Molecular Hydrogen in Infrared Cirrus

We combine data from our recent FUSE survey of interstellar molecularhydrogen absorption toward 50 high-latitude AGN with COBE-corrected IRAS 100micron emission maps to study the correlation of infrared cirrus with H2. Aplot of the H2 column density vs. IR cirrus intensity shows the same transitionin molecular fraction, f_H2, as seen with total hydrogen column density, N_H.This transition is usually attributed to H2 self-shielding, and it suggeststhat many diffuse cirrus clouds contain H2 in significant fractions, f_H2 =1-30%. These clouds cover approximately 50% of the northern sky at latitudes b> 30 degrees, at temperature-corrected 100 micron intensities D_100 > 1.5MJy/sr. The sheetlike cirrus clouds, with hydrogen densities n_H > 30 cm^-3,may be compressed by dynamical processes at the disk-halo interface, and theyare conducive to H2 formation on grain surfaces. Exploiting the correlationbetween N(H2) and 100 micron intensity, we estimate that cirrus clouds at b >30 contain approximately 3000 M_sun in H2. Extrapolated over the inner MilkyWay, the cirrus may contain 10^7 M_sun of H2 and 10^8 M_sun in total gas mass.If elevated to 100 pc, their gravitational potential energy is ~10^53 erg.Comment: 22 pages, 8 figures, submitted to Ap