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2.2. Metallicity Distributions

Information on the GCS metallicity distribution function (MDF) is now available for many galaxies. A rough general trend for mean cluster metallicity to increase with galaxy size was suspected to exist at a very early stage in the subject (Brodie & Huchra 1991; Harris 1991) and mimics the same trend for the halo light in the galaxies themselves. However, as more data have accumulated, it has become increasingly clear that large galaxy-to-galaxy differences in the MDF exist even between otherwise similar ellipticals. Bimodal MDFs are commonly, but not universally, found in large galaxies of both elliptical and spiral type (cf. Ashman & Zepf 1998; Ajhar et al. 1994; Kissler-Patig et al. 1997). When present, a bimodal MDF is usually interpreted as an indicator of at least two major phases of galaxy formation at early times, but the exact mechanism (whether by in situ formation, major mergers, or accretion) is still very much under debate. No one process (see below) can be said to conveniently answer all the observed situations equally well.

In an important recent study, Forbes et al. (1997) find that for bimodal MDFs in giant E galaxies, the redder, more metal-rich component (denoted MRC in parallel with the Milky Way above) is more closely correlated with galaxy size than the bluer, metal-poor component (MPC). The mean metallicity of the MPC differs considerably, and without obvious patterns, from one such galaxy to another, whereas in most gE galaxies the MRC consistently peaks at [Fe/H] ~ -0.2 or varies only slightly with MVT. By contrast, the MPC in most spiral galaxies and dwarfs peaks consistently at [Fe/H] appeq -1.5, and the MRC (if it exists) at typically [Fe/H] appeq -0.5, at lower levels than in the giant ellipticals. Forbes et al. use this, among other evidence, to argue against the hypothesis originally laid out by Schweizer (1987) and Ashman & Zepf (1992) that giant E galaxies are the result of major mergers between large disk galaxies (but see the discussion below).