A detection of H${}_2$ in a high velocity cloud toward the Large Magellanic Cloud

Abstract

This work presents a new detection of H${}_2$ absorption arising in a high velocity cloud (HVC) associated with either the Milky Way or the Large Magellanic Cloud (LMC). The absorber was found in an archival Far Ultraviolet Spectroscopic Explorer spectrum of the LMC star Sk-70${}^{\circ }$32. This is the fifth well-characterized H${}_2$ absorber to be found in the Milky Way's halo and the second such absorber outside the Magellanic Stream and Bridge. The absorber has a local standard of rest central velocity of $+$140 km s${}^{-1}$ and a H${}_2$ column density of $10^{17.5}$ cm${}^{-2}$. It is most likely part of a cool and relatively dense inclusion ($T\approx 75$ K, $n_{\mathrm{H}}\sim 100$ cm${}^{-3}$) in a warmer and more diffuse halo cloud. This halo cloud may be part of a still-rising Milky Way Galactic fountain flow or an outflow from the Large Magellanic Cloud.

Accepted to ApJ. 13 pages, 6 figures

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• ALMACAL X: Constraints on molecular gas in the low-redshift circumgalactic medium
• The Baryonic Mass Estimates of the Milky Way Halo in the form of High Velocity Clouds
• One H2 molecule per ten million H-atoms reveals sub-pc scale cold overdensities at z~4