Galactic diffuse H2 rotational lines with JWST

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.