OVI Traces Photoionized Streams With Collisionally Ionized Boundaries in
  Cosmological Simulations of $z \sim 1$ Massive Galaxies
  We analyse the distribution and origin of OVI in the Circumgalactic Medium
(CGM) of dark-matter haloes of $\sim 10^{12}$ M$_\odot$ at $z\sim1$ 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 ($\gtrsim 0.3 R_{\rm vir}$), 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. 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.
