A face-on view of the first galactic quadrant in molecular clouds

A face-on map of 440 molecular clouds in the first quadrant of the inner Galaxy is presented. The map shows two main spiral arms, or rings, at 0.75Ro and 0.50Ro which extend to the far side of the Galaxy. It is found that the dominant feature, at 0.50Ro, contains half of the total mass in these clouds in contrast with previous results in which the feature is almost completely missing. The arm-interarm contrast for these clouds is 5:1. It is shown that the cooler clouds within this sample show less arm-interarm contrast and a flatter radial distribution than the warmer clouds, supporting the hypothesis that spiral structure is correlated with peak molecular cloud temperature. These cooler clouds also have a lower average mass than the warmer clouds indicating a mass-temperature and mass-spiral arm correlation, although there is a wide range of mass in both groups. 15 refs.