Abstract:
Investigating spatially-resolved physical properties of molecular gas in individual regions (e.g., center, bar, and arm) is necessary to understand star formation and evolution of galaxies. We observed 13CO(1-0), CH3CO(2-1), and CS(2-1) lines and 3 mm continuum emission toward the disk of the nearby starburst galaxy NGC 253 with ALMA, and examined the physical properties of molecular gas. We obtained spatial distributions in each emission, and calculated CH3OH/13CO, CS/13CO, and 3 mm/13CO ratios at ~35 pc scale. We found an extremely high CH3OH/13CO ratio, 0.7, in the bar, whereas such a high CH3OH/13CO ratio has not been reported for other external galaxy disks. We discuss what environments can enhance CH3OH in the bar of NGC 253. We found that 3 mm/13CO ratio is lower in the high CH3OH/13CO ratio region compared to other regions with moderate CH3OH/13CO ratio (typically 0.1-0.2). Thus we suggest a possibility to explain the extremely high CH3OH/13CO ratio; molecular gas is dense and cold, and the gas experienced mild shock and effectively forms CH3OH molecule.