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H3O+ line emission from starbursts and AGNs

Susanne Aalto (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Francesco Costagliola (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; F. van der Tak ; R. Meijerink
Astronomy and Astrophysics (0004-6361). Vol. 527 (2011),
[Artikel, refereegranskad vetenskaplig]

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 3(2)(+)-2(2)(-) 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(H-2) greater than or similar to 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+-aswell as Herschel observations of water abundances - will help to further constrain the models.

Nyckelord: galaxies: evolution, galaxies: starburst, galaxies: active, radio, lines: galaxies, ISM: molecules, ultraluminous infrared galaxies, dense molecular gas, active galactic, nucleus, high-spatial-resolution, young stellar objects, star-formation, ngc 1068, ic 342, nearby galaxies, x-ray

Denna post skapades 2011-04-14. Senast ändrad 2016-04-11.
CPL Pubid: 139052


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