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

Abstract

Abstract We present the results of high-resolution ${{13\atop} \mathrm{CO}}$ ($ J = 1 \hbox{--} 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 ${{13\atop} \mathrm{CO}}$ ($ J = 1 \hbox{--} 0$) emission, the circumnuclear ring (radius $ \sim$$ {10{}^{\mathrm {\prime \prime }}}$–$ {20{}^{\mathrm {\prime \prime }}}$), and the spiral arm structure. The arm-to-interarm ratio of the ${{13\atop} \mathrm{CO}}$ ($ J = 1 \hbox{--} 0$) integrated intensity is 2–4. We also have found a feature different from that found in the ${{12\atop} \mathrm{CO}}$ results. For example, the ${{13\atop} \mathrm{CO}}$ distribution shows a depression in part of the spiral arm. The $ {{12\atop} \mathrm{CO}}/{{13\atop} \mathrm{CO}}$ ratio spatially varies, and shows high values ($ \sim 20$) for the interarm and the central region, but low values ($ \sim 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 ${{13\atop} \mathrm{CO}}$ shows a better correspondence with that of the $ \mathrm{H}\alpha$ emission than with the ${{12\atop} \mathrm{CO}}$ in the disk region, except for the central region. We found that the ${{13\atop} \mathrm{CO}}$ emission is located on the downstream side of the ${{12\atop} \mathrm{CO}}$ arm, namely there is an offset between the ${{12\atop} \mathrm{CO}}$ and the ${{13\atop} \mathrm{CO}}$ as well as the $ \mathrm{H}\alpha$ 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 $ \sim 10^7 \,\mathrm{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.