Complex, quiescent kinematics in a highly filamentary infrared dark cloud

Infrared dark clouds (IRDCs) host the initial conditions under which massive stars and stellar clusters form. It is therefore important to study the kinematics, as well as the physical and chemical properties of these regions. Their complex structure however posits challenges in the data interpretation. We have obtained high-sensitivity and high-spectral-resolution observations with the Instituto de Radioastronomia Milimetrica 30 m antenna, which allowed us to perform detailed analysis of the kinematics within one IRDC, G035.39-00.33. This cloud has been selected for its highly filamentary morphology and the presence of extended quiescent regions, characteristics of dynamical youth. We focus on the J = 1 -> 0 and J = 3 -> 2 transitions of N2H+, (CO)-O-18 (1-0), and make comparison with SiO (2-1) observations and extinction mapping. Three interacting filaments of gas are found. We report large-scale velocity coherence throughout the cloud, evidenced through small velocity gradients and relatively narrow line widths. This suggests that the merging of these filaments is somewhat 'gentle', possibly regulated by magnetic fields. This merging of filaments may be responsible for the production of weak, parsec-scale SiO emission, via grain mantle vaporization. A systematic velocity shift between the N2H+ (1-0) and (CO)-O-18 (1-0) gas throughout the cloud of 0.18 +/- 0.04 km s(-1) is also found, consistent with a scenario of collisions between filaments which is still ongoing. The N2H+ (1-0) is extended throughout the IRDC, in contrast to low-mass star forming regions, where N2H+ is known to trace only the dense cores. The average H-2 number density across the IRDC is similar or equal to 5 x 10(4) cm(-3), at least one order of magnitude larger than in nearby molecular clouds where low-mass stars are forming. A temperature gradient perpendicular to the filament is found. From our study, we conclude that G035.39-00.33 (clearly seen in the extinction map and in N2H+) has been formed via the collision between two relatively quiescent filaments with average densities of similar or equal to 5 x 10(3) cm(-3), moving with relative velocities of similar or equal to 5 km s(-1). The accumulation of material at the merging points started greater than or similar to 1 Myr ago and it is still ongoing.