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M. Rejkuba, D. R. Silva (ESO, Germany), D. Minniti (PUC, Chile), T. Bedding (University of Sydney, Australia)
Using the VLT with FORS1 and ISAAC we resolve stars in the halo and NE shell of the nearest giant elliptical galaxy NGC~5128. The shell is likely to be the stellar debris of a smaller galaxy accreted by NGC~5128 approximately 160 to 500 Myr ago (Quinn 1984, ApJ, 297, 596; Israel 1998, A&ARv, 8, 237). We indeed observe a higher stellar density at the position of the shell. On the shell a string of young blue stars is superimposed (see also Graham 1998, ApJ, 502, 245 and Mould et al.\ 2000, ApJ, 536, 266). Comparison of our UV color-magnitude diagram (CMD) with the theoretical isochrones of the Padua group gives an age of 10-15 Myr for this recent star formation. The bluest stars (U-V<-0.75) are mainly aligned with the direction of the radio and X-ray jet, but there is also a vertical structure located at the edge of the large HI cloud found next to the shell (Schiminovich 1994, ApJ, 432, L101). The optical (U-V) and near-IR (V-Ks) CMDs for the rest of the shell stars are consistent with an old population similar to the one observed in the halo field of NGC~5128.
The halo population of NGC~5128 is mainly old and has a significant metallicity spread. The optical-near IR (V-Ks) CMDs reveal stars as metal-poor as [Fe/H]~2 dex. Unfortunately, the incompleteness in the V-band photometry prevents the determination of the upper end of the metallicity distribution. At the assumed distance of 3.6 Mpc (Soria et al.\ 1996, ApJ, 465, 79), the K-band magnitude of the tip of the RGB is expected to be found at K~21. We observe a large number of stars between 19.5 and 21 mag in Ks, corresponding to a significant intermediate-age AGB population.
This work was partially sponsored by FONDECYT grants No. 07990048 and 01990440.