The W51 Giant Molecular Cloud

Abstract

We present 45AÈ47A angular resolution maps at 50A sampling of the 12CO and 13CO J \ 1È0 emission toward a region in the W51 H II region complex. These data permit the spatial and kine-1¡ .39 ] 1¡ .33 matic separation of several spectral features observed along the line of sight to W51 and establish the presence of a massive (1.2 ] 106 large (*l ] *b \ 83 ] 114 pc) giant molecular cloud (GMC), M _), deÐned as the W51 GMC and centered at (l, b, 61 km s~1). A second massive V) c B (49¡ .5, [0¡ .2, elongated (22 ] 136 pc) molecular cloud is found at velocities of D68 km s~1 along the (1.9 ] 105M _), southern edge of the W51 GMC. Of the Ðve radio continuum sources that classically deÐne the W51 region, the brightest source at j6 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 Ðfth 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%È10% by mass. While the W51 GMC is larger and more massive than any nearby molecular cloud, the average column density is not unusual given its size, and the H 2 mean volume density is comparable to that in nearby clouds. The W51 GMC is also similar to other H 2 clouds in that most of the molecular mass is contained in a di use 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.