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), H${}^{13}$CN (1-0), HCO${}^{+}$ (1-0), H${}^{13}$CO${}^{+}$ (1-0), HOC${}^{+}$ (1-0), HCO${}^{+}$ (4-3), CS (2-1), C${}_2$H (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, H${}^{13}$CN, HCO${}^{+}$, and H${}^{13}$CO${}^{+}$ in the CND is dense ($n_{{\mathrm{H}}_2}$~$10^5$ cm${}^{-3}$) and warm (20 K$<T_{\mathrm{k}}$<100 K). The calculations yield a low average gas density of $n_{{\mathrm{H}}_2}$~$10^2-10^3$ cm${}^{-3}$ for a temperature of $T_{\mathrm{k}}\ge 30$ K in the nuclear outflow. Dense gas tracers HCN (1-0), HCO${}^{+}$ (1-0), CS (2-1), and C${}_2$H (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.

Accepted for publication in ApJ