H II region--molecular cloud complex W51

We have made new observations of the H II region--molecular cloud complex W51 in the spectral lines of lambda=2.6 mm CO, lambda=3.1 mm CS, lambda=2 cm H2CO, and the recombination line H76α. Since the line of sight to this H II region lies nearly tangent to the Sagittarius spiral arm, W51 offers an excellent opportunity to study the activity associated with the large-scale spiral structure of our Galaxy. Our molecular observations are most consistent with a picture in which three streams of cold gas flow through the complex. These streams have been placed within the structure of the hydrogen observed in this neighborhood of the galactic plane. This study shows that the H II regions are now near the outside of these molecular streams, ionizing cold gas that flows into the low-pressure regions between the clouds. Simple models of the H II regions in the W51 complex have been constructed which reproduce many of the continuum and spectral line observations that have been obtained