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

We present high-resolution (~ 4") 12CO (J = 1 - 0) mapping observations withhigh - velocity resolution (~ 2.6 km s^{-1}) toward the disk of flocculentgalaxy NGC 5055, using the Nobeyama Millimeter Array in order to study thephysical properties of the molecular clouds in the arm and the interarm. Theobtained map shows clumpy structures. Although these are mainly distributedalong a spiral arm seen in near-infrared observations, some clouds are locatedfar from the arm, namely in the interarm. These clouds in both the arm and theinterarm have a typical size and mass of a few 100 pc and a few 10^6 Mo,respectively. These correspond to the largest Giant Molecular Cloud (GMC) inour Galaxy, and are slightly smaller than Giant Molecular Associations (GMAs)in the grand design spiral M 51. Their CO flux-based masses show good agreementwith their virial masses. A size - velocity dispersion relation is also plottedon an extension of the relation for the Galactic GMCs. These facts suggest thatthe properties of these clouds are similar to that of the Galactic GMCs. Wealso found no clear systematic offset between the molecular gas and HII regionsunlike M 51. This fact and no existense of GMAs suggest the view that, in NGC5055, cloud formation and following star formation in both the arm and theinterarm are due to enhancement of gas by local fluctuation. On the other hand,in grand design spiral galaxies, such as M 51, GMA formations may occur only inthe arm due to a strong density wave also enhanced star formation in GMAformation may also occur. These may control the optical morphology of spiralarms in spiral galaxies.Comment: 16 pages, 8 figure