4.3. The G Dwarf Problem: A Peaked Abundance Distribution

The abundance distribution (number of stars vs. [Fe/H]) in the Milky Way galaxy is more strongly peaked than the simplest closed-box model (see Section 1) with constant yield predicts. This is known as the G dwarf problem (van den Bergh 1962; Pagel 1997; Cowley 1995). It seems almost certain now that other galaxies share this "problem" of having a relatively peaked abundance distribution. Part of the evidence comes from integrated light (Bressan, Chiosi, & Fagotto 1994; Worthey, Dorman, & Jones 1996) via three lines of evidence.

First, around 2600 Å there is a paucity of ultraviolet flux which would otherwise be greater if large numbers of metal-poor main-sequence stars are present. Second, in small compact ellipticals for which data exist, a high-resolution Ca II index (Rose 1994 system, not Lick system) detects few A-type horizontal-branch stars, where more of these objects would be expected if a large metal-poor population exists. Third, if a metal-sensitive index like Fe4668 is modeled using the broad simple model predictions, the strong line strengths in large galaxies are very difficult to attain and require improbably high yield values. Clinching the integrated light results, recent CMD studies of individual red giants in the compact elliptical M32 (Grillmair et al. 1996), large elliptical NGC 5128 (Soria et al. 1996), and the disk of M31 (Grillmair et al. 1996) all indicate a very peaked abundance distribution similar to or more peaked than that of the Milky Way.

These empirical findings represent important constraints on some galaxy formation and chemical evolution models, and their implications are only starting to be explored (Larson 1998).