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 one to two magnitudes 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 which are calibrated using the Galactic globular clusters, and also using 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 simple, single-component model of chemical evolution with the effective yields increasing with galaxy luminosity. However, metallicity distribution
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