2006ApJ...636..908G

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 H₂column density versus IR cirrus intensity shows the same transition in molecular fraction, f_H2, 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 H₂in significant fractions, f_H2~1%-30%. These clouds cover ∼50% of the northern sky at b>30°, at temperature-corrected 100 µm intensities D^T₁₀₀≥1.5 MJy/sr. The sheetlike cirrus clouds, with hydrogen densities n_H≥30/cm3, may be compressed by dynamical processes at the disk-halo interface, and they are conducive to H₂formation on grain surfaces. Exploiting the correlation between N_H2and 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 H₂and 10⁸ M_☉in total gas mass. If elevated to 100 pc, their gravitational potential energy is ∼10⁵³ ergs.