Distinguishing between photoionized and collisionally ionized gas in the circumgalactic medium

Abstract

Most studies of highly ionized plasmas have historically assumed ions are either in photoionization equilibrium, PIE, or collisional ionization equilibrium, CIE, sometimes including multiple phases with different relevant mechanisms. Simulation analysis packages, on the other hand, tend to use precomputed ion fraction tables which 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.

Cosmic metal invaders: Intergalactic O VII as a tracer of the warm-hot intergalactic medium within cosmic filaments in the EAGLE simulation

“…In these conditions O vii is mainly produced via collisional ionisation (e.g. Strawn et al 2023). The demonstration that the oxygen density also peaks in these regions (see Sect.…”

“…Towards lower densities, the UV background photon density per particle increases and thus a larger fraction of O vii is produced via photoionisation (e.g. Nelson et al 2018;Wijers et al 2019;Strawn et al 2023). However, only the high-energy tail of the UV background photons exceeds the ionisation energy of O vii (Haardt & Madau 2001), and therefore this effect does not compensate for the inefficiency of CIE in the production of O vii.…”

The AGORA High-resolution Galaxy Simulations Comparison Project. VI. Similarities and Differences in the Circumgalactic Medium

“…Each phaseplot is repeated four times vertically, and plotted over each are the 1% and 20% ion fraction contours for the four ions being analyzed in this work. As argued in Strawn et al (2021) and Strawn et al (2023a), in the horizontal "upper" ridge, each ion should be considered collisionally ionized (CI), and in the diagonal "lower" ridge, it should be considered photoionized (PI). 53 The data in Galbiati et al (2023) are generally reported as equivalent widths rather than column densities.…”