H₃O$^+$ line emission from starbursts and AGNs}

Context. The H₃O$^{+}$ molecule probes the chemistry and the ionization rate of dense circumnuclear gas in galaxies. Aims: We use the H₃O$^{+}$ molecule to investigate the impact of starburst and AGN activity on the chemistry of the molecular interstellar medium. Methods: Using the JCMT, we have observed the 3₂$^{+}$ - 2₂$^{+}$ 364 GHz line of p-H₃O$^{+}$ towards the centres of seven active galaxies. Results: We have detected p-H₃O$^{+}$ towards IC 342, NGC 253, NGC 1068, NGC 4418, and NGC 6240. Upper limits were obtained for IRAS 15250 and Arp 299. We find large H₃O$^{+}$ abundances (N(H₃O$^{+}$)/N(H₂) {\gsim} 10$^{-8}$) in all detected galaxies apart from in IC 342 where it is about one order of magnitude lower. We note, however, that uncertainties in N(H₃O$^{+}$) may be significant due to lack of definite information on source size and excitation. We furthermore compare the derived N(H₃O$^{+}$) with N(HCO$^{+}$) and find that the H₃O$^{+}$ to HCO$^{+}$ column density ratio is large in NGC 1068 (24), moderate in NGC 4418 and NGC 253 (4-5), slightly less than unity in NGC 6240 (0.7) and lowest in IC 342 (0.2-0.6). We compare our results with models of X-ray and photon dominated regions (XDRs and PDRs). Conclusions: For IC 342 we find that a starburst PDR chemistry can explain the observed H₃O$^{+}$ abundance. For the other galaxies, the large H₃O$^{+}$ columns are generally consistent with XDR models. In particular for NGC 1068 the elevated N(H₃O$^{+}$)/N(HCO$^{+}$) ratio suggests a low column density XDR. For NGC 4418 however, large HC₃N abundances are inconsistent with the XDR interpretation. An alternative possibility is that H₃O$^{+}$ forms through H₂O evaporating off dust grains and reacting with HCO$^{+}$ in warm, dense gas. This scenario could also potentially fit the results for NGC 253. Further studies of the excitation and distribution of H₃O$^{+}$ - as well as Herschel observations of water abundances - will help to further constrain the models.