ar X iv : a st ro - p h / 02 05 55 6 v 1 3 1 M ay 2 00 2 On the Physical Origin of O vi Absorption - Line Systems

We present a unified analysis of the Ovi 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 Ovi 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 Ovi clouds obey this relation, we infer that they can not 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 IGM is consistent with available sources. We show that the cooling column densities for the Oiv, Ov, Nev, and Nevi ions are comparable to those seen in Ovi. This is also true for the Li-like ions Neviii, Mgx, and Sixii (if the gas is cooling from T 10 6 K). All these ions have strong resonance lines in the extreme-ultraviolet spectral range, and would be accessible to F USE at z 0.2 to 0.8. We also show that the Li-like ions can be used to probe radiatively cooling gas at temperatures an 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 ∼ 10 17 (few×10 16) cm −2. The properties of the Ovii, Oviii, and Neix X-ray absorption-lines towards PKS 2155-304 may be consistent with a scenario of radiatively cooling gas in the Galactic disk or halo.