Effect of cosmic rays and ionizing radiation on observational ultraviolet plasma diagnostics in the circumgalactic medium

Abstract

The relevance of some galactic feedback mechanisms, in particular cosmic ray feedback and the hydrogen ionizing radiation field, has been challenging to definitively describe in a galactic context, especially far outside the galaxy in the circumgalactic medium (CGM). Theoretical and observational uncertainties prevent conclusive interpretations of multiphase CGM properties derived from ultraviolet (UV) diagnostics. We conduct three dimensional magnetohydrodynamic simulations of a section of a galactic disk with star formation and feedback, including radiative heating from stars, a UV background, and cosmic ray feedback. We utilize the temperature phases present in our simulations to generate Cloudy models to derive spatially and temporally varying synthetic UV diagnostics. We find that radiative effects without additional heating mechanisms are not able to produce synthetic diagnostics in the observed ranges. For low cosmic ray diffusivity ${\kappa }_{\mathrm{cr}}=10^{28}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, cosmic ray streaming heating in the outflow helps our synthetic line ratios roughly match observed ranges by producing transitional temperature gas ($T\sim 10^{5-6}$ K). High cosmic ray diffusivity ${\kappa }_{\mathrm{cr}}=10^{29}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, with or without cosmic ray streaming heating, produced transitional temperature gas. The key parameter controlling the production of this gas phase remains unclear, as the different star formation history and outflow evolution itself influences these diagnostics. Our work demonstrates the use of UV plasma diagnostics to differentiate between galactic/circumgalactic feedback models.

Published in MNRAS

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