Dense molecular gas in the starburst nucleus of NGC 1808
  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
$n_{\mathrm{H}_2}$~$10^2\mathrm{-}10^3$ cm$^{-3}$ for a temperature of
$T_\mathrm{k}\geq30$ 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.
