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 mu m emission maps to study the correlation of infrared cirrus with H-2. A plot of the H2 column density versus IR cirrus intensity shows the same transition in molecular fraction, fH(2), 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 H-2 in significant fractions, fH(2) approximate to 1%-30%. These clouds cover similar to 50% of the northern sky at b > 30 degrees, at temperature-corrected 100 mu m intensities D-T(100) >= 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 NH2 and 100 mu m intensity, we estimate that cirrus clouds at b > 30 degrees contain similar to 3000 M-circle dot in H-2. Extrapolated over the inner MilkyWay, the cirrus may contain 10(7) M-circle dot of H-2 and 10(8) M-circle dot in total gas mass. If elevated to 100 pc, their gravitational potential energy is similar to 10(53) ergs.