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

Abstract

Context. A statistically significant sampling of H$_2$ rotational excitation in the diffuse interstellar medium (ISM) is essential to identifying its excitation mechanisms and assessing the importance of H$_2$ in the cooling of the gas and the regulation of thermal pressure. Aims. To complement the statistics provided by ancillary telescopes, we conducted 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. Methods. 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$_2$ 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$_2$ and to dust emission derived from HI4PI, Planck, and WISE data. Results. 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 $T_{34}$ and $T_{35}$ excitation temperatures vary between 200 and $\sim$1000 K, are correlated with each other, and are anticorrelated with the column density of H$_2$ , as expected from ancillary data. All lines of sight in the sample have undergone the H-H$_2$ transition, at $N_{\rm{H}} \gtrsim 10^{20} \ \rm{cm}^{-2}$, and are partly molecular, with $f_{\rm H_2} \gtrsim 0.1$. 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 $\sim 4\times10^{-27}$ erg s$^{-1}$ H$^{-1}$. Conclusions. 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.

Figures (7)

• Figure 1: Aitoff projection of the MRS background lines of sight analyzed in this paper. Filled circles indicate detections of the 0--0 S(1) line of $\rm H_2$, color-coded by integrated intensity, while triangles show non-detections. All lines of sight are overlaid on the total hydrogen column density map derived from the dust opacity at 353 GHz measured by PlanckPlanck2014.
• Figure 2: Integrated intensities of the 0--0 S(1) line of $\rm H_2$ as a function of the absolute Galactic latitude, $|b|$. Green squares and blue stars correspond to values inferred from absorption measurements toward extragalactic targets and nearby OB stars observed with FUSE Wakker2006aShull2021a, using Eq. \ref{['Eq:cdint']} and the column densities of the $J=3$ rotational level. Yellow triangles show measurements obtained with Spitzer by Ingalls2011a. Gray and red points represent intensities measured along flagged and un-flagged MRS background lines of sight, respectively, while red arrows indicate upper limits set by the sensitivity of the MRS over the wide range of exposure times in the sample. The dashed line shows a geometrically motivated analytical estimate of the maximum expected emission (Eq. \ref{['Eq:max']}).
• Figure 3: Excitation diagrams of $\rm H_2$ derived along the MRS background lines of sight, excluding the flagged data. The column densities of each rotational level are derived from the corresponding line intensities using Eq. \ref{['Eq:cdint']} and are normalized and color-coded by the $\rm H_2$ column density estimated in Appendix \ref{['append:ancillary']}. For visual clarity, data points are slightly offset along the energy axis by small random values. Detections, along a given line of sight, of all successive rotational transitions up to at least the S(3) line are connected by segments.
• Figure 4: Excitation temperature $T_{34}$ between the $J=3$ and $J=4$ levels as a function of the excitation temperature $T_{35}$ between the $J=3$ and $J=5$ levels. Green squares and blue stars correspond to values inferred from absorption measurements toward extragalactic targets and nearby OB stars observed with FUSE Wakker2006aShull2021a. Red points and arrows show measurements and upper limits, respectively, obtained along MRS background lines of sight after excluding flagged data. The dashed black line shows a power-law relation derived between the two temperatures (Eq. \ref{['eq:T34T35']}), and the shaded region is an envelope obtained by varying each fit parameter independently within its marginalized $\pm 3\sigma$ uncertainty, which encloses 78% of the data points.
• Figure 5: Flux of the 0--0 S(1) line of $\rm H_2$ as a function of the total hydrogen column density, $N_{\rm H}$ (Table \ref{['tab:ancillary']}). Filled squares correspond to values inferred from absorption measurements toward extragalactic targets observed with FUSE Wakker2006a. Filled circles show intensities measured along MRS background lines of sight after excluding flagged data. Points are color-coded by the ratio of the S(1) integrated intensity to the PAH surface brightness at 12 $\mu$m measured by WISE (Table \ref{['tab:ancillary']}). The three dotted gray lines indicate constant values of the S(1) cooling rate per hydrogen atom. The dashed red line shows a fit to the S(1) cooling rate derived from the MRS sample only (Eq. \ref{['Eq:cool']}).