Variation of molecular cloud properties across the spiral arm in M 51

Abstract

We present the results of high-resolution 13CO (J = 1-0) mapping observations with the NRO 45 m telescope of the area toward the southern bright arm region of M 51, including the galactic center, in order to study the physical conditions of the molecular clouds in the arm and the interarm. The obtained map shows the central depression of the 13CO (J = 1-0) emission, the circumnuclear ring (radius ∼ 10″-20″), and the spiral arm structure. The arm-to-interarm ratio of the 13CO (J = 1-0) integrated intensity is 2-4. We also have found a feature different from that found in the 12CO results. For example, the 13CO distribution shows a depression in part of the spiral arm. The 12CO/13CO ratio spatially varies, and shows high values (∼ 20) for the interarm and the central region, but low values (∼ 10) for the arm. Their values indicate that there is a denser gas in the spiral arm than in the interarm. The distribution of the 13CO shows a better correspondence with that of the Hα emission than with the 12CO in the disk region, except for the central region. We found that the 13CO emission is located on the downstream side of the 12CO arm, namely there is an offset between the 12CO and the 13CO as well as the Hα emission. This suggests that there is a time delay between the accumulation of gas caused by the density wave and dense gas formation, accordingly star formation. This time delay is estimated to be ∼ 107 yr based on the assumption of galactic rotation derived by the rotation curve and the pattern speed of the M 51 spiral pattern. It is similar to the growth timescale of a gravitational instability in the spiral arm of M 51, suggesting that the gravitational instability plays an important role for dense gas formation.