Feeding the fire: Tracing the mass-loading of 10^7 K galactic outflows with O VI absorption

TL;DR

This study investigates the multi-phase structure of galactic outflows at z~2.9 using ultraviolet absorption lines, revealing that O VI traces a hot, escaping wind phase distinct from cooler, photoionized gas.

Contribution

It provides new insights into the formation and mass-loading of galactic outflows by analyzing O VI absorption and its relation to other gas phases, highlighting the role of hot winds in galaxy evolution.

Findings

O VI gas is co-spatial with low-ionization gas at certain velocities.

O VI outflows can exceed the galaxy's escape velocity, indicating potential for gas escape.

O VI absorption is likely a by-product of conduction transferring mass to hot winds.

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 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 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.