2014MNRAS.439.1996J

Some theories of dense molecular cloud formation involve dynamical environments driven by converging atomic flows or collisions between preexisting molecular clouds. The determination of the dynamics and physical conditions of the gas in clouds at the early stages of their evolution is essential to establish the dynamical imprints of such collisions, and to infer the processes involved in their formation. We present multitransition 13CO and C18O maps towards the IRDC G035.39-00.33, believed to be at the earliest stages of evolution. The 13CO and C18O gas is distributed in three filaments (Filaments 1, 2 and 3), where the most massive cores are preferentially found at the intersecting regions between them. The filaments have a similar kinematic structure with smooth velocity gradients of ∼ 0.4-0.8 km/s/pc. Several scenarios are proposed to explain these gradients, including cloud rotation, gas accretion along the filaments, global gravitational collapse and unresolved sub-filament structures. These results are complemented by HCO+, HNC, H13CO+ and HN13C single-pointing data to search for gas infall signatures. The 13CO and C18O gas motions are supersonic across G035.39-00.33, with the emission showing broader linewidths towards the edges of the infrared dark cloud (IRDC). This could be due to energy dissipation at the densest regions in the cloud. The average H2 densities are ∼ 5000-7000cm-3, with Filaments 2 and 3 being denser and more massive than Filament 1. The C18O data unveil three regions with high CO depletion factors (fD ∼ 5-12), similar to those found in massive starless cores.