Spiral Arm Tangencies in the Milky Way

Abstract

The historical directions of spiral arm tangencies in the Milky Way are presented and compared to results of mid-infrared star counts using the Spitzer Space Telescope. While the Scutum and Centaurus tangency directions show a 20-30% excess of star counts, all other expected tangency directions show no similar increases. These two tangencies are probably associated with a density wave arm that comes off the near side of the bar of the Galaxy while the other arms whose tangencies are not detected may be compression in the gas, but not in the old stellar disk.