OVI Traces Photoionized Streams With Collisionally Ionized Boundaries in Cosmological Simulations of $z \sim 1$ Massive Galaxies

TL;DR

This study uses cosmological simulations to analyze the ionization states and distribution of OVI in the circumgalactic medium of massive galaxies at redshift ~1, revealing the roles of photoionization and collisional ionization in different regions.

Contribution

It provides a detailed analysis of OVI ionization mechanisms and spatial distribution in simulated galaxy halos, linking simulation results with analytic models and observations.

Findings

OVI in inflowing streams is mainly photoionized.

Collisionally ionized OVI dominates closer to the galaxy.

Most collisional OVI resides in stream boundaries.

Abstract

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.