Arclike Distribution of High CO (J = 3-2)/CO (J = 1-0) Ratio Gas Surrounding the Central Star Cluster of the Supergiant H II Region NGC 604

We report the discovery of a high CO (J = 3-2)/CO (J = 1-0) ratio gas with an arclike distribution ("high-ratio gas arc") surrounding the central star cluster of the supergiant H II region NGC 604 in the nearby spiral galaxy M33, based on multi-J CO observations of a 5' × 5' region of NGC 604 conducted using the ASTE 10 m and NRO 45 m telescopes. The discovered "high-ratio gas arc" extends to the southeast-northwest direction with a size of ~200 pc. The western part of the high-ratio gas arc closely coincides with the shells of the H II regions traced by Hα and radio continuum peaks. The CO (J = 3-2)/CO (J = 1-0) ratio, R3-2/1-0, ranges between 0.3 and 1.2 in the observed region, and the R3-2/1-0 values of the high-ratio gas arc are around or higher than unity, indicating very warm (Tkin ≥ 60 K) and dense (n ≥ 103-104 cm-3) conditions of the high-ratio gas arc. We suggest that the dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.

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