Dense molecular gas towards W49A: A template for extragalactic starbursts?

Context. The HCN, HCO+, and HNC molecules are commonly used as tracers of dense star-forming gas in external galaxies, but such observations are spatially unresolved. Reliably inferring the properties of galactic nuclei and disks requires detailed studies of sources whose structure is spatially resolved. Aims. To understand the origin of extragalactic molecular line emission, we compare the spatial distributions and abundance ratios of HCN, HCO+, and HNC in W49A, the most massive and luminous star-forming region in the Galactic disk. Methods. Maps of a 2′ (6.6 pc) field at 14′′ (0.83 pc) resolution of the J=4–3 transitions of HCN, H13CN, HC15N, HCO+, H13CO+, HC18O+, and HNC are combined with supplementary observations of the J=5–4 transition of DCN and the J=3–2 transitions of HCN and H13CO+. Most of the data are from HARP/ACSIS, with supplementary data from JCMT Receiver A and the SCUBA archive. We use maps of the integrated intensity and line-profiles to pick out regions of the source to study in more detail. We compare column densities and abundance ratios towards these regions with each other and with predictions from gas-phase chemical models. Results. The kinematics of the molecular gas in W49A appears complex, with a mixture of infall and outflow motions. Both the line profiles and comparison of the main and rarer species show that the main species are optically thick. Two “clumps” of infalling gas that we look at in more detail appear to be at ∼40 K, compared to ≥100 K at the source centre, and may be ∼10× denser than the rest of the outer cloud. The chemical modelling suggests that the HCN/HNC ratio probes the current gas temperature, while the HCN/HCO+ ratio and the deuterium fractionation were set during an earlier, colder phase of evolution. Conclusions. The similarity in the derived physical conditions in W49A and those inferred for the molecular gas in external galaxies suggest W49A is an appropriate analogue of an extragalactic star forming region. Our data show that the use of HCN/HNC/HCO+ line ratios as proxies for the abundance ratios is incorrect for W49A , suggesting that using these line ratios as abundance ratios in galactic nuclei is invalid too. On the other hand, our observed isotopic line ratios such as H13CN/H13CO+ approach our modeled abundance ratios quite well in W49A. Second, the 4–3 lines of HCN and HCO+ are much better tracers of the dense star-forming gas in W49A than the 1–0 lines, confirming similar indications for galactic nuclei. Finally, our observed HCN/HNC and HCN/HCO+ ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating that irradiation does