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

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