Dense gas formation triggered by spiral density wave in M31

We present the high-resolution CO(J = 1−0), CO(J = 1−0) and CO(J = 3−2) maps toward a GMA located on the southern arm region of M31 using Nobeyama 45 m and ASTE 10 m telescopes. The GMA consists of two velocity-components, i.e., red and blue. The blue component shows a strong and narrow peak, whereas the red one shows a weak and broad profile. The red component has a lower CO(J = 1 − 0)/CO(J = 1 − 0) ratio (∼ 5) than that of the blue one (∼ 16), indicating that the red component is denser than the blue one. The red component could be the decelerated gas if we consider the galactic rotational velocity in this region. We suggest that the red component is “post shock” dense gas decelerated due to a spiral density wave. This could be observational evidence of dense molecular gas formation due to galactic shock by spiral density waves. We also present results from on-going observations toward NGC 604, which is the supergiant HII region of M33, using Nobeyama 45 m and ASTE 10 m telescopes. The ratio of CO(J = 3 − 2) to CO(J = 1 − 0) ranges from 0.3 to 1.2 in NGC 604. The CO(J = 1 − 0) map shows the clumpy structure while CO(J = 3− 2) shows a strong peak near to the central star cluster of NGC 604. The high ratio gas is distributed on the arc-like or shell-like structure along with Hα emission and HII region detected by radio continuum. These suggest that the dense gas formation and second generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova in central star cluster.