We combine data from our recent FUSE survey of interstellar molecular hydrogen absorption toward 50 high-latitude AGN with COBE-corrected IRAS 100 \mu m emission maps to study the correlation of infrared cirrus with H _ { 2 } .
A plot of the H _ { 2 } column density vs. IR cirrus intensity shows the same transition in molecular fraction , f _ { H 2 } , as seen with total hydrogen column density , N _ { H } .
This transition is usually attributed to H _ { 2 } “ self-shielding ” , and it suggests that many diffuse cirrus clouds contain H _ { 2 } in significant fractions , f _ { H 2 } \approx 1–30 % .
These clouds cover \sim 50 % of the northern sky at b > 30 ^ { \circ } , at temperature-corrected 100 \mu m intensities D _ { 100 } ^ { ( T ) } \geq 1.5 MJy sr ^ { -1 } .
The sheetlike cirrus clouds , with hydrogen densities n _ { H } \geq 30 cm ^ { -3 } , may be compressed by dynamical processes at the disk-halo interface , and they are conducive to H _ { 2 } formation on grain surfaces .
Exploiting the correlation between N _ { H 2 } and 100 \mu m intensity , we estimate that cirrus clouds at b > 30 ^ { \circ } contain \sim 3000 ~ { } M _ { \odot } in H _ { 2 } .
Extrapolated over the inner Milky Way , the cirrus may contain 10 ^ { 7 } ~ { } M _ { \odot } of H _ { 2 } and 10 ^ { 8 } ~ { } M _ { \odot } in total gas mass .
If elevated to 100 pc , their gravitational potential energy is \sim 10 ^ { 53 } erg .