부산대학교 도서관

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 |$[10,40]\, \rm km\, s^{-1}$| and an arc at velocities of |$[-20,10]\, \rm km\, s^{-1}$|⁠. 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 |$[40,85]\, \rm km\, s^{-1}$|⁠ , and are between or along the edges of SiO gas features at |$[100,120]\, \rm km\, s^{-1}$|⁠ , 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 |$n_{\rm H_2}\sim 10^5\, \rm cm^{-3}$| and |$\sim 30\, \rm K$|⁠ , respectively, towards three positions of Sgr B2. The average values of the SiO relative abundances, integrated line intensities, and line widths are ∼10−9, |$\sim 11\, \rm K\, km\, s^{-1}$|⁠ , and |$\sim 31\, \rm km\, s^{-1}$|⁠ , 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 |$\lesssim 0.5\, \rm Myr$| ago can explain the density distribution with a mean column density of |$\bar{N}_{\rm H_2}\gtrsim 5\times 10^{22}\, \rm cm^{-2}$|⁠ , and the morphology and kinematics of shocked gas in different velocity channels. Colliding clouds are efficient at producing internal shocks with velocities |$\sim 5\!-\!50\, \rm km\, s^{-1}$|⁠. 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. [ABSTRACT FROM AUTHOR]