The COS Absorption Survey of Baryon Harbors: Unveiling the Physical Conditions of Circumgalactic Gas through Multiphase Bayesian Ionization Modeling

TL;DR

This study uses Bayesian multiphase ionization modeling to analyze quasar absorption systems, revealing complex, multi-temperature gas phases and challenging simple single-phase models, thereby improving understanding of circumgalactic media.

Contribution

Develops a Bayesian framework for multiphase ionization modeling that accounts for diverse ions and physical conditions in circumgalactic gas, surpassing traditional single-phase approaches.

Findings

At least two ionization phases are needed to explain observations.

Some ions originate from multiple phases within a single absorption component.

Collisionally ionized phases help achieve thermal pressure equilibrium.

Abstract

Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g., Mg II, S II--S VI, O II--O VI, Ne VIII), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modeling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favor three) distinct ionization phases ranging from $T \approx 10^{4}$ K photoionized gas to warm-hot phases at $T \lesssim 10^{5.8}$ K. For some ions, an apparently single absorption "component" includes contributions from more than one phase, and up to 30% of the H I is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with sub-solar N/$\alpha$, and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model.