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

We present 13CO(J = 1-0) line emission observations with the Nobeyama 45-m telescope toward the giant HII region NGC 604 in the spiral galaxy M 33. We detected 13CO(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, 13CO(J = 1-0)=12CO(J = 1-0), R13/12, and 12CO(J = 3-2)=12CO(J = 1-0), R31, for each GMC at an angular resolution of 25″ (100 pc). The averaged values of R13/12 and R31 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 R13/12 = 0.09 ± 0.02 and R31 = 0.76 ± 0.06 at the 12CO(J = 1-0) peak position of the GMC-B. Under the Large Velocity Gradient approximation, we determined a gas density of 2.8 × 10 3 cm-3 and a kinetic temperature of 33+9-5 K at the 12CO(J = 1-0) peak position of the GMC-B. Moreover, we determined 2.5 × 103cm-3 and 25±2K 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 103cm-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 HII regions are lopsided to the northern part of the GMC-B; thus, OB stars can heat only the northern part, including the 12CO(J = 1-0) peak position, of this GMC. © 2012. Astronomical Society of Japan.