Dense Clouds and Star Formation on Spiral Arm in M33 -deep CO and HCN Observation in NGC604-

We investigated properties of giant molecular clouds (GMCs) in the massive star forming region NGC604 on the spiral arm in M33. CO(J=1-0), HCN and 90G Hz continuum emissions were observed at the two field toward NGC604 with the Nobeyama Millimeter Array (NMA). Eight clouds are identified from our highresolution CO images using the CLFIND algorithm, with similar masses and sizes (M∼ 0.2−2.9×10M , D∼21-38pc ). The physical properties of these clouds follow the Larson’s law and are comparable to those of Galactic GMCs. Several of these clouds compose two large complexes. From our HCN and continuum results as well as the CO results, we classify these clouds into 4 categories; the clouds which have (1) no sign of star formation (2) association with dense gas but no HII region, (3) association with both dense gas and star cluster formation, and (4) association with HII regions. Category (3) has a larger rotation velocity than that predicted by the galactic rotation curve and that of category (2), although both categories locate at the same galactic radius. The reason is likely to be the acceleration by the galactic potential. In fact there are intensive star formation activity in category (3), which suggest that the molecular gas is being accumulated at the bottom of the gravitational potential. This scenario is also supported by a rough estimate of the time scale from category (2) to (3).