13CO(J = 1–0) On-the-Fly Mapping of the Giant H II Region NGC 604: Variation in Molecular Gas Density and Temperature due to Sequential Star Formation

Abstract

Abstract We present $^{13}$CO($J$$=$ 1–0) line emission observations with the Nobeyama 45-m telescope toward the giant H II region NGC 604 in the spiral galaxy M 33. We detected $^{13}$CO($J$$=$ 1–0) line emissions in 3 major giant molecular clouds (GMCs) labeled as GMC-A, B, and C beginning at the north. We derived two line intensity ratios, $^{13}$CO($J$$=$ 1–0)$/$$^{12}$CO($J$$=$ 1–0), $R_{13/12}$, and $^{12}$CO($J$$=$ 3–2)$/$$^{12}$CO($J$$=$ 1–0), $R_{31}$, for each GMC at an angular resolution of 25$”$ (100 pc). The averaged values of $R_{13/12}$ and $R_{31}$ are 0.06 and 0.31 within the whole GMC-A, 0.11 and 0.67 within the whole GMC-B, and 0.05 and 0.36 within the whole GMC-C, respectively. In addition, we obtained $R_{13/12}$$=$ 0.09$\ \pm\ $0.02 and $R_{31}$$=$ 0.76$\ \pm\ $0.06 at the $^{12}$CO($J$$=$ 1–0) peak position of the GMC-B. Under the Large Velocity Gradient approximation, we determined a gas density of 2.8 $\times$ 10$^{3}\ $cm$^{-3}$ and a kinetic temperature of 33$^{+9}_{-5}\ $K at the $^{12}$CO($J$$=$ 1–0) peak position of the GMC-B. Moreover, we determined 2.5 $\times$ 10$^{3}\ $cm$^{-3}$ and 25$\ \pm\ $2 K as the averaged values within the whole GMC-B. We concluded that dense molecular gas is formed everywhere in the GMC-B because the derived gas density not only at the peak position of the GMC, but also averaged over the whole GMC, exceeds 10$^{3}\ $cm$^{-3}$. On the other hand, the kinetic temperature averaged over the whole GMC-B, 25 K, is significantly lower than that at the peak position, 33 K. This is because the H II regions are lopsided to the northern part of the GMC-B; thus, OB stars can heat only the northern part, including the $^{12}$CO($J$$=$ 1–0) peak position, of this GMC.