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

Abstract

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 OI, tracing neutral gas, up to OVI, tracing transitional phase gas. The OVI profile mimics weak low-ionization profiles at low velocities, and strong saturated profiles at high velocities. These trends indicate that OVI 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 OVI column density rises, suggesting that OVI is created as the low-ionization gas is destroyed. Photoionization models do not reproduce the observed OVI, but adequately match the low-ionization gas, indicating that the phases have different formation mechanisms. Photoionized outflows are more massive than OVI outflows for most of the observed velocities, although the OVI 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 OVI 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.