Letter to the Editor

Constraining the cosmic-ray ionization rate and spectrum with NIR spectroscopy of dense clouds

A testbed for JWST

¹ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
e-mail: sbialy@umd.edu
² Harvard Smithsonian Center for Astrophysics, 60 Garden st., Cambridge, MA 02138, USA
³ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy

Received: 8 November 2021
Accepted: 14 January 2022

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 H₂ 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_H2 ≈ 10²² cm⁻².

Results. We derived 3σ upper limits on the H₂ (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 H₂ lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H₂ 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.