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Originally published in Science Express on 1 June 2006
Science 23 June 2006:
Vol. 312. no. 5781, pp. 1773 - 1777
DOI: 10.1126/science.1128455
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Reports

The Spiral Structure of the Outer Milky Way in Hydrogen

E. S. Levine,* Leo Blitz, Carl Heiles

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 kiloparsecs, 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°.

Astronomy Department, University of California, 601 Campbell Hall, Berkeley, CA 94720, USA.

* To whom correspondence should be addressed. E-mail: elevine{at}astron.berkeley.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)