Molecular Hydrogen in Infrared Cirrus

We combine data from our recent FUSE survey of interstellar molecular hydrogen absorption toward 50 high-latitude AGNs with COBE-corrected IRAS 100 μm emission maps to study the correlation of infrared cirrus with H₂. A plot of the H₂ column density versus IR cirrus intensity shows the same transition in molecular fraction, f, as seen with total hydrogen column density, N_H. This transition is usually attributed to H₂ "self-shielding," and it suggests that many diffuse cirrus clouds contain H₂ in significant fractions, f ≈ 1%-30%. These clouds cover ~50% of the northern sky at b > 30°, at temperature-corrected 100 μm intensities D ≥ 1.5 MJy sr^-1. The sheetlike cirrus clouds, with hydrogen densities n_H ≥ 30 cm^-3, may be compressed by dynamical processes at the disk-halo interface, and they are conducive to H₂ formation on grain surfaces. Exploiting the correlation between N and 100 μm intensity, we estimate that cirrus clouds at b > 30° contain ~3000 M_☉ in H₂. Extrapolated over the inner Milky Way, the cirrus may contain 10⁷ M_☉ of H₂ and 10⁸ M_☉ in total gas mass. If elevated to 100 pc, their gravitational potential energy is ~10⁵³ ergs.