H₂ absorption in a dense interstellar filament in the Milky Way halo

¹ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Center for Astrophysics & Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA

Corresponding author: P. Richter, richter@arcetri.astro.it

Received: 6 March 2003
Accepted: 12 May 2003

Abstract

We investigate interstellar absorption from molecular hydrogen (H₂) 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. H₂ absorption from the Lyman and Werner bands is detected in 30 lines at radial velocities  km s^-1 in this IVC that is presumably located in the Milky Way halo. We obtain a total logarithmic H₂ column density of log H along with a very low Doppler parameter of  km s^-1. The presence of molecular material in this cloud is suprising, given the fact that the O i column density (log O i) implies a very low neutral gas column density of  cm^-2 (assuming a solar oxygen abundance). If the H₂ 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, H i 21 cm absorption, and in CO emission.