Evolution of Molecular Clouds in the Superwind Galaxy NGC 1808 Probed by
  ALMA Observations
  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 $^{12}$CO (1-0) and $^{12}$CO (3-2), and continuum (93 and 350 GHz)
across the central 1 kpc starburst region at high resolution of $\sim1$ arcsec.
A molecular gas torus (radius $\sim30$ pc) is discovered in the circumnuclear
disk (CND; central 100 pc), with a high CO (3-2)/CO (1-0) ratio of $\sim1$,
surrounded by massive ($10^6$-$10^7$ $M_\odot$) clouds with high star formation
efficiency (SFE$\sim10^{-8}$ yr$^{-1}$), molecular spiral arms, and a 500 pc
pseudoring. 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
$\sim180$ km s$^{-1}$, and show evidence of a velocity gradient of $\sim+0.4$
km s$^{-1}$ pc$^{-1}$ along the axis. In addition, supershells expanding from
the 500 pc ring with maximum velocities of $\sim75$ 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.
