The metallicity distribution in the halo stars of NGC 5128: Implications for galaxy formation

We have used the Hubble Space Telescope to obtain WFPC2 V, I photometry for stars in the halo of NGC 5128, the nearest giant elliptical galaxy. The resulting color-magnitude diagram (CMD) of this field, which lies similar to 21 kpc from the center of the galaxy, contains more than 10,000 stars and reaches almost 3 mag down the red giant branch (RGB). From the sharply defined RGB tip at I = 24.1 +/- 0.1 and M-I(tip)= -4.1, we obtain a distance to NGC 5128 of 3.9 Mpc. Comparison with the fiducial RGBs of Milky Way globular clusters and model isochrones demonstrates that this outer-halo population of NGC 5128 is completely dominated by old stars, with an extremely broad metallicity range extending from the most metal-poor Galactic globular clusters at [Fe/H] less than or similar to -2 up to above-solar abundance. The relative contribution from any younger, bright asymptotic branch component is negligible. The shape of the metallicity distribution function (MDF), derived from the CMD by interpolation within the isochrones, can be remarkably well matched by a simple two-component model of closed-box chemical enrichment, where the first component starts with an initial abundance Z(0) = O and the second component with Z(0) similar or equal to 0.25 Z(.). Two-thirds of the stars belong to the metal-richer component, and one-third to the metal-poorer one; the mean metallicity of the entire sample is ([Fe/H]) = -0.41, consistent with the colors of the integrated halo light. The metal-rich component also coincides strikingly in mean and dispersion with the metal-rich peak of the halo globular clusters in NGC 5128, suggesting that both of these halo subsystems formed contemporaneously. A discussion of various models of E galaxy formation leads us to suggest that a basic in situ formation picture with two distinct epochs of star formation best fits-the observations; other models involving major contributions from accretions or mergers are less satisfactory. The timing of the events we suggest is that the first, more metal-poor star-forming epoch took place while the protogalaxy was still in a clumpy, fragmented state, leaving most of the gas unused. The second and larger star formation epoch took place after the majority of the now preenriched gas had recollected into the fully formed potential well of the new giant elliptical galaxy.