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

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.

💡 Research Summary

This study presents a novel and extensive survey of molecular hydrogen (H2) pure rotational line emission in the diffuse interstellar medium (ISM) of our Galaxy, leveraging serendipitous data from the James Webb Space Telescope (JWST). The core methodology involved repurposing the typically discarded background observations taken by the Mid-Infrared Instrument’s Medium Resolution Spectrometer (MIRI-MRS). Instead of using these observations solely for sky subtraction in the standard pipeline, the team independently reprocessed 276 uncalibrated background files to create science-ready spectral cubes, effectively turning calibration data into a primary scientific dataset.

The primary goal was to build a statistically significant sample of H2 rotational excitation in diffuse gas to understand its driving mechanisms (collisional vs. ultraviolet fluorescent pumping) and assess H2’s role in cooling the ISM and regulating thermal pressure. To focus on the diffuse ISM, lines of sight likely intersecting star-forming complexes were filtered out based on total hydrogen column density and dust temperature, resulting in a final sample of 233 directions.

The analysis yielded several groundbreaking results. The study reports the first detections of the pure rotational S(4), S(5), and S(7) lines of H2 in the diffuse ISM, transitions that had previously eluded detection. The more commonly observed S(1) line was detected along 84 lines of sight, corresponding to a 41% detection rate. The integrated intensity of the S(1) line exhibits a steep decline with increasing Galactic latitude, spanning nearly two orders of magnitude. This trend shows remarkable quantitative agreement with intensities inferred from independent FUSE absorption measurements, validating the emission analysis.

The excitation temperatures derived from different rotational level ratios (e.g., T34, T35) range from 200 to ~1000 K. They are correlated with each other and show the expected anticorrelation with H2 column density, consistent with ancillary data. All lines of sight in the sample have undergone the atomic-to-molecular (H-H2) transition, with total hydrogen column densities N_H ≳ 10^20 cm^-2, and are partially molecular (f_H2 ≳ 0.1). Under these conditions, a key finding is that the cooling rate per hydrogen atom associated with the S(1) line is remarkably constant across the sample, with a characteristic value of ~4×10^-27 erg s^-1 H^-1. This suggests a regulated cooling process in the diffuse ISM that may be governed by large-scale Galactic properties rather than local variations.

In conclusion, this work demonstrates the transformative power of JWST’s sensitivity, enabling emission-line measurements that both reinforce and complement findings from traditional absorption studies. By mining data collected over just a fraction of JWST’s operational lifetime, the research has significantly expanded the statistical sample of H2 rotational excitation in the diffuse ISM. This new dataset provides crucial constraints for models of multiphase turbulent gas, the dissipation of mechanical energy, and the thermal balance of the interstellar medium on Galactic scales.