Dense molecular gas in the starburst nucleus of NGC 1808
Abstract
Dense molecular gas tracers in the central 1 kpc region of the superwind galaxy NGC 1808 have been imaged by ALMA at a resolution of 1" (~50 pc). Integrated intensities and line intensity ratios of HCN (1-0), H13CN (1-0), HCO+ (1-0), H13CO+ (1-0), HOC+ (1-0), HCO+ (4-3), CS (2-1), C2H (1-0), and previously detected CO (1-0) and CO (3-2) are presented. SiO (2-1) and HNCO (4-3) are detected toward the circumnuclear disk (CND), indicating the presence of shocked dense gas. There is evidence that an enhanced intensity ratio of HCN(1-0)/HCO+(1-0) reflects star formation activity, possibly in terms of shock heating and electron excitation in the CND and a star-forming ring at radius ~300 pc. A non-LTE analysis indicates that the molecular gas traced by HCN, H13CN, HCO+, and H13CO+ in the CND is dense (nH2~105 cm-3) and warm (20 K<Tk<100 K). The calculations yield a low average gas density of nH2~102-103 cm-3 for a temperature of Tk≥30 K in the nuclear outflow. Dense gas tracers HCN (1-0), HCO+ (1-0), CS (2-1), and C2H (1-0) are detected for the first time in the superwind of NGC 1808, confirming the presence of a velocity gradient in the outflow direction.
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