Distinguishing between photoionized and collisionally ionized gas in the circumgalactic medium

Abstract

ABSTRACT Most studies of highly ionized plasmas have historically assumed that ions are either in photoionization equilibrium (PIE) or in collisional ionization equilibrium (CIE), sometimes including multiple phases with different relevant mechanisms. Simulation analysis packages, on the other hand, tend to use pre-computed ion fraction tables that include both mechanisms, among others. Focusing on the low-density, high-temperature phase space likely to be most relevant in the circumgalactic medium, in this work we show that most ions can be classified as ‘PI’ or ‘CI’ on an ion-by-ion basis. This means that for a cloud at a particular point in phase space, some ions will be created primarily by PI and others by CI, with other mechanisms playing only very minor roles. Specifically, we show that ions are generally CI if the thermal energy per particle is greater than ∼6 per cent of their ionization energy, and PI otherwise. We analyse the accuracy of this ansatz compared to usual PIE/CIE calculations, and show the surprisingly minor dependence of this conclusion on redshift and ionizing background.