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Abstract

Infrared dark clouds (IRDCs) are cold, dense regions of high (optical and infrared) extinction, believed to be the birthplace of high-mass stars and stellar clusters. The physical mechanisms leading to the formation of these IRDCs are not completely understood and it is thus important to study their molecular gas kinematics and chemical content to search for any signature of the IRDCs formation process. Using the 30-m-diameter antenna at the Instituto de Radioastronomía Milimétrica (IRAM), we have obtained emission maps of dense gas tracers (H¹³CO⁺ and HN¹³C) and typical shock tracers (SiO and CH₃OH) towards three IRDCs, G028.37+00.07, G034.43+00.24, and G034.77-00.55 (clouds C, F, and G, respectively). We have studied the molecular gas kinematics in these clouds and, consistent with previous works towards other IRDCs, the clouds show complex gas kinematics with several velocity-coherent substructures separated in velocity space by a few km s^-1. Correlated with these complex kinematic structures, widespread (parsec-scale) emission of SiO and CH₃OH is present in all the three clouds. For clouds C and F, known to be actively forming stars, widespread SiO and CH₃OH is likely associated with on-going star formation activity. However, for cloud G, which lacks either 8 or 24 μm sources and 4.5 μm H₂ shock-excited emission, the detected widespread SiO and CH₃OH emission may have originated in a large-scale shock interaction, although a scenario involving a population of low- mass stars driving molecular outflows cannot be fully ruled out.