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 \sim 21 kpc from the center of the galaxy , contains more than 10,000 stars and reaches almost 3 magnitudes down the red-giant branch ( RGB ) .
From the sharply defined RGB tip at I = 24.1 \pm 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 globulars at [ Fe/H ] { } ^ { < } _ { \sim } -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 } = 0 and the second component with Z _ { 0 } \simeq 0.18 Z _ { \odot } .
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 \langle [ Fe/H ] \rangle = -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 pre-enriched gas had recollected into the fully formed potential well of the new giant elliptical .