Isolating signatures of major cloud–cloud collisions using position–velocity diagrams

Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally and distinguishing it from other mechanisms. We produce synthetic position–velocity diagrams from models of cloud–cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high-velocity cloud–cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5 Myr after the formation of the first massive (ionizing) star. However for a head-on 10 km s⁻¹ collision, we find that this will only be observable from 20 to 30 per cent of viewing angles. Such broad–bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud–cloud collision by Torii et al., and also towards star formation in the outer Milky Way by Izumi et al.