2016A&A...595A.128M

We report the detection of OH⁺ and H₂O⁺ in the z=0.89 absorber toward the lensed quasar PKS 1830-211. The abundance ratio of OH⁺ and H₂O⁺ is used to quantify the molecular hydrogen fraction (f_H2) and the cosmic-ray ionization rate of atomic hydrogen (ζ_H) along two lines of sight, located at ∼2kpc and ∼4kpc to either side of the absorber's center. The molecular fraction decreases outward, from ∼0.04 to ∼0.02, comparable to values measured in the Milky Way at similar galactocentric radii. For ζ_H , we find values of ∼2x10^–14s^–1 and ∼3x10^–15s^–1, respectively, which are slightly higher than in the Milky Way at comparable galactocentric radii, possibly due to a higher average star formation activity in the z=0.89 absorber. The ALMA observations of OH⁺, H₂O⁺, and other hydrides toward PKS 1830-211reveal the multi-phase composition of the absorbing gas. Taking the column density ratios along the southwest and northeast lines of sight as a proxy of molecular fraction, we classify the species ArH⁺, OH⁺, H₂Cl⁺, H₂O⁺, CH, and HF as tracing gases increasingly more molecular. Incidentally, our data allow us to improve the accuracy of H₂O⁺ rest frequencies and thus refine the spectroscopic parameters.