Molecular Hydrogen in Infrared Cirrus

We combine data from our recent FUSE survey of interstellar molecular hydrogen absorption toward 50 high-latitude AGN with COBE-corrected IRAS 100 micron emission maps to study the correlation of infrared cirrus with H2. A plot of the H2 column density vs. IR cirrus intensity shows the same transition in molecular fraction, fH2, as seen with total hydrogen column density, NH. This transition is usually attributed to H2 self-shielding, and it suggests that many diffuse cirrus clouds contain H2 in significant fractions, fH2 = 1-30%. These clouds cover approximately 50% of the northern sky at latitudes b > 30 degrees, at temperature-corrected 100 micron intensities D₁00 > 1. 5 MJy/sr. The sheetlike cirrus clouds, with hydrogen densities nH > 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 (H2) and 100 micron intensity, we estimate that cirrus clouds at b > 30 contain approximately 3000 Mₛun in H2. Extrapolated over the inner Milky Way, the cirrus may contain 10⁷ Mₛun of H2 and 10⁸ Mₛun in total gas mass. If elevated to 100 pc, their gravitational potential energy is ~10⁵3 erg.