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

We analyse the distribution and origin of Ovi in the Circumgalactic Medium (CGM) of dark-matter haloes of similar to 10(12) M-circle dot at z similar to 1 in the VELA cosmological zoom-in simulations. We find that the Ovi 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 (greater than or similar to 0.3R(vir)), while the collisionally ionized component dominates closer in. We find that most of the collisional Ovi, 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 Ovi 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 Ovi and other ions in future CGM observations and provides a toy model for interpreting them.

Automated summary

In the VELA cosmological zoom-in simulations, the distribution and origin of Ovi in the CGM of dark-matter haloes are analyzed, showing that the ionization of Ovi is primarily through photoionization and collisional ionization. The distinction between the two ionization mechanisms coincides with different phases of the CGM, which provides insights into predicting the profiles of Ovi and other ions in future CGM observations.