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Should the Outer Milky Way be Axisymmetric?

Published online by Cambridge University Press:  25 May 2016

James Binney
James Binney*
Affiliation:
Department of Physics, 1 Keble Road, Oxford, 0X1 3NP, England

Abstract

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The shape of the outer Milky Way should be heavily influenced by the shape of our heavy halo. Simulations of the formation of collisionless halos indicate that they should be highly triaxial, with a tendency to pro-lateness. If our halo were as strongly triaxial as these simulations suggest, the outer disk would have ellipticity ϵ disk ≃ 0.06.

Simulations of galaxy formation which include an appropriately small fraction of gaseous matter give rise to halos of much smaller ellipticity, and predict ϵ disk ≃ 0.02. These simulations are not as reliable as purely collisionless simulations, and we should treat their results with some caution. However, it is not incredible that a small fraction of gas could radically reduce the ellipticity of a halo by making the system's potential more centrally concentrated.

The distribution of the apparent ellipticities of spiral galaxies indicates that their disks have slightly eliptical disks, ϵ disk ≃ 0.05. This may merely reflect ephemeral features such as spiral arms, however. The velocity fields of disks provide a more robust probe of the ellipticity ϵ Φ of galaxy potentials. The narrowness of the Tully–Fisher relation places an interesting upper limit ϵ Φ < 0.1. More detailed studies of individual velocity fields will be required if we are unambiguously to demonstrate ϵ Φ > 0 in some cases.

Type
Chapter 1: Are the Disk and Halo Axisymmetric?
Information
Symposium - International Astronomical Union , Volume 169: Unsolved Problems of the Milky Way , 1996 , pp. 1 - 10
Copyright
Copyright © Kluwer 1996 

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