Large-scale molecular shocks in galaxies

We review our on-going study of the shock-driven molecular gas chemistry in star-forming galaxies, based on observations with the IRAM Plateau de Bure Interferometer. We have obtained high-resolution (∼5″) images of the emission of silicon monoxide (SiO) in the nuclei of the nearby galaxies NGC 253, IC 342 and M 82. From observations in the Galaxy and theoretical models, SiO is known to be a privileged tracer of molecular shock chemistry. The large SiO abundances, 〈X(SiO)〉 ∼ 10−10–10−9, found along several hundreds of pc imply that large-scale shock chemistry must be at play in the inner disks of the three surveyed galaxies. Noticeable differences in the morphology of the SiO emission call to different driving mechanisms, however. In NGC 253 and IC 342, the most plausible scenario is that of shocks arising in cloud–cloud collisions, dynamically triggered along the bar potential. In the case of M 82, shocks arise in the disk–halo interface, probably boosted by local episodes of mass ejection from the disk. These dissimilarities are explained in terms of the evolutionary stage of the starburst episodes. This work illustrates how high-resolution imaging of specific chemical tracers provide useful inputs to the understanding of galaxy evolution.