Turning JWST/MIRI backgrounds into a survey of diffuse molecular hydrogen

A statistically significant sampling of ̋H rotational excitation in the diffuse interstellar medium (ISM) is essential to identifying its excitation mechanisms and assessing the importance of ̋H in the cooling of the gas and the regulation of thermal pressure. To complement the statistics provided by ancillary telescopes, we conducted a search for pure rotational ̋H emission lines in all publicly available background observations obtained with the Medium Resolution Spectrometer (MRS) aboard the JWST. The sample consists of 276 background observations acquired over the past three years. Departing from the standard pipeline, each uncalibrated MRS background file was reprocessed, enabling the analysis of ̋H pure rotational emission. Lines of sight likely associated with star-forming complexes were excluded to focus on emission from the diffuse ISM. The results were compared with FUSE absorption data and were analyzed in relation to the column densities of H and ̋H and to dust emission derived from HI4PI, Planck, and WISE data. This analysis reveals widespread ̋H emission throughout the Galaxy. We report the first detections of the pure rotational S (4), S (5), and S (7) lines in the diffuse ISM. The S (1) line is detected along 84 lines of sight, corresponding to a detection rate of 41%. Its integrated intensity decreases steeply with Galactic latitude, spanning nearly two orders of magnitude, in remarkable agreement with absorption measurements. The T₃4 and T₃5 excitation temperatures vary between 200 and ∼1000 K, are correlated with each other, and are anticorrelated with the column density of ̋H, as expected from ancillary data. All lines of sight in the sample have undergone the H--̋H transition, at H ≳ 10^ 20 cm ^ -2, and are partly molecular, with f_ ̋H ≳ 0. 1. Under these conditions, the cooling rate associated with the S (1) line, expressed per hydrogen atom, is found to be remarkably constant, with a characteristic value of ∼ 4 erg s -27 -1 H^ -1. This study demonstrates that the high sensitivity of the JWST enables measurements that both strengthen and complement those from absorption studies. Observations collected over just a fraction of JWST’s lifetime have already yielded detections along dozens of lines of sight, significantly expanding the statistical sample of ̋H rotational excitation in the diffuse ISM.