2002ApJ...577..691H

We present a unified analysis of the O VI absorption lines seen in the disk and halo of the Milky Way, high-velocity clouds, the Magellanic Clouds, starburst galaxies, and the intergalactic medium. We show that these disparate systems define a simple relationship between the O VI column density and absorption-line width that is independent of the oxygen abundance over the range O/H∼10% to twice solar. We show that this relation is exactly that predicted theoretically as a radiatively cooling flow of hot gas passes through the coronal temperature regime–independent of its density or metallicity (for O/H≳0.1 solar). Since most of the intergalactic O VI clouds obey this relation, we infer that they cannot have metallicities less than a few percent solar. In order to be able to cool radiatively in less than a Hubble time, the intergalactic clouds must be smaller than ∼1 Mpc in size. The implied global heating rate of the warm/hot intergalactic medium is consistent with available sources. We show that the cooling column densities for the O IV, O V, Ne V, and Ne VI ions are comparable to those seen in O VI. This is also true for the Li-like ions Ne VIII, Mg X, and Si XII (if the gas is cooling from T≳10⁶ K). All these ions have strong resonance lines in the extreme-ultraviolet spectral range and would be accessible to Far Ultraviolet Spectroscopic Explorer at z≳0.2-0.8. We also show that the Li-like ions can be used to probe radiatively cooling gas at temperatures 1 order of magnitude higher than where their ionic fraction peaks. We calculate that the H-like (He-like) O, Ne, Mg, Si, and S ions have cooling columns of ∼10¹⁷ (a few times 10¹⁶) cm^–2. The properties of the O VII, O VIII, and Ne IX X-ray absorption lines toward PKS 2155-304 may be consistent with a scenario of radiatively cooling gas in the Galactic disk or halo.