  We present the first infrared color-magnitude diagram (CMD) for the halo of a
giant elliptical galaxy. The CMD for the stars in the halo of NGC 5128
(Centaurus A) was constructed from HST NICMOS observations of the WFPC2 CHIP-3
field of Soria et al. (1996) to a 50% completeness magnitude limit of
[F160W]=23.8. This field is located at a distance of 08'50" (~9 kpc) south of
the center of the galaxy. The luminosity function (LF) shows a marked
discontinuity at [F160W]=20.0. This is 1-2 mag above the tip of the red giant
branch (TRGB) expected for an old population (~12 Gyr) at the distance modulus
of NGC 5128. We propose that the majority of stars above the TRGB have
intermediate ages (~2 Gyr), in agreement with the WFPC2 observations of Soria
et al. (1996). Five stars with magnitudes brighter than the LF discontinuity
are most probably due to Galactic contamination. The weighted average of the
mean giant branch color above our 50% completeness limit is
[F110W]-[F160W]=1.22+-0.08 with a dispersion of 0.19 mag. From our
artificial-star experiments we determine that the observed spread in color is
real, suggesting a real spread in metallicity. We estimate the lower and upper
bounds of the stellar metallicity range by comparisons with observations of
Galactic star clusters and theoretical isochrones. Assuming an old population,
we find that, in the halo field of NGC 5128 we surveyed, stars have
metallicities ranging from roughly 1% of solar at the blue end of the color
spread to roughly solar at the red end, with a mean of [Fe/H]=-0.76 and a
dispersion of 0.44 dex.
