Arc-like Distribution of High Co(j=3–2)/co(j=1–0) Ratio Gas Surrounding the Central Star Cluster of the Supergiant Hii Region Ngc

We report the discovery of a high CO(J=3–2)/CO(J=1–0) ratio gas with an arc-like distribution (“high-ratio gas arc”) surrounding the central star cluster of the supergiant Hii region NGC 604 in the nearby spiral galaxy M 33, 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 south-east to north-west direction with a size of ∼ 200 pc. The western part of the high-ratio gas arc closely coincides well with the shells of the Hii 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 (nH2 ≥ 10 cm) 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. Subject headings: galaxies: individual (M 33) — ISM: Hii regions — stars: formation — ISM: individual (NGC 604) — ISM: molecules