2026-06-03T07:40:43Zivo://CDS.VizieR/J/A+A/709/A1522026-05-19T14:14:26ZJ.A_AIVOAGalactic diffuse H2 rotational lines with JWSTA statistically significant sampling of H_2_ rotational excitation in the diffuse interstellar medium (ISM) is essential to identify its dominant excitation mechanisms and assess the importance of H_2_ in the cooling of the gas and the regulation of thermal pressure. To expand upon the statistics provided by ancillary telescopes, we conduct a search for pure rotational H_2_ 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 is reprocessed, enabling the analysis of H_2_ pure rotational emission. Lines of sight likely associated with star-forming complexes are excluded in order to focus on emission from the diffuse ISM. The results are compared with FUSE absorption data and are analyzed in relation to the column densities of H and H_2_ and dust emission derived from HI4PI, Planck, and WISE data. This analysis reveals widespread H_2_ 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 are correlated, and vary between 200 and ~1000K, showing an anticorrelation with the column density of H_2_ as expected from ancillary data. All lines of sight in the sample have undergone the H-H_2_ transition, at NH>~10^+20^cm^-2^, and are partly molecular with f_H2_>~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 ~4x10^-27^erg/s/H. 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_2_ rotational excitation in the diffuse ISM.interstellar-mediumspectroscopyinfrared-astronomyinterdisciplinary-astronomyobservational-astronomyhttps://dc.g-vo.org/rr/q/lp/custom/CDS.VizieR/J/A+A/709/A152https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/709/A1522026A&A...709A.152Nhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/709/A152Cone search capability for table J/A+A/709/A152/tablea1 (Rotational lines detections)Cone search capability for table J/A+A/709/A152/tableb1 (Diffuse medium Galactic properties)Nigou E.Godard B.Guillard P.Pineau des Forets G.,Miville-Deschenes M.A.Lesaffre P.CDS2026https://cds.unistra.fr/vizier-org/licences_vizier.htmlCDS support team <cds-question@unistra.fr>DatasetAstroObjectsastronomy