2011A&A...525A.107R

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. To understand the origin of extragalactic molecular line emission, we compare the spatial distributions and abundance ratios of HCN, HCO⁺, and HNC in W 49A, the most massive and luminous star-forming region in the Galactic disk. Maps of a 2' (6.6pc) field at 14'' (0.83pc) 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. The kinematics of the molecular gas in W 49A 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 ∼40K, compared to ≥100K at the source centre, and may be ∼10x 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. The similarity in the derived physical conditions in W 49A and those inferred for the molecular gas in external galaxies suggest that W 49A 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 W 49A, 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 W 49A. Second, the 4-3 lines of HCN and HCO⁺ are much better tracers of the dense star-forming gas in W 49A than the 1-0 lines, confirming similar indications for galactic nuclei. Finally, our observed HCN/HNC and HCN/HCO⁺ ratios in W 49A are inconsistent with homogeneous PDR or XDR models, indicating that irradiation does not strongly affect the gas chemistry in W 49A. Overall, the W 49A region appears to be a useful template for starburst galaxies.