We present 45''–47'' angular resolution maps at 50'' sampling of the 12CO and 13CO J = 1–0 emission toward a 139 × 133 region in the W51 H II region complex. These data permit the spatial and kinematic separation of several spectral features observed along the line of sight to W51 and establish the presence of a massive (1.2 × 106 M⊙), large (Δl × Δb = 83 × 114 pc) giant molecular cloud (GMC), defined as the W51 GMC and centered at (l, b, V)c ≈ (495, -02, 61 km s-1). A second massive (1.9 × 105M⊙), elongated (22 × 136 pc) molecular cloud is found at velocities of ~68 km s-1 along the southern edge of the W51 GMC. Of the five radio continuum sources that classically define the W51 region, the brightest source at λ6 cm (G49.5-0.4) is spatially and kinematically coincident with the W51 GMC and three (G48.9-0.3, G49.1-0.4, and G49.2-0.4) are associated with the 68 km s-1 cloud. Published absorption-line spectra indicate that the fifth prominent continuum source (G49.4-0.3) is located behind the W51 molecular cloud. The W51 GMC is among the upper 1% of clouds in the Galactic disk by size and the upper 5%–10% by mass. While the W51 GMC is larger and more massive than any nearby molecular cloud, the average H2 column density is not unusual given its size, and the mean H2 volume density is comparable to that in nearby clouds. The W51 GMC is also similar to other clouds in that most of the molecular mass is contained in a diffuse envelope that is not currently forming massive stars. We speculate that much of the massive star formation activity in this region has resulted from a collision between the 68 km s-1 cloud and the W51 GMC.
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机译：我们在W51 H II区域复合体中向139×133区域的12CO和13CO J = 1–0发射的50英寸采样处呈现了45''– 47''的角分辨率图。这些数据允许沿W51视线观察到的几个光谱特征进行空间和运动学分离，并建立了巨大的（1.2×106M⊙），大的（Δl×Δb= 83×114 pc）巨型分子云（ GMC），定义为W51 GMC，并以（l，b，V）c≈（495，-02，61 km s-1）为中心。在W51 GMC的南边缘以约68 km s-1的速度发现了第二个大块（1.9×105M⊙），细长（22×136 pc）的分子云。在经典定义W51区域的五个无线电连续谱源中，λ6cm（G49.5-0.4）处最亮的源在空间和运动学上与W51 GMC和三个（G48.9-0.3，G49.1-0.4，和G49.2-0.4）与68 km s-1云有关。已发表的吸收线光谱表明，第五个突出的连续谱源（G49.4-0.3）位于W51分子云后面。 W51 GMC的大小在银河系盘中云层的上1％，在质量上的5％-10％之间。尽管W51 GMC比附近的任何分子云都更大，更重，但考虑到其大小，平均H2柱密度并不罕见，并且平均H2体积密度与附近的云相当。 W51 GMC也与其他云类似，因为大部分分子质量都包含在目前尚未形成大质量恒星的弥漫性包膜中。我们推测该地区的大量恒星形成活动主要是由68 km s-1云与W51 GMC碰撞造成的。
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