CO (J = 2-1) Line Observations of the Galactic Center Molecular Cloud Complex. II Dynamical Structure and Physical Conditions
CO (J = 2-1) Line Observations of the Galactic Center Molecular Cloud Complex. II Dynamical Structure and Physical Conditions
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CO (J = 2-1) 银河系中心分子云复合体的线观测。
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
S. Sakamoto
中科院分区：
文献类型：
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作者：
T. Oka;T. Hasegawa;M. Hayashi;T. Handa;S. Sakamoto
利用东京大学野山射电天文台的60厘米巡天望远镜，对银河系内部几百秒差距进行了大规模的12C16O （J = 2-1）巡天。我们在-2区取了大约70012C16O （J = 2-1）的光谱。°5≤l≤2。|≤1°，带0。°125栅格间距，覆盖了银河系中心巨大分子云复合体的整个区域。参考哥伦比亚1.2 m望远镜采集的CO （J = 1-0）数据，计算出J = 2-1与J = 1-0的强度比。给出了CO （J = 2-1）线的速度通道图和经速图，并给出了对应的J = 2-1/J = 1-0强度比图。大尺度CO图使我们能够识别出几个巨大的分子云复合体和分子气体的许多特征。我们在CO （J = 2-1）数据中发现了15个大于~ 30pc的分子云配合物。它们的虚质量比CO光度估计的质量至少大一个数量级。如果我们注意到它们可能没有重力束缚，而是与该区域的热气体和/或磁场处于压力平衡，则可以消除这种差异。利用压力平衡情况下云的维里质量和CO质量的表达式，我们得到银河系中心分子云的X因子为X = 0.24 × 1020 cm-2 (K km s-1)-1，比银河系盘的X因子（X = 3.0 × 1020 cm-2 [K km s-1]-1）低一个数量级。我们估计银河系中心的总分子质量为M（H2） × 2 × 107 M☉作为下限；银河系中央400pc内的实际总气体质量必须是M(H2) = (2-6) × 107 M☉。我们利用J = 2-1和J = 1-0线之间的强度比来诊断银河系中心分子气体的物理状态。虽然中层CO J = 2-1/J = 1-0线强度比高（~0.74），但|b处的分子气体|≥0。°25表现出低的J = 2-1/J = 1-0比值（~0.6）。如果我们把|≤1°，-2范围内的所有发射都包括在内，则J = 2-1/J = 1-0的总光度比为R(2-1)/(1-0) = 0.64±0.01。°5≤l≤2。|≤150km s-1。这表明，在银河系中央400pc的CO总光度中，距离平面小于50pc的低密度气体占主导地位。每个云复合体中R(2-1)/(1-0)的分子气体的分数分布清楚地表明，比值很高的气体[R(2-1)/(1-0)≥1.0]与相关紫外源之间存在密切关系。
A large-scale 12C16O (J = 2-1) survey of the inner few hundred parsecs of the Galaxy has been conducted using the University of Tokyo-Nobeyama Radio Observatory 60 cm survey telescope. We have taken about 70012C16O (J = 2-1) spectra in the region -2.°5 ≤ l ≤ 2.°5 and |b| ≤ 1° with 0.°125 grid spacing, covering the entire region of the huge molecular cloud complex in the Galactic center. We refer to the CO (J = 1-0) data taken with the Columbia 1.2 m telescope and calculate the J = 2-1 to J = 1-0 intensity ratio. Velocity channel maps and longitude-velocity maps of CO (J = 2-1) line are presented, with corresponding maps of J = 2-1/J = 1-0 intensity ratio. Large-scale CO maps enable us to identify several giant molecular cloud complexes and many characteristic features of molecular gas. We identify 15 molecular cloud complexes larger than ~30 pc in our CO (J = 2-1) data. Their virial masses are at least 1 order of magnitude larger than the masses estimated from the CO luminosity. This discrepancy can be removed if we notice that they may not be gravitationally bound but are in pressure equilibrium with the hot gas and/or magnetic field in this region. Using the expressions of virial mass and CO mass for a cloud in the pressure equilibrium case, we get the X-factor for the Galactic center molecular clouds as X = 0.24 × 1020 cm-2 (K km s-1)-1, which is 1 order of magnitude lower than that in the Galactic disk (X = 3.0 × 1020 cm-2 [K km s-1]-1). We estimate the total molecular mass in the Galactic center as M(H2) ≅ 2 × 107 M☉ as a lower limit; the actual total gas mass within the central 400 pc of the Galaxy must be M(H2) = (2-6) × 107 M☉. We diagnose the physical conditions of the molecular gas in the Galactic center using the intensity ratio between the J = 2-1 and J = 1-0 lines. Although the CO J = 2-1/J = 1-0 line intensity ratio is high (~0.74) in the midplane, molecular gas at |b| ≥ 0.°25 exhibits low J = 2-1/J = 1-0 ratios (~0.6). The overall J = 2-1/J = 1-0 luminosity ratio is R(2-1)/(1-0) = 0.64 ± 0.01 if we include all the emission within |b| ≤ 1°, -2.°5 ≤ l ≤ 2.°5, and |VLSR| ≤ 150 km s-1. This indicates that low-density gas ≲50 pc away from the plane dominates the total CO luminosity of the central 400 pc of the Galaxy. The fractional distribution of the molecular gas with R(2-1)/(1-0) for each cloud complex clearly demonstrates the close relationship between the gas with a very high ratio [R(2-1)/(1-0) ≥ 1.0] and associated UV sources.