The nature of the halo population of NGC 5128 resolved with NICMOS on the Hubble Space Telescope

We present the first infrared (IR) color-magnitude diagram (CMD) for the ha lo of a giant elliptical galaxy. The CMD for the stars in the halo of NGC 5 128 (Centaurus A) was constructed from Hubble Space Telescope NICMOS observ ations of the WFPC2 CHIP-3 held of Soria et al. to a 50% completeness magni tude limit of [F160W] = 23.8. This field is located at a distance of 08'50 "(similar to 9 kpc) south of the center of the galaxy. The luminosity funct ion (LF) shows a marked discontinuity at [F160W] approximate to 20.0. This is 1-2 mag above the tip of the red giant branch (TRGB) expected for an old population (similar to 12 Gyr) at the distance modulus of NGC 5128. We pro pose that the majority of stars above the TRGB have intermediate ages (simi lar to 2 Gyr), in agreement with the WFPC2 observations of Soria et al. Fiv e stars with magnitudes brighter than the LF discontinuity are most probabl y due to Galactic contamination. The weighted average of the mean giant bra nch 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 d etermine that the observed spread in color is real, suggesting a real sprea d in metallicity. We estimate the lower and upper bounds of the stellar met allicity range by comparisons with observations of Galactic star clusters a nd 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.