gas from Strawn et al. (2020) and the ionization equilibrium software TRIDENT, we analyzed the distribution between PI and CI components for each ionization state of oxygen (OI, OII, ⋯ OIX) within 12 cosmological zoom-in simulations (the VELA suite).
Using astrophysical analysis software YT, we plotted the mass fraction of each ion species throughout the CGM, as well as the distribution of each species into PI and CI dominated phases. To facilitate more direct comparison to observations, we also compared 2D covering fractions of total, PI, and CI oxygen at different detectability thresholds by creating projections of each ion species’ column density and using a color analyzer to output the percentage of the image at a particular order of magnitude. We separated these simulations into different galaxy mass bins to analyze any change resulting from different galaxy properties. The main results that we find are: (1) OVII and OIX are the only ion states which are predominantly CI in the CGM. This is significant because many current studies assume OVIII is in CI equilibrium when it might in fact be PI. (2) OVI is a local minimum in both PI and CI gas, and remains so in each galaxy studied. (3) The column density distributions emphasize higher ion states compared to the 3D ion distribution. (4) When the covering fraction plots are fixed to a higher threshold, the shape begins to resemble the 3D plots. (5) CI gas becomes more dominant in higher mass galaxy simulations.