Ambient Column Densities of Highly Ionized Oxygen in Precipitation-Limited Circumgalactic Media

Abstract: Many of the baryons associated with a galaxy reside in its circumgalactic medium (CGM), in a diffuse volume-filling phase at roughly the virial temperature. Much of the oxygen produced over cosmic time by the galaxy's stars also ends up there. The resulting absorption lines in the spectra of UV and X-ray background sources are powerful diagnostics of the feedback processes that prevent more of those baryons from forming stars. This paper presents predictions for CGM absorption lines (O VI, O VII, O VIII, Ne VIII, N V) that are based on precipitation-regulated feedback models, which posit that the radiative cooling time of the ambient medium cannot drop much below 10 times the freefall time without triggering a strong feedback event. The resulting predictions align with many different observational constraints on the Milky Way's ambient CGM and explain why N_OVI ~ 10^14 cm^-2 over large ranges in halo mass and projected radius. Within the precipitation framework, the strongest O~VI absorption lines result from vertical mixing of the CGM that raises low-entropy ambient gas to greater altitudes, because adiabatic cooling of the uplifted gas then lowers its temperature and raises the fractional abundance of O^5+. Condensation stimulated by uplift may also produce associated low-ionization components. The observed velocity structure of the O VI absorption suggests that galactic outflows do not expel circumgalactic gas at the halo's escape velocity but rather drive circulation that dissipates much of the galaxy's supernova energy within the ambient medium, causing some of it to expand beyond the virial radius.