Variation of Molecular Cloud Properties across the Spiral Arm in M 51

We present the results of high-resolution 13 CO ( J = 1–0) mapping observations with the NRO 45m 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 13 CO ( J = 1–0) emission, the circumnuclear ring (radius ∼ 10 ′′ – 20 ′′ ), and the spiral arm structure. The arm-to-interarm ratio of the 13 CO ( J = 1–0) integrated intensity is 2 – 4. We also have found a feature different from that found in the 12 CO results. For example, the 13 CO distribution shows a depression in part of the spiral arm. The 12 CO/ 13 CO 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 13 CO shows a better correspondence with that of the H α emission than with the 12 CO in the disk region, except for the central region. We found that the 13 CO emission is located on the downstream side of the 12 CO arm, namely there is an offset between the 12 CO and the 13 CO as well as the H α 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 ∼ 10 7 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.