Outflowing OH⁺ in Markarian 231: The Ionization Rate of the Molecular Gas

Abstract

The oxygen-bearing molecular ions OH⁺, H₂O⁺, and H₃O⁺ are key species that probe the ionization rate of (partially) molecular gas that is ionized by X-rays and cosmic-rays permeating the interstellar medium. We report Herschel far-infrared and submillimeter spectroscopic observations of OH⁺ in Mrk 231, showing both ground-state P-Cygni profiles, and excited line profiles with blueshifted absorption wings extending up to ≈1000 km s^-1. In addition, OH⁺ probes an excited component peaking at central velocities, likely arising from the torus probed by the OH centimeter-wave megamaser. Four lines of H₂O⁺ are also detected at systemic velocities, but H₃O⁺ is undetected. Based on our earlier OH studies, we estimate an abundance ratio of {OH}/{OH}}⁺∼ 5{--}10 for the outflowing components and ≈20 for the torus, and an OH⁺ abundance relative to H nuclei of ≳10^-7. We also find high OH⁺/H₂O⁺ and OH⁺/H₃O⁺ ratios; both are ≳4 in the torus and ≳10-20 in the outflowing gas components. Chemical models indicate that these high OH⁺ abundances relative to OH, H₂O⁺, and H₃O⁺ are characteristic of gas with a high ionization rate per unit density, \zeta /{n}_{{H}}∼ (1{--}5)× {10}^-17 cm³ s^-1 and ∼(1-2) × 10^-16 cm³ s^-1 for the above components, respectively, an ionization rate of ζ ∼ (0.5-2) × 10^-12 s^-1, and a low molecular fraction, {f}_{{{H}}₂}∼ 0.25. X-rays appear to be unable to explain the inferred ionization rate, and thus we suggest that low-energy (10-400 MeV) cosmic-rays are primarily responsible for the ionization, with {\dot{M}}_CR}∼ 0.01 M _⊙ yr^-1 and {\dot{E}}_CR}∼ {10}⁴⁴ erg s^-1 the latter corresponds to ∼1% of the luminosity of the active galactic nucleus and is similar to the energetics of the molecular outflow. We suggest that cosmic-rays accelerated in the forward shock associated with the molecular outflow are responsible for the ionization, as they diffuse through the outflowing molecular phase downstream.