2024A&A...685A.101L

Aims. W43-main is a massive molecular complex undergoing starburst activities, located at the interaction of the Scutum arm and the Galactic bar. We aim to investigate the gas dynamics, in particular, the prevailing shock signatures from cloud to clump scales. We also look to assess the impact of shocks on the formation of dense gas and early-stage cores in OB cluster formation processes.
Methods. We carried out NOEMA and IRAM-30 m observations at 3 mm towards five molecular gas clumps in W43 main located within large-scale interacting gas components. We used CH₃CCH and H₂CS lines to trace the extended gas temperature and CH₃OH lines to probe the volume density of the dense gas components (≳10⁵ cm^–3). We adopted multiple tracers that are sensitive to different gas density regimes to reflect the global gas motions. The density enhancements constrained by CH₃OH and a population of NH₂D cores are correlated (in the spatial and velocity domains) with SiO emission, which is a prominent indicator of shock processing in molecular clouds.
Results. The emission of SiO (2-1) is extensive across the region (∼4 pc) and it is contained within a low-velocity regime, hinting at a large-scale origin for the shocks. Position-velocity maps of multiple tracers show systematic spatio-kinematic offsets supporting the cloud-cloud collision-merging scenario. We identified an additional extended velocity component in the CCH emission, which coincides with one of the velocity components of the larger scale ¹³CO (2-1) emission, likely representing an outer, less-dense gas layer in the cloud merging process. We find that the 'V-shaped', asymmetric SiO wings are tightly correlated with localised gas density enhancements, which is direct evidence of dense gas formation and accumulation in shocks. The dense gas that is formed in this way may facilitate the accretion of the embedded, massive pre-stellar and protostellar cores. We resolved two categories of NH₂D cores: those exhibiting only subsonic to transonic velocity dispersions and those with an additional supersonic velocity dispersion. The centroid velocities of the latter cores are correlated with the shock front seen via SiO. The kinematics of the ∼0.1 pc NH₂D cores are heavily imprinted by shock activities and may represent a population of early-stage cores forming around the shock interface.