The W51 Giant Molecular Cloud

     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.