Parsec-scale SiO emission in an infrared dark cloud

We present high-sensitivity 2 x 4 arcmin2 maps of the J = 2 -> 1 rotational lines of SiO, CO, 13CO and C18O, observed towards the filamentary infrared dark cloud (IRDC) G035.39-00.33. Single-pointing spectra of the SiO J = 2 -> 1 and J = 3 -> 2 lines towards several regions in the filament are also reported. The SiO images reveal that SiO is widespread along the IRDC (size >= 2 pc), showing two different components: one bright and compact arising from three condensations (N, E and S) and the other weak and extended along the filament. While the first component shows broad lines (linewidths of similar to 4-7 km s-1) in both SiO J = 2 -> 1 and SiO J = 3 -> 2, the second one is only detected in SiO J = 2 -> 1 and has narrow lines (similar to 0.8 km s-1). The maps of CO and its isotopologues show that low-density filaments are intersecting the IRDC and appear to merge towards the densest portion of the cloud. This resembles the molecular structures predicted by flow-driven, shock-induced and magnetically-regulated cloud formation models. As in outflows associated with low-mass star formation, the excitation temperatures and fractional abundances of SiO towards N, E and S increase with velocity from similar to 6 to 40 K and from similar to 10-10 to >= 10-8, respectively, over a velocity range of similar to 7 km s-1. Since 8 mu m and 24 mu m sources and/or extended 4.5 mu m emission are detected in N, E and S, broad SiO is likely produced in outflows associated with high-mass protostars. The excitation temperatures and fractional abundances of the narrow SiO lines, however, are very low (similar to 9 K and similar to 10-11, respectively), and consistent with the processing of interstellar grains by the passage of a shock with v(s) similar to 12 km s-1. This emission could be generated (i) by a large-scale shock, perhaps remnant of the IRDC formation process, (ii) by decelerated or recently processed gas in large-scale outflows driven by 8- and 24-mu m sources or (iii) by an undetected and widespread population of lower mass protostars. High-angular-resolution observations are needed to disentangle between these three scenarios.