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 ( 4 \farcs 2 \times 2 \farcs 6 , corresponding to 17 pc \times 11 pc physical size ) of CO emission allowed us to uncover ten individual molecular clouds that have masses of ( 0.8 – 7.4 ) \times 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 ” .
The HCN and 89 GHz continuum emission show offset from the CO peak and are distributed to the direction of the central cluster .
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 .
The CO clouds show varieties in star formation efficiencies ( SFEs ) , which are estimated from the 89-GHz and combination of H \alpha and Spitzer 24- \micron data .
Furthermore , we 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 .
( 48 ) 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 .