O vi traces photoionized streams with collisionally ionized boundaries in cosmological simulations of z ∼1 massive galaxies

We analyse the distribution and origin of O vi in the Circumgalactic Medium (CGM) of dark-matter haloes of ∼1012 MO at z ∼1 in the VELA cosmological zoom-in simulations. We find that the O vi in the inflowing cold streams is primarily photoionized, while in the bulk volume it is primarily collisionally ionized. The photoionized component dominates the observed column density at large impact parameters (≥0.3Rvir), while the collisionally ionized component dominates closer in. We find that most of the collisional O vi, by mass, resides in the relatively thin boundaries of the photoionized streams. Thus, we predict that a reason previous work has found the ionization mechanism of O vi so difficult to determine is because the distinction between the two methods coincides with the distinction between two significant phases of the CGM. We discuss how the results are in agreement with analytic predictions of stream and boundary properties, and their compatibility with observations. This allows us to predict the profiles of O vi and other ions in future CGM observations and provides a toy model for interpreting them.