Spitzer IRS Detection of Molecular Hydrogen Rotational Emission Towards
  Translucent Clouds
  Using the Infrared Spectrograph on board the Spitzer Space Telescope, we have
detected emission in the S(0), S(1), and S(2) pure-rotational (v=0-0)
transitions of molecular hydrogen (H2) towards 6 positions in two translucent
high Galactic latitude clouds, DCld 300.2-16.9 and LDN 1780. The detection of
these lines raises important questions regarding the physical conditions inside
low-extinction clouds that are far from ultraviolet radiation sources. The
ratio between the S(2) flux and the flux from PAHs at 7.9 microns averages
0.007 for these 6 positions. This is a factor of about 4 higher than the same
ratio measured towards the central regions of non-active Galaxies in the
Spitzer Infrared Nearby Galaxies Survey (SINGS). Thus the environment of these
translucent clouds is more efficient at producing rotationally excited H2 per
PAH-exciting photon than the disks of entire galaxies. Excitation analysis
finds that the S(1) and S(2) emitting regions are warm (T >300K), but comprise
no more than 2% of the gas mass. We find that UV photons cannot be the sole
source of excitation in these regions and suggest mechanical heating via shocks
or turbulent dissipation as the dominant cause of the emission. The clouds are
located on the outskirts of the Scorpius-Centaurus OB association and may be
dissipating recent bursts of mechanical energy input from supernova explosions.
We suggest that pockets of warm gas in diffuse or translucent clouds,
integrated over the disks of galaxies, may represent a major source of all
non-active galaxy H2 emission.
