2007ApJ...664L..27T

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'x5' 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, R_3–2/1–0, ranges between 0.3 and 1.2 in the observed region, and the R_3–2/1–0 values of the high-ratio gas arc are around or higher than unity, indicating very warm (T_kin≥60 K) and dense (n_H2≥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.