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

Brad Koplitz; Sanchayeeta Borthakur; Timothy Heckman; Frances H. Cashman; Evan Scannapieco; Jason Tumlinson; Hsiao-Wen Chen; Molly Peeples

arXiv · 2610.06662

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

Brad Koplitz·Sanchayeeta Borthakur·Timothy Heckman·Frances H. Cashman·Evan Scannapieco·Jason Tumlinson·Hsiao-Wen Chen·Molly Peeples

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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$)/\mathrm{cm}^{-2}] = 18.52^{+0.13}_{-3.58}$). 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 ($245^{+25}_{-21}$ 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.

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Brad Koplitz, Sanchayeeta Borthakur, Timothy Heckman, Frances H. Cashman, Evan Scannapieco, Jason Tumlinson, Hsiao-Wen Chen, Molly Peeples. 2026-10-05. Forming Molecular Hydrogen in a Galaxy Halo Through Star Formation Wind Interactions. https://arxiv.org/abs/2610.06662

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