SiO emission as a probe of cloud–cloud collisions in infrared dark clouds

Infrared Dark Clouds (IRDCs) are very dense and highly extincted regions that host the initial conditions of star and stellar cluster formation. It is crucial to study the kinematics and molecular content of IRDCs to test their formation mechanism and ultimately characterise these initial conditions. We have obtained high-sensitivity Silicon Monoxide, SiO(2-1), emission maps toward the six IRDCs, G018.82$-$00.28, G019.27+00.07, G028.53$-$00.25, G028.67+00.13, G038.95$-$00.47 and G053.11+00.05 (cloud A, B, D, E, I and J, respectively), using the 30-m antenna at the Instituto de Radioastronomia Millimetrica (IRAM30m). We have investigated the SiO spatial distribution and kinematic structure across the six clouds to look for signatures of cloud-cloud collision events that may have formed the IRDCs and triggered star formation within them. Toward clouds A, B, D, I and J we detect spatially compact SiO emission with broad line profiles which are spatially coincident with massive cores. Toward the IRDCs A and I, we report an additional SiO component that shows narrow line profiles and that is widespread across quiescent regions. Finally, we do not detect any significant SiO emission toward cloud E. We suggest that the broad and compact SiO emission detected toward the clouds is likely associated with ongoing star formation activity within the IRDCs. However, the additional narrow and widespread SiO emission detected toward cloud A and I may have originated from the collision between the IRDCs and flows of molecular gas pushed toward the clouds by nearby HII regions.