Feeding the fire: Tracing the mass-loading of 10⁷ K galactic outflows with O vi absorption

Galactic outflows regulate the amount of gas galaxies convert into stars. However, it is difficult to measure the mass outflows remove because they span a large range of temperatures and phases. Here, we study the rest-frame ultraviolet spectrum of a lensed galaxy at z ~ 2.9 with prominent interstellar absorption lines from O i, tracing neutral gas, up to O vi, tracing transitional phase gas. The O vi profile mimics weak low-ionization profiles at low velocities, and strong saturated profiles at high velocities. These trends indicate that O vi gas is co-spatial with the low-ionization gas. Further, at velocities blueward of -200 km s^-1 the column density of the low-ionization outflow rapidly drops while the O vi column density rises, suggesting that O vi is created as the low-ionization gas is destroyed. Photoionization models do not reproduce the observed O vi, but adequately match the low-ionization gas, indicating that the phases have different formation mechanisms. Photoionized outflows are more massive than O vi outflows for most of the observed velocities, although the O vi mass outflow rate exceeds the photoionized outflow at velocities above the galaxy's escape velocity. Therefore, most gas capable of escaping the galaxy is in a hot outflow phase. We suggest that the O vi absorption is a temporary by-product of conduction transferring mass from the photoionized phase to an unobserved hot wind, and discuss how this mass-loading impacts the observed circum-galactic medium.