UC Santa Cruz

The Circumgalactic Medium (CGM) is highly ionized and very low density, and therefore difficult to observe. This means analysis of it depends very sensitively on our software and definitions used in interpretation, which can be significantly improved by study of observable quantities in either idealized or cosmological simulations. We analyzed the ion fraction tables in CLOUDY, to define photoionized (PI) and collisionally ionized (CI) gas on an ion-by-ion basis, and showed that this definition is very straightforwardly related to the ionization energy of the ion, and that the relevant mechanism is highly insensitive to ionizing background hardness or redshift. We then applied this definition to O VI in massive halos from the cosmological simulation VELA3, showing that O VI is primarily PI in cool inflowing streams, CI on an interface layer between the streams and the bulk, and basically negligible in the bulk. Finally, we used the same tools to analyze the CGM of the eight AGORA halos, which use the same initial conditions but different feedback systems and code architectures, and show that they create significantly different CGMs, both in metal content and gas phase. While this does suggest caution is needed in interpretation of modern cosmological simulations, the AGORA simulations' controlled creation helps us form new insights into interpretation of observable results.