Annu. Rev. Astron. Astrophys. 1997. 35: 637-675
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2.5. General Properties of the Local Group Disk Galaxies

The diversity of properties of bulges, haloes, and disks evident in the four largest disk galaxies in the Local Group is striking. The essential properties seem to be the following. The two latest type galaxies (M 33, LMC) have no convincingly detected bulge, but both have at least some evidence for a small population of very old metal-poor stars. Both have old metal-poor globular clusters. The intermediate-type Milky Way galaxy contains what can be termed both a halo (metal-poor, old, extended, narrow abundance distribution, containing globular clusters) and a bulge (metal-rich, mostly, and perhaps exclusively, fairly old, with a very broad metallicity distribution function, and extremely compact in spatial scale). The earlier type M 31 has a prominent and extended bulge, which is both quite metal-rich and fairly old, and has a broad abundance distribution function. The only evidence for a metal-poor old halo in M 31 comes from its globular clusters and its - very few - RR Lyrae stars and BHB stars. In all cases, haloes are supported against gravitational gradients by their velocity dispersion (pressure-supported systems), very unlike disks, though this is perhaps as much a definition as an observation.

Thus, whereas the Local Group Spiral galaxies have a definable halo:disk ratio, which is apparently rather similar for all three, only the two earlier types have a definable bulge-to-disk ratio, which is greater for M 31 than for the Milky Way.

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