Spitzer IRS Detection of Molecular Hydrogen Rotational Emission Towards Translucent Clouds

TL;DR

This study detects molecular hydrogen emission in translucent clouds using Spitzer, revealing that mechanical heating, not UV radiation, likely excites H2, and suggesting such clouds significantly contribute to galaxy-wide H2 emission.

Contribution

First detection of H2 rotational emission in translucent clouds, highlighting mechanical heating as the excitation mechanism and its potential impact on galaxy H2 emission.

Findings

H2 emission detected in 6 positions within translucent clouds.

Mechanical heating likely dominates H2 excitation over UV photons.

Warm H2 gas constitutes less than 2% of the total gas mass.

Abstract

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.