Milky Way Spiral Structure: A New Look

Abstract

The spiral structure of the outer Milky Way based on two recent HI 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 ℓ-u 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.

The mapping of the spiral structure of the Milky Way has been a notoriously difficult task. Among the many problems have been the confusion caused by velocity streaming (Burton 1971, 1972), the distance ambiguity in the inner Galaxy, and the lack of objective criteria for identifying spiral arms. The last difficulty has introduced an unwelcome degree of subjectivity to spiral structure studies, and there is as yet no consensus on what the global appearance of the Milky Way is when viewed normal to the disk. For example, one would like to know to what degree the spiral arms are regular and symmetric, what is the length scale of the spiral features, and to what degree the structure is dominated by “spurs:” features that can be identified only over a small fraction of a radian.

Because there is no distance ambiguity for material beyond the solar circle, analysis of the HI in the outer Galaxy has the potential for providing a clearer picture of the spiral arm pattern than can be obtained in the inner Galaxy. Two independent, complementary analyses of the HI beyond the solar circle by Kulkarni, Blitz and Heilies (1982 hereafter KBH) and Henderson, Jackson and Kerr (1982 hereafter HJK) are used here to examine the spiral structure in the outer Galaxy. The observed structure in the outer Galaxy is then used to predict the appearance of longitude-velocity plots of the gas in the inner Galaxy, assuming that the spiral pattern is unchanged inwards to a distance of 4 kpc from the center.