Detection of Molecular Clouds in the Interarm of the Flocculent Galaxy NGC 5055

Abstract

Abstract We present high-resolution ($\sim {4{}^{\mathrm {\prime \prime }}}$)${{12\atop} \mathrm{CO}}$ ($J = 1 \hbox{--} 0$) mapping observations with high-velocity resolution (${\sim 2.6 \,\mathrm{km} \,\mathrm{s}^{-1}}$) toward the disk of flocculent galaxy NGC 5055, using the Nobeyama Millimeter Array in order to study the physical properties of the molecular clouds in the arm and the interarm. The map shows clumpy structures. Although these are mainly distributed along an arm seen in NIR, some are located in the interarm. These clouds in the arm and the interarm have a typical size and mass of a few 100 pc and a few $10^6 \,{{{M}_{\odot}}}$. These correspond to the largest Giant Molecular Cloud (GMC) in our Galaxy, and are slightly smaller than Giant Molecular Associations (GMAs) in the grand design spiral M 51. Their CO flux-based masses show good agreement with their virial masses. A size–velocity dispersion relation is plotted on an extension of the relation for the Galactic GMCs. It suggest that the properties of these clouds are similar to that of the Galactic GMCs. We found no clear systematic offset between the molecular gas and H ii regions unlike M 51. This and no existense of GMAs suggest that, in NGC 5055, cloud formation and star formation in the arm and the interarm are due to enhancement of gas by local fluctuation; while, in grand design spiral galaxies, such as M 51, GMA formations may occur only in the arm due to a strong density wave and enhanced star formation in GMA formation may occur. These may control the optical morphology of spiral arms in spiral galaxies.