| Summary: | 碩士 === 國立臺灣大學 === 天文物理研究所 === 99 === Giant molecular clouds (GMCs) are the birthplaces of most stars, so their physical properties is important to understand star formation and galaxy evolution. In this study, the spiral galaxy NGC 6946 was observed over a 10 kpc2 area along its major axis in J=2–1 rotational transition line of carbon monoxide (CO) by the Submillimeter Array (SMA). This 10-pointing mosaic data have been combined with the observations of IRAM-30m
single-dish telescope so that the total flux is recovered. The combined data have ∼ 2 arcsecond (50 pc) resolution, and 532 GMCs have been identified by the cloud-identifying algorithm CPROPS. Those GMCs in the inner disk (< 1.2 kpc) and the outer disk (1.2–4 kpc) have similar properties; the medians for virial mass, size, velocity dispersion, surface density, and temperature are ∼ 8 × 10^5 Msun, 60 pc, 4 km s^−1, 100 Msun pc^−2, and 2 K, respectively. In contrast, GMCs near the center have larger values for these properties. Correlations between cloud size, velocity dispersion, and luminosity have been compared with those observed in Local Group galaxies, and some differences have been found. The size-line width relation shows different slopes so that the velocity dispersions for disk GMCs are nearly independent of their sizes. Moreover, the surface densities does not remain constant like other galaxies but instead varies with cloud sizes. The CO-to-H2 conversion factor is found to be ∼ 4 × 10^20 cm^−2(K km s^−1)^−1 if GMCs are assumed to be in virial equilibrium. Finally, a significant fraction of molecular gas in the disk of NGC 6946 does not belong to the GMCs identified in this study.
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