Mapping the Cosmic-Ray Ionization Rate in the Local Galaxy with H$_3^+$

Abstract

Chemistry in diffuse molecular clouds relies primarily on rapid ion-molecule reactions. Formation of the initial ions, H$^+$ and H$_2^+$, is dominated by cosmic-ray ionization of H and H$_2$, making the cosmic-ray ionization rate (denoted $\zeta({\rm X})$ for species X) an important parameter for chemical modeling. We have made observations targeting absorption lines of H$_3^+$, one of the most reliable tracers of $\zeta({\rm H_2})$, toward diffuse molecular cloud sight lines where the H$_2$ column density has been directly measured in the ultraviolet, detecting H$_3^+$ in 12 out of 27 sight lines. The 3D-PDR modeling method introduced by Obolentseva et al. (2024) was used to infer cosmic-ray ionization rates in the clouds along these sight lines, and our combined sample has a mean ionization rate of $5.3\times10^{-17}$ s$^{-1}$ with standard deviation $2.5\times10^{-17}$ s$^{-1}$. By associating H$_3^+$ absorption with gas density peaks derived from the differential extinction maps of Edenhofer et al. (2024) we have constructed a sparsely sampled 3D map of the cosmic-ray ionization rate in targeted regions within about 1~kpc of the Sun. Specific regions show reasonably uniform ionization rates over length scales of tens of parsecs, with the average ionization rate in each region being different. Large differences (factor of 5) in $\zeta({\rm H_2})$ are found over length scales of about 100 pc. This supports a picture where the cosmic-ray ionization rate varies smoothly over small size scales, but is not uniform everywhere in the Galactic disk, likely being controlled by proximity to particle acceleration sites.