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

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 approximate 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+ similar to 5-10 for the outflowing components and approximate to 20 for the torus, and an OH+ abundance relative to H nuclei of greater than or similar to 10(-7). We also find high OH+/H2O+ and OH+/H3O+ ratios; both are greater than or similar to 4 in the torus and greater than or similar to 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/nH similar to (1-5) x 10(-17) cm(3) s(-1) and similar to(1-2) x 10(-16) cm(3) s(-1) for the above components, respectively, an ionization rate of zeta similar to (0.5-2) x 10(-12) s(-1), and a low molecular fraction, fH(2) similar to 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 <(M)over dot>(CR) similar to 0.01 M-circle dot yr(-1) and <(E)over dot>(CR) similar to 10(44) erg. s(-1); the latter corresponds to similar 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 asso