Multi-line Imaging of the Starburst Galaxy NGC 1808 with ALMA
  Revealing the properties of molecular gas in starburst galaxies is key to
understand the process of star formation across cosmic history. In this paper,
we present our recent high-resolution ($\sim1''$ or 50 pc) ALMA observations of
the nearby, barred starburst galaxy NGC 1808, and highlight some of the main
findings: (1) the discovery of a double peak revealed in the distributions of
CO (3-2) and CS (2-1) in the circumnuclear disk indicating the presence of a
molecular gas torus with a radius of $r\sim30$ pc; inside the torus, we found a
compact source of dust continuum at 0.87 mm and CO, coincident with the
location of the low-luminosity AGN revealed by X-ray observations; (2) the
excitation of CO gas, traced by the line intensity ratio of CO (3-2) to CO
(1-0) is elevated to $\sim1$ in the star-forming disk; the ratio is between 0.3
and 1 in the 500 pc ring; (3) the ratio of HCN (1-0) to HCO$^{+}$ (1-0) in the
central 1 kpc exhibits a radial gradient: while the ratio decreases from
$\sim1.5$ in the center to $\sim0.9$ in the 500 pc disk, it exhibits a peak of
$\sim1.2$ in a ring-like structure at a galactocentric radius of $r\sim300$ pc.
The detection of SiO (2-1) in the circumnuclear disk suggests the presence of
shocks, likely generated by intense nuclear star formation (supernova
explosions and winds from massive stars), as well as inflows and outflows of
turbulent gas in the central 100 pc.
