H₃O⁺line emission from starbursts and AGNs

Abstract

Context. The H 3 O + molecule probes the chemistry and the ionization rate of dense circumnuclear gas in galaxies. Aims. We use the H 3 O + molecule to investigate the impact of starburst and AGN activity on the chemistry of the molecular interstellar medium. where it is about one order of magnitude lower. We note, however, that uncertainties in N(H 3 O + ) may be significant due to lack of definite information on source size and excitation. We furthermore compare the derived N(H 3 O + ) with N(HCO + ) and find that the H 3 O + 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 H 3 O + abundance. For the other galaxies, the large H 3 O + columns are generally consistent with XDR models. In particular for NGC 1068 the elevated N(H 3 O + )/N(HCO + ) ratio suggests a low column density XDR. For NGC 4418 however, large HC 3 N abundances are inconsistent with the XDR interpretation. An alternative possibility is that H 3 O + forms through H 2 O 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 H 3 O + -as well as Herschel observations of water abundances -will help to further constrain the models.

Hot, metastable hydronium ion in the Galactic centre: formation pumping in X-ray-irradiated gas?

“…In the context of X-ray models, the high rotational temperature observed may be naturally explained by the + -emitting gas is not constrained by the present observations. While the large H 3 O + columns derived towards the four galaxies are generally consistent with predictions of XDR models, Aalto et al [31] also consider an alternative scenario, in which H 3 O + can be formed from H 2 O evaporating from dust grains and reacting with HCO + in a warm, dense gas. Shocks would help remove water molecules from grain mantles, resulting in an enhanced gas-phase water abundance.…”

THE CHEMISTRY OF INTERSTELLAR OH+, H2O+, AND H3O+: INFERRING THE COSMIC-RAY IONIZATION RATES FROM OBSERVATIONS OF MOLECULAR IONS

“…Extragalactic H 3 O + was first detected through JCMT observations of the p-H 3 O + 364 GHz emission line towards the prototypical ultraluminous infrared galaxy Arp 220, and towards M82, an evolved starburst (van der Tak et al 2008). H 3 O + was subsequently detected towards IC342, NGC253, NGC1068, NGC4418 and NGC 6240, and upper limits obtained towards IRAS 15250 and Arp 299 galaxies (Aalto et al 2011). Except for IC342 and M82, the typical H 3 O + column densities (∼ 10 15 − 10 16 cm −2 ) derived from extragalactic H 3 O + detections are much larger than the predictions of our models for single clouds.…”

Molecular line emission in NGC 1068 imaged with ALMA

“…García-Burillo et al 2002;Wang 2004;Usero et al 2004;Fuente et al 2008;Aalto et al 2011;Martín et al 2011;Aladro et al 2013;Costagliola et al 2011;Kamenetzky et al 2011;Watanabe et al 2014). A common trend observed in recent surveys is a chemical diversity and complexity that is hard to explain with a one-component gas phase.…”

“…Observations seem to indicate that abundance ratios of HCN/HCO + may help trace the dominating energetic processes (e.g. Krips et al 2008Krips et al , 2011Aalto et al 2011;Loenen et al 2008); however, the chemistry of both these species is highly dependent on the physical conditions of the gas as well as on its energetics. Galaxies where starbursts and AGN are both present therefore require both high spatial resolution and targeted multi-species, multi-line observations to disentangle and characterize the energetics and chemical effects of the molecular gas.…”