Constraining the cosmic-ray ionization rate and spectrum with NIR spectroscopy of dense clouds
Shmuel Bialy, Sirio Belli, M. Padovani
Abstract
Context. Low-energy cosmic rays (CRs) control the thermo-chemical state and the coupling between gas and magnetic fields in dense molecular clouds. However, current estimates of the low-energy CR spectrum ( E ≲ 1 GeV) and the associated CR ionization rate are highly uncertain. Aims. We apply, for the first time, a new method for constraining the CR ionization rate and the CR spectral shape using H 2 rovibrational lines from cold molecular clouds. Methods. Using the MMIRS instrument on the MMT, we obtained deep near-infrared (NIR) spectra in six positions within four dense cores, namely, G150, G157, G163, G198, with column densities of N H 2 ≈ 10 22 cm −2 . Results. We derived 3 σ upper limits on the H 2 (1 − 0)S(0) line (2.22 μm) brightness in the range I = 5.9 × 10 −8 to 1.2 × 10 −7 erg cm −2 s −1 sr −1 for the different targets. Using both an analytic model and a numerical model of CR propagation, we convert these into upper limits on the CR ionization rate in the clouds’ interior, ζ = 1.5 to 3.6 × 10 −16 s −1 , and lower limits on the low-energy spectral slope of interstellar CR protons, α = −0.97 to −0.79. We show that while MMT was unable to detect the H 2 lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H 2 lines, making it the ideal method for constraining the otherwise elusive low-energy CRs and shedding light on the sources and propagation modes of CRs.