Halos of spiral galaxies. III. Metallicity distributions

We report results of a campaign to image the stellar populations in the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc (projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We unambiguously resolve the stellar halos 1 to 2 mag fainter than the tip of the red giant branch. Extended halo populations are detected in all galaxies. The color-magnitude diagrams appear to be completely dominated by giant branch stars, with no evidence for the presence of young stellar populations in any of the fields. The metallicity distribution function for the galaxy sample is derived from interpolation within an extensive grid of red giant branch loci. These loci are derived from theoretical sequences that are calibrated using the Galactic globular clusters and from empirical sequences for metal-rich stellar populations. We find that the metallicity distribution functions are dominated by metal-rich populations, with a tail extending toward the metal-poor end. To first order, the overall shapes of the metallicity distribution functions are similar to what is predicted by a simple, single-component model of chemical evolution with the effective yields increasing with galaxy luminosity. However, metallicity distributions significantly narrower than the simple model are observed for a few of the most luminous galaxies in the sample. The discrepancies are similar to those previously observed for NGC 5128, the halo of M31, and the Galactic bulge. Our observations can be used to help distinguish between models for the formation of spiral galaxies. It appears that more luminous spiral galaxies also have more metal-rich stellar halos. The increasingly significant departures from the closed-box model for the more luminous galaxies indicate that a parameter in addition to a single yield is required to describe chemical evolution. This parameter, which could be related to gas infall or outflow either in situ or in progenitor dwarf galaxies that later merge to form the stellar halo, tends to make the metallicity distributions narrower at high metallicity.