旋渦星系NGC 6946中的巨分子雲的物理性質

Abstract

巨分子雲(giant molecular clouds)被認為是恆星形成的場所，因此它們的物理性質與恆星形成以及星系演化息息相關。在這份論文中，我們利用次毫米波陣列(SMA)觀測旋渦星系NGC 6946。觀測波長為一氧化碳因轉動能階由二躍遷到一所放出的1.3毫米輻射，視野涵蓋範圍約為10^7平方秒差距。我們將SMA的觀測資料與IRAM-30m的觀測資料組合在一起，以得到完整的亮度資訊。組合完後的影像其解析度約為2角秒，對應到NGC6946的距離則相當於50秒差距。我們利用CPROPS程式套件找到532個巨分子雲。在星系盤面上的巨分子雲有著大致相似的性質，質量約為8*10^5太陽質量，半徑約為60秒差距，譜線寬度約為每秒4公里，表面密度約為每平方秒差距100個太陽質量，溫度則為2 K。至於在星系中心的巨分子雲，這些性質的數值都比位在盤面上的巨分子雲要高。我們也觀察到巨分子雲的半徑、線寬、光度之間的關係和本星系群有一些差異，像是在盤面上的巨分子雲其線寬幾乎與半徑無關，同時巨分子雲的表面密度也不像其他星系為一個定值，而是隨半徑大小而改變。另外，若假設巨分子雲皆處於維里平衡(virial equilibrium)，則一氧化碳強度與氫氣的柱密度之間的換算因子約為4*10^20 cm^-2(K km s^-1)^-1。最後，在星系盤面中，大部分的分子氣體並不屬於巨分子雲，而是巨分子雲外圍一些比較稀薄的氣體。

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.