AAS 207th Meeting, 8-12 January 2006
Session 40 Galactic Structure with WIMPS, STARS and Gas
Oral, Monday, 10:00-11:30am, January 9, 2006, Salon 1

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[40.05] The Shape of the HI Warp in the Outer Milky Way Disk

E.S. Levine, L. Blitz, C. Heiles (UC Berkeley), M. Weinberg (University of Massachusetts, Amherst)

Although the warping of the disk of the Milky Way has been known since 1957, our work represents the first time the Milky Way warp has been quantitatively described and we find it to be both elegant and surprising. We examine the outer Galactic HI disk for deviations from the b=0 plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency 1 and 2, all of which grow with Galactocentric radius. The global warp demonstrates approximately an order of magnitude more power in each mode with azimuthal wavenumber m=0,1, and 2 than in any higher frequency mode; thus three and only three modes are necessary to describe the large-scale behavior of the warp. The power in the m=0 and m=2 modes grows starting from around 15 kpc; the m=1 mode is the most powerful everywhere in the outer disk. We outline six observational conclusions regarding the warp that any potential theoretical mechanism must satisfy. We will also show a movie that demonstrates the evolution of the three modes with time.

ESL and LB are supported by NSF grant AST 02-28963. CH is supported by NSF grant AST 04-06987.

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