On the Physical Origin of O vi Absorption‐Line Systems

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 intregalactic O{ vi} 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. 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 $FUSE$ 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 ~10¹⁷ cm⁻². The O{ vii}, O{ viii}, and Ne{ ix} X-ray absorption-lines towards PKS 2155-304 may arise in radiatively cooling gas in the Galactic disk or halo.