Tracing Coherent Gas Structures in the Central Region of the Starburst Galaxy NGC 253. II. Gas Excitation and Star Formation

Abstract Molecular gas in the central region of the starburst galaxy NGC 253 is complicated because of the combination of different gas kinematics, including stellar feedback, noncircular motions, and magnetic activity. In paper I, we used the latest multitransition, multi-isotope CO-line data sets obtained with the Atacama Large Millimeter/submillimeter Array to classify the molecular gas within the central 850 pc in radius into 28 coherent structures, and we proposed a three-dimensional geometric structure for the gas in the central region of this galaxy. In this paper, we utilize intensity ratios between 12 CO ( J = 3–2), 12 CO ( J = 1–0), HCN ( J = 4–3), and SiO ( J = 2–1) to investigate the physical properties of each coherent structure, comparing the distributions with (proto-)star clusters, known as superbubble candidates, and stars. We found that the excitation state of the gas tends to increase toward the galactic center, and we identified high-excitation states in the gas with noncircular motions. From analyses of HCN ( J = 4–3) and SiO ( J = 2–1), we found that the dense, star-forming clumps that drive the starburst reside either at the points connecting any two streams or in the central region where two oppositely directed, accretion flows terminate. Based on analyses of the intensity ratios and the proposed geometric structure, we suggest that some superbubbles may instead be the overlap of the coherent structures rather than an expanding shell. From all these results, we propose a scenario in which the starburst was triggered by cloud–cloud collisions caused by interactions of streams and/or by magnetic flotation and/or efficient accretion along a stellar bar.