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

Abstract

The oxygen-bearing molecular ions OH + , H 2 O + , and H 3 O + 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 H 2 O + are also detected at systemic velocities, but H 3 O + is undetected. Based on our earlier OH studies, we estimate an abundance ratio of 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 + /H 2 O + and OH + /H 3 O + 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, H 2 O + , and H 3 O + are characteristic of gas with a high ionization rate per unit density, cm 3 s −1 and ∼(1–2) × 10 −16 cm 3 s −1 for the above components, respectively, an ionization rate of ζ  ∼ (0.5–2) × 10 −12 s −1 , and a low molecular fraction, . 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 M ⊙  yr −1 and 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.