What Is a Halo? Observations of Faint Starlight around Disk Galaxies

Early observations of the stellar halos of nearby disk galaxies are discussed. Since most of the currently available data come from deep CCD surface photometry, and it can be diicult to discriminate between spatially overlapping populations with this technique, all non-thin-disk populations (bulges, halos, thick disks, and any component with low surface brightness) are considered. The sample of nine nearby disk galaxies covers a large range in bulge-to-disk ratio, and splits cleanly into 3 groups: (1) The galaxies with large bulges show r 1=4 law luminosity distributions out to very large radii. Diierences between globular clusters and eld stars at the same radius in M31 suggest that these r 1=4 components formed diierently from the globular cluster systems of these galaxies, which may represent their true halos. (2) Galaxies with medium-sized bulges have large, spatially extended thick disks, which have so far prevented detection of the signiicantly fainter halos, if they exist. However , they do have globular cluster systems. (3) Galaxies with very small bulges are the easiest place to look for halos like the Milky Way's, because of the lack of other stellar components outside the thin disk. However, nothing resembling our faint, centrally concentrated halo has been seen.