Evidence for a two-armed spiral in the Milky Way

Emission profiles of the Galactic plane in K and at 240 mum are presented, and features associated with the tangents of the spiral arms are identified. In the K band, which traces stellar emission and suffers little from absorbtion, features associated with the arm tangents indicate that a two-armed logarithmic spiral dominates the nonaxisymmetric structure of the Milky Way. In contrast, the 240 mum emission from dust entrained in the interstellar gas is consistent with a four-armed model, in concordance with radio data and optical spiral tracers. This suggests that the non-axisymmetric mass perturbation responsible for the four-armed spiral structure in the gas has a two rather than four-armed structure.