H 2 absorption in a dense interstellar filament in the Milky Way halo

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 (upsilon)(LSR) ~~ +50 km s^(-1) in this IVC that is presumably located in the Milky Way halo. We obtain a total logarithmic H 2 column density of log  N(H2)= 16.6 +- 0.5 along with a very low Doppler parameter of b=1.5(+0.8)(-0.2) km s^(-1). The presence of molecular material in this cloud is suprising, given the fact that the O I column density (log N(O I)= 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, H I 21 cm absorption, and in CO emission.