Evolution of Molecular Clouds in the Superwind Galaxy NGC 1808 Probed by ALMA Observations

Abstract

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^\prime\prime . 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-{10}⁷ {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.