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

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 N̄H₂ 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.