The H2 Glow of a Quiescent Molecular Cloud Observed with JWST

We report JWST MIRI/MRS observations of the H2 S(1) 17.04 μm transition in two regions in the boundary of the Taurus Molecular Cloud. The “Edge” (near the relatively sharp boundary of the 13CO J = 1 → 0 emission) and “Peak” (the location of the strongest H2 emission observed with Spitzer) regions have average intensities of 14.5 MJy sr−1 and 32.1 MJy sr−1. We find small scale structures of characteristic size 1.0′′–2.5′′, corresponding to 140 AU–350 AU, with characteristic intensity above the extended background of 10 MJy sr−1, corresponding to a J = 3 column density of 1.6×1017 cm−2. The most plausible explanation for the observed intensities from level 845 K above the ortho–H2 ground state is excitation by collisions with H2 molecules (the hydrogen in this region being predominantly molecular). Two mechanisms, turbulent dissipation and shocks, have been proposed for heating of localized regions of the ISM to temperatures ≃1000 K to explain abundances of and emission from particular molecules. While we cannot determine unique values of density and kinetic temperature, the solutions in best agreement with predictions of shock models are n(H2) = 370 cm−3 and Tkin = 1000 K. The total H2 column density of the small–scale structures under these conditions is ≃8×1017 cm−2. This first direct detection of tiny scale structure in the quiescent molecular interstellar medium has significant implications for the physical structure of this phase of the ISM and the maintaining of supersonic motions within it.