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Quasar absorption systems encode a wealth of information about theabundances, ionization structure, and physical conditions in intergalactic andcircumgalactic media. Simple (often single-phase) photoionization models arefrequently used to decode such data. Using five discrete absorbers from the COSAbsorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range ofdetected ions (e.g., Mg II, S II--S VI, O II--O VI, Ne VIII), we show severalexamples where single-phase ionization models cannot reproduce the full set ofmeasured column densities. To explore models that can self-consistently explainthe measurements and kinematic alignment of disparate ions, we develop aBayesian multiphase ionization modeling framework that characterizes discretephases by their unique physical conditions and also investigates variations inthe shape of the UV flux field, metallicity, and relative abundances. Ourmodels require at least two (but favor three) distinct ionization phasesranging 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% ofthe H I is not from the lowest ionization phase. If we assume that all of thephases are photoionized, we cannot find solutions in thermal pressureequilibrium. By introducing hotter, collisionally ionized phases, however, wecan achieve balanced pressures. The best models indicate moderatemetallicities, often with sub-solar N/$\alpha$, and, in two cases, ionizingflux fields that are softer and brighter than the fiducial Haardt & Madau UVbackground model.

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