Distortions in the Galactic Disk

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Session 60 -- Galactic Structure
Oral presentation, Thursday, January 13, 10:15-11:45, Crystal Forum Room (Crystal City Marriott)

[60.04] Distortions in the Galactic Disk

David N. Spergel (Princeton University), Leo Blitz, Peter Teuben (University of Maryland, College Park)

Using the newly released COBE data, we show that the disk of the Milky Way has a far more complex distribution of stars than has been previously thought. Specifically, neither an axisymmetric or simple elliptical distribution seems to be able to account for the distribution of stars. We have analyzed the data from the four shortest infrared bands from the DIRBE instrument, and have produced images corrected for dust extinction at K, L, and M bands. By comparing the emission at various longitudes with that at particular co-longitudes, we find that there are large scale asymmetries in surface brightness of order 20\% in amplitude spanning \sim$40^o$ in each of the three wavelength bands which are consistent from one image to the next. These asymmetries do not correspond to the variations in CO or IRAS 100 micron emission, which are expected to be strongly influenced by spiral arms, and the asymmetries can be shown to be little affected by uncertainties in the dust subtraction. If they are due to the variations in the large scale distribution of stars, the amplitude of the asymmetry is surprisingly large. The sign of the asymmetry is seen to change three times between $ l = 0\deg and 180\deg$. The first correspnds to the bar or triaxial bulge; the two other changes of sign are newly detected. It may be that the Milky Way has an ellipsoidal distribution of stars in the disk with an orientation that changes with radius, or that the DIRBE data is seeing a complex distribution of modes with $m = 1, m = 2$, or both. Currently existing dynamical models do not appear to be good representations of the DIRBE data.

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