Aperture Synthesis Observations of CO, HCN, and 89GHz Continuum Emission toward NGC 604 in M 33: Sequential Star Formation Induced by Supergiant Hii region

Abstract: We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and 89-GHz continuum emissions toward NGC 604, known as the supergiant H ii region in a nearby galaxy M 33. Our high spatial resolution images of CO emission allowed us to uncover ten individual molecular clouds that have masses of (0.8 -7.4) 10$^5$M$_{\sun }$ and sizes of 5 -- 29 pc, comparable to those of typical Galactic giant molecular clouds (GMCs). Moreover, we detected for the first time HCN emission in the two most massive clouds and 89 GHz continuum emission at the rims of the "H${\alpha}$ shells". Three out of ten CO clouds are well correlated with the H${\alpha}$ shells both in spatial and velocity domains, implying an interaction between molecular gas and the expanding H ii region. Furthermore, we estimated star formation efficiencies (SFEs) for each cloud from the 89-GHz and combination of H${\alpha}$ and 24-${\mu}$m data, and found that the SFEs decrease with increasing projected distance measured from the heart of the central OB star cluster in NGC 604, suggesting the radial changes in evolutionary stages of the molecular clouds in course of stellar cluster formation. Our results provide further support to the picture of sequential star formation in NGC604 initially proposed by Tosaki et al. (2007) with the higher spatially resolved molecular clouds, in which an isotropic expansion of the H ii region pushes gases outward and accumulates them to consecutively form dense molecular clouds, and then induces massive star formations.