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

TL;DR

This study uses ALMA observations to analyze the molecular gas distribution and dynamics in NGC 1808, revealing a nuclear gas torus, outflows, and an evolutionary sequence of starburst activity.

Contribution

First high-resolution ALMA imaging of molecular gas in NGC 1808, discovering a nuclear torus and detailed gas kinematics related to starburst feedback.

Findings

Discovered a molecular gas torus (~30 pc) in the circumnuclear disk.

Confirmed nuclear gas outflow with velocity ~180 km/s.

Revealed supershells expanding from the 500 pc ring.

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 $^{12}$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.