Milky Way spiral structure - A new look

The spiral structure of the outer Milky Way based on two recent H I studies is shown to imply that beyond R = 10 kpc, the Galaxy is a regular, four-armed spiral galaxy. The spiral pattern is extrapolated inward and compared to the results of northern and southern hemisphere CO surveys. The positions of the maxima of the CO emissivity can be reproduced, which suggests that at least part of the emissivity of the 'molecular ring' is related to spiral structure. Comparison of survey results with predicted longitude-velocity diagrams indicates that, although a number of features seen in the CO surveys can be explained, the inner Galaxy is more complex than an extrapolation of the relatively simple outer structure would suggest.