We report on Herschel/PACS observations of absorption lines of OH ^ { + } , H _ { 2 } O ^ { + } and H _ { 3 } O ^ { + } in NGC 4418 and Arp 220 .
Excited lines of OH ^ { + } and H _ { 2 } O ^ { + } with E _ { \mathrm { lower } } of at least 285 and \sim 200 K , respectively , are detected in both sources , indicating radiative pumping and location in the high radiation density environment of the nuclear regions .
Abundance ratios \mathrm { OH ^ { + } / H _ { 2 } O ^ { + } } of 1 - 2.5 are estimated in the nuclei of both sources .
The inferred OH ^ { + } column and abundance relative to H nuclei are ( 0.5 - 1 ) \times 10 ^ { 16 } cm ^ { -2 } and \sim 2 \times 10 ^ { -8 } , respectively .
Additionally , in Arp 220 , an extended low excitation component around the nuclear region is found to have \mathrm { OH ^ { + } / H _ { 2 } O ^ { + } \sim 5 - 10 } .
H _ { 3 } O ^ { + } is detected in both sources with N ( H _ { 3 } O ^ { + } ) \sim ( 0.5 - 2 ) \times 10 ^ { 16 } cm ^ { -2 } , and in Arp 220 the pure inversion , metastable lines indicate a high rotational temperature of \sim 500 K , indicative of formation pumping and/or hot gas .
Simple chemical models favor an ionization sequence dominated by \mathrm { H ^ { + } \rightarrow O ^ { + } \rightarrow OH ^ { + } \rightarrow H _ { 2 } O ^ { + } % \rightarrow H _ { 3 } O ^ { + } } , and we also argue that the H ^ { + } production is most likely dominated by X-ray/cosmic ray ionization .
The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density ( \gtrsim 10 ^ { 4 } cm ^ { -3 } ) interclump medium , in which case the ionization rate per H nucleus ( including secondary ionizations ) is \zeta > 10 ^ { -13 } s ^ { -1 } , a lower limit that is \mathrm { several } \times 10 ^ { 2 } times the highest rate estimates for Galactic regions .
In Arp 220 , our lower limit for \zeta is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission , and also with the expected ionization rate produced by X-rays .
In NGC 4418 , we argue that X-ray ionization due to an AGN is responsible for the molecular ion production .