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] kms-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]kms-1 and are between or along the edges of SiO gas features at [100,120]kms-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 nm H2∼ 105cm-3 and ∼ 30K, 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 Kkms-1, and ∼ 31kms-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 NrmH2 5 x 1022cm-2, and the morphology and kinematics of shocked gas in different velocity channels. Colliding clouds are efficient at producing internal shocks with velocities ∼ 5-50kms-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.