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 \approx 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 \mathrm { OH / OH ^ { + } } \sim 5 - 10 for the outflowing components and \approx 20 for the torus , and an OH ^ { + } abundance relative to H nuclei of \gtrsim 10 ^ { -7 } .
We also find high OH ^ { + } /H _ { 2 } O ^ { + } and OH ^ { + } /H _ { 3 } O ^ { + } ratios , both are \gtrsim 4 in the torus and \gtrsim 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 , \zeta / n _ { \mathrm { H } } \sim ( 1 - 5 ) \times 10 ^ { -17 } cm ^ { 3 } s ^ { -1 } and \sim ( 1 - 2 ) \times 10 ^ { -16 } cm ^ { 3 } s ^ { -1 } for the above components , respectively , an ionization rate of \zeta \sim ( 0.5 - 2 ) \times 10 ^ { -12 } s ^ { -1 } , and a low molecular fraction , f _ { \mathrm { H } _ { 2 } } \sim 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 } _ { \mathrm { CR } } \sim 0.01 M _ { \odot } yr ^ { -1 } and \dot { E } _ { \mathrm { CR } } \sim 10 ^ { 44 } erg s ^ { -1 } , the latter corresponding to \sim 1 % of the AGN luminosity and 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 .