We present the first infrared ( IR ) 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 \scriptscriptstyle \prime 50 \scriptscriptstyle \prime \scriptscriptstyle \prime ( \sim 9 kpc ) south of the center of the galaxy .
The luminosity function ( LF ) shows a marked discontinuity at [ F160W ] \approx 20.0 .
This is 1 - 2 mag above the tip of the red giant branch ( TRGB ) expected for an old population ( \sim 12 Gyr ) at the distance modulus of NGC 5128 .
We propose that the majority of stars above the TRGB have intermediate ages ( \sim 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 \pm 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 .