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 \arcsec ( \sim 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 \sim 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 } } } \sim 10 ^ { 5 } ~ { } \mathrm { cm } ^ { -3 } ) and warm ( 20 ~ { } \mathrm { K } \lesssim T _ { \mathrm { k } } \lesssim 100 ~ { } \mathrm { K } ) .
The calculations yield a low average gas density of n _ { \mathrm { H _ { 2 } } } \sim 10 ^ { 2 } -10 ^ { 3 } ~ { } \mathrm { cm } ^ { -3 } for a temperature of T _ { \mathrm { k } } \gtrsim 30 ~ { } \mathrm { 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 .