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D. A. Hurley-Keller, P. Harding, H. L. Morrison (Case Western Reserve Univ.), G. Jacoby (WIYN)
Does the nearest large spiral to us, M31, have a thick disk and halo, as the Milky Way does? We now know that other galaxies with large bulges (e.g. NGC 891) also have a thick disk, and that some galaxies do not have them (e.g. NCG 4244). We also know that the Milky Way's halo is not an extension of its bulge/bar, but a kinematically distinct population. However, the opportunities to observe these faint populations are rare, and their existence and properties in even our nearest neighbor remain uncertain. We have velocities for a sample of ~120 planetary nebulae (PNe) near the minor axis of M31 out to a distance of ~20 kpc from the galaxy. With this sample, we can identify both the elusive thick disk and halo populations, if present.
Careful work on the ages, metallicities, and kinematics of thick disk and halo stars in the Milky Way has greatly improved our understanding of the properties of these populations, and has provided a first step in distinguishing between competing formation scenarios. But the similarity of faint stellar populations in other spiral galaxies to those of the Milky Way remains in question, and M31 will provide a stepping stone to more distant galaxies, as well as those with larger bulge-to-disk ratio.
The author(s) of this abstract have provided an email address for comments about the abstract: denise@smaug.astr.cwru.edu