We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk, demonstrating that the Galaxy is a non-axisymmetric multiarmed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kiloparsec, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20° to 25°.
- Pub Date:
- June 2006
- 10 pages with 4 color figures. Accepted for publication in Science. Embargoed for discussion in the popular press until publication in ScienceXpress. Higher resolution versions of figures 3 and 4 are available at http://astron.berkeley.edu/~elevine