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H_2 absorption in a dense interstellar filament in the Milky Way halo

P. Richter·K. R. Sembach·J. C. Howk
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Abstract

We investigate interstellar absorption from molecular hydrogen (H_2) and metals in an intermediate-velocity cloud (IVC) in the direction of the LMC star Sk -68 80 (HD 36521), based on data from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. H_2 absorption from the Lyman- and Werner bands is detected in 30 lines at radial velocities v=+50 km/s in this IVC that is presumably located in the Milky Way halo. We obtain a total logarithmic H_2 column density of log N(H_2)=16.6 (0.5) along with a very low Doppler parameter of b=1.5 km/s. The presence of molecular material in this cloud is suprising, given the fact that the OI column density (log N(OI)=14.8 (0.1)) implies a very low neutral gas column density of ~10^{18} cm^-2 (assuming a solar oxygen abundance). If the H_2 column density represents its abundance in a formation-dissociation equilibrium, the data imply that the molecular gas resides in a small, dense filament at a volume density of ~800 cm^-3 and a thickness of only 41 Astronomical Units (AU). The molecular filament possibly corresponds to the tiny-scale atomic structures (TSAS) in the diffuse interstellar medium observed in high-resolution optical data, HI 21cm absorption, and in CO emission.

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P. Richter, K. R. Sembach, J. C. Howk. 2003-05-01. H_2 absorption in a dense interstellar filament in the Milky Way halo. https://doi.org/10.1051/0004-6361%3A20030722

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