Forming Molecular Hydrogen in a Galaxy Halo Through Star Formation Wind Interactions

Abstract

Molecular hydrogen (H${}_2$) is the key ingredient for star formation in galaxies. However, how H${}_2$ is distributed in and around galaxies is an open question in astronomy. We report the detection of H${}_2$ (covering excitation levels $J=0$ to 3) in a unique environment at a distance of 139 kpc from a starbursting luminous infrared galaxy. This is the furthest from a galaxy that H${}_2$ has been directly detected to date. This H${}_2$ system is unlike any seen before, with a high molecular-to-atomic hydrogen ratio (2%) and an accompanying low neutral hydrogen column density (log${}_{10}[N($H I$)/{\mathrm{cm}}^{-2}]=18.52_{-3.58}^{+0.13}$). Our measurements reveal that the H${}_2$ bearing cloud is traveling at $\sim$100 km s${}^{-1}$ relative to the host galaxy, has a super-solar metallicity, and excites at a single, cold temperature ($245_{-21}^{+25}$ K) indicating a lack of the ''typical'' core-envelope structure seen in other H${}_2$ clouds. Furthermore, our analysis points to the H${}_2$ likely being formed through interactions between galactic winds and the gaseous halo surrounding the galaxy. The properties of this cloud suggest that the H${}_2$ is short-lived, yet it presents crucial information on the microphysics of the wind-CGM interactions.

14 pages, 3 figures, 2 tables, Accepted for publication in ApJL