# The first detection of cosmic-ray excited H$_2$ in interstellar space

## TL;DR

This paper reports the first direct detection of cosmic-ray excited H$_2$ emission in a molecular cloud, enabling direct measurement of cosmic-ray ionization rates and advancing understanding of star formation and galaxy evolution.

## Contribution

It provides the first observational confirmation of cosmic-ray excitation of H$_2$, allowing direct measurement of cosmic-ray ionization rates in molecular clouds.

## Key findings

- Detection of vibrational H$_2$ emission matching theoretical predictions
- Confirmation of cosmic-ray excitation as a dominant process in dense clouds
- Enabling direct measurement of cosmic-ray ionization rate $6$

## Abstract

Stars and planets form within cold, dark molecular clouds. In these dense regions, where starlight cannot penetrate, cosmic rays (CRs) are the dominant source of ionization -- driving interstellar chemistry(Dalgarno (2006, PNAS, 103, 12269)), setting the gas temperature(Goldsmith et al. (1969, ApJ, 158, 173)), and enabling coupling to magnetic fields(McKee & Ostriker (2007, ARA&A, 45, 565; arXiv:0707.3514)). Together, these effects regulate the collapse of clouds and the onset of star formation. Despite this importance, the cosmic-ray ionization rate, $\zeta$, has never been measured directly. Instead, this fundamental parameter has been loosely inferred from indirect chemical tracers and uncertain assumptions, leading to published values that span nearly two orders of magnitude and limiting our understanding of star formation physics. Here, we report the first direct detection of CR-excited vibrational H$_2$ emission, using \textit{James Webb Space Telescope} (JWST) observations of the starless core Barnard 68 (B68). The observed emission pattern matches theoretical predictions for CR excitation precisely, confirming a decades-old theoretical proposal long considered observationally inaccessible. This result enables direct measurement of $\zeta$, effectively turning molecular clouds into natural, light-year-sized, cosmic-ray detectors. It opens a transformative observational window into the origin, propagation, and role of cosmic rays in star formation and galaxy evolution.