2017ApJ...849...90S

ALMA imaging of the cold molecular medium in the nearby starburst galaxy NGC 1808 is presented. The observations reveal the distribution of molecular gas, traced by ¹²CO (1-0) and ¹²CO (3-2), and continuum (93 and 350 GHz) across the central 1 kpc starburst region at a high resolution of ∼1^′′ . A molecular gas torus (radius ∼ 30 pc) is discovered in the circumnuclear disk (CND; central 100 pc), with a high CO (3-2)/CO (1-0) ratio of ∼1, surrounded by massive (10⁶-10⁷ M_☉) clouds with high star formation efficiency (SFE∼10^–8 yr^–1), molecular spiral arms, and a 500 pc pseudo-ring. The CND harbors a continuum core and molecular gas exhibiting peculiar motion. The new data confirm the line splitting along the minor galactic axis, interpreted as a nuclear gas outflow with average velocity ∼180 km s^–1, and show evidence of a velocity gradient of ∼+0.4 km s^–1 pc^–1 along the axis. In addition, supershells expanding from the 500 pc ring with maximum velocities of ∼75 km s^–1 are revealed. The distribution and CO luminosities of molecular clouds in the central 1 kpc starburst region indicate an evolutionary sequence, from gas accretion onto the 500 pc ring from the large-scale bar to enhanced star formation in the ring, and outflow as feedback.