Structure and kinematics of shocked gas in Sgr B2: further evidence of a cloud–cloud collision from SiO emission maps. Issue 4 (9th October 2020)

• Record Type: Journal Article
• Authors: Armijos-Abendaño, J; Banda-Barragán, W E; Martín-Pintado, J; Dénes, H; Federrath, C; Requena-Torres, M A
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 499:Issue 4(2020)
• Page Start: 4918
• Page End: 4939
• Publication Date: 2020-10-09
• Subjects: methods: numerical -- ISM: clouds -- ISM: molecules -- Galaxy: centre; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa3119
• Languages: English
• ISSNs: 0035-8711

Abstract

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 $[10, 40]\, \rm km\, s^{-1}$ 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.