  Formation paradigms for massive galaxies have long centered around two
antipodal hypotheses -- the monolithic-collapse and the accretion/merger
scenarios. Empirical data on the stellar contents of galaxy halos is crucial in
order to develop galaxy formation and assembly scenarios which have their root
in observations, rather than in numerical simulations. Hubble Space Telescope
(HST) has enabled us to study directly individual stars in the nearby E/S0
galaxies Cen A, NGC 3115, NGC 5102, and NGC 404. We here present and discuss
HST single-star photometry in V and I bands. Using color-magnitude diagrams and
stellar luminosity functions, we gauge the galaxies' stellar contents. This can
be done at more than one position in the halo, but data with deeper limiting
magnitudes are desired to quantify the variation of metallicity with
galactocentric radius. We here compare the color distributions of red giant
stars with stellar isochrones, and we intercompare the galaxies' halo
populations, noting that their total absolute V magnitudes cover the range from
about -21.5 to -17.5. In the future, we plan to model the stellar metallicity
distributions with the aim to constrain chemical enrichment scenarios, a step
towards unravelling the evolutionary history of elliptical and lenticular
galaxies.
