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 two regions, denoted “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), have average intensities of 14.5 and 32.1 MJy sr⁻¹, respectively. We find small-scale structures of characteristic size 1 . ″ 0-2 . ″ 5, corresponding to 140-350 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 this level 845 K above the J = 1 ortho-H₂ ground-state level 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 the heating of localized regions of the interstellar medium (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 T_kin = 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 significantly heated tiny-scale structures in the quiescent molecular ISM has significant implications for the physical structure of this phase of the ISM and the maintaining of supersonic motions within it.