Structure and kinematics of shocked gas in Sgr B2: Further evidence of a cloud-cloud collision from SiO emission maps

Abstract

We present SiO J = 2–1 maps of the Sgr B2 molecular cloud, which show shocked gas with a turbulent substructure comprising at least three cavities at velocities of [10, 40] km s−1 and an arc at velocities of [−20, 10] km s−1. The spatial anticorrelation of shocked gas at low and high velocities, and the presence of bridging features in position-velocity diagrams suggest that these structures formed in a cloud–cloud collision. Some of the known compact H II regions spatially overlap with sites of strong SiO emission at velocities of [40, 85] km s−1, and are between or along the edges of SiO gas features at [100, 120] km s−1, suggesting that the stars responsible for ionizing the compact H II regions formed in compressed gas due to this collision. We find gas densities and kinetic temperatures of the order of nH2 ∼ 105 cm−3 and ∼ 30 K, respectively, towards three positions of Sgr B2. The average values of the SiO relative abundances, integrated line intensities, and line widths are ∼10−9, ∼ 11 K km s−1, and ∼ 31 km s−1, respectively. These values agree with those obtained with chemical models that mimic grain sputtering by C-type shocks. A comparison of our observations with hydrodynamical simulations shows that a cloud–cloud collision that took place 0.5 Myr ago can explain the density distribution with a mean column density of N¯H2 5 × 1022 cm−2, and the morphology and kinematics of shocked gas in different velocity channels. Colliding clouds are efficient at producing internal shocks with velocities ∼ 5–50 km s−1. High-velocity shocks are produced during the early stages of the collision and can readily ignite star formation, while moderate- and low-velocity shocks are important over longer time-scales and can explain the widespread SiO emission in Sgr B2. Show more