Multiphase turbulence as the origin of OH+, H2O+ and H3+ column density scatter in the local ISM

Abstract

Observations of the reactive ions OH+, H2O+ and H3+ in the Galactic interstellar medium reveal large sight-line-to-sight-line scatter in their column densities, commonly interpreted as evidence for substantial variations in the cosmic-ray ionization rate (CRIR). We revisit this interpretation using high-resolution three-dimensional magneto-hydrodynamic simulations of the multiphase ISM with time-dependent chemistry for H, H2, H+ and electrons, building on the fiducial model of Godard et al. (2023). We find that a single CRIR of ~2 10^{-16} s^{-1}, together with standard Galactic-scale parameters, naturally produces broad column-density distributions for all three tracers in good agreement with the observed medians and percentile widths, with no fine tuning. Reaching this match requires that the post-processing of OH+, H2O+ and H3+ retain the time-dependent H2 field generated by the turbulent flow rather than assume chemical equilibrium: turbulence drives long-lived H2 enhancements in the unstable neutral medium where OH+ and H2O+ predominantly reside, and an equilibrium treatment under-predicts their columns substantially. H3+, which receives most of its column from denser CNM gas closer to equilibrium, is much less affected. Our results caution against interpreting sight-line-to-sight-line scatter as direct evidence for large CRIR fluctuations, and motivate a shift from independent 1D equilibrium analyses toward 3D dynamical frameworks when inferring ionization conditions in the ISM.