1 Introduction

Galaxies show a wide range of star formation activity and a large range of metallicities and ages in their stellar populations. Understanding the physical nature and origins of their stars is fundamental to understanding the formation and evolution of galaxies. The current knowledge of star formation histories of galaxies along the Hubble sequence is summarized by Kennicutt (1998). This knowledge is mainly based on the integrated photometric and spectroscopic studies through the predictions of populations synthesis of the integrated light of star clusters (Bica 1988) and galaxies (Bruzual & Charlot 1993; Maraston 1998). The predictions of population synthesis can now be tested directly not only for globular clusters (e.g. Vazdekis et al. 2001), but also for nearby galaxies.

Stellar evolution theory provides predictions of the features expected in color-magnitude diagrams (CMDs) for stellar populations with different ages and metallicities (Renzini & Fusi Pecci 1988; Chiosi et al. 1992; Gallart 1998; Aparicio 1998; Tolstoy 1998). Coupled with the improvements in telescope sizes, detector sensitivity, field of view and spatial resolution, compared with those a decade ago, the direct observations of the stellar content of nearby galaxies is becoming one of the most active areas of extragalactic research.

In the Local Group, indicators of the old stellar populations such as old main sequence turn-offs or at least the horizontal branch magnitudes, are within reach of available instrumentation (e.g. Hurley-Keller et al. 1998; Gallart et al. 1999; Held et al. 2000; Rejkuba et al. 2000). The recent results on the studies of the Local Group galaxies have been summarized by Mateo (1998), van den Bergh (1999,2000) and Grebel (2000).

The Local Group contains galaxies representative of almost all the classes, except the important giant elliptical class of galaxies. The closest giant elliptical is NGC 5128, the dominant galaxy in the Centaurus group at distance of $3.6 \pm 0.3$ Mpc (Harris et al. 1999; Soria et al. 1996; Hui et al. 1995; Tonry & Schechter 1990). It has been extensively studied over the last 50 years (for an exhaustive review see Israel 1998). Its popularity is not only due to its brightness, but also to its many unusual features: (i) there is a prominent dust band containing young stars and H II regions (Unger et al. 2000; Wild & Eckart 2000; Graham 1979); (ii) there is an active nucleus with a radio and X-ray jet, radio lobes and optical filaments (Cooper et al. 1965; Feigelson et al. 1981; Kraft et al. 2000; Schreier et al. 1981; Clarke et al. 1992; Blanco et al. 1975; Dufour & van den Bergh 1978); (iii) Malin et al. (1983) discovered a large number of faint narrow shells of stars surrounding the galaxy; (iv) Schiminovich et al. (1994) detected $4\times10^8$ $M_\odot$ of H I gas associated with the stellar shells, but slightly displaced outside the shells; and (v) most recently, molecular CO gas has been found associated with the H I gas and the stellar shells (Charmandaris et al. 2000). All of these are clear indications of a recent interaction with a gas-rich galaxy.

The high resolution and sensitivity of WFPC2 on the Hubble Space Telescope (HST) enabled the first studies of the resolved old stellar populations in the halo of NGC 5128 (Soria et al. 1996; Harris et al. 1999; Harris & Harris 2000; Mould et al. 2000). NICMOS on HST was used to resolve the stars in the near IR (Marleau et al. 2000) in the same field as the optical study of Soria et al. (1996; $\sim$9 kpc south of the center of the galaxy). In these two studies, a small intermediate-age population of $\sim$5 Gyr has been found, comprising up to $\sim$$10 \%$ of the total stellar population in the halo. On the other hand, there are almost no intermediate-age stars in the field further out in the halo, at $\sim$20 kpc from the galaxy center (Harris et al. 1999) nor at $\sim$31 kpc (Harris & Harris 2000). The comparison of the two results may indicate the presence of a gradient in the stellar population in the halo, as suggested by Marleau et al. (2000). However, the small field of view of HST puts serious limitations to the conclusions in the cases where the strong gradients in galaxy populations exist (see, for example, the case of Local Group dwarf galaxies like Leo I (Gallart et al. 1999; Held et al. 2000) or WLM (Minniti & Zijlstra 1996,1997)).

We present here a wide wavelength range photometry of the resolved stellar populations in NGC 5128 obtained from the ground with Very Large Telescope (VLT) in U, V and $K_{\rm s}$ band. The deep and high-resolution VLT imaging, coupled with the much larger field of view than HST, enables us to address the questions of the gradients in stellar populations in the halo of this giant elliptical galaxy. The proximity of NGC 5128 provides an unusual opportunity for a direct study of shell stars. We use infrared-optical colour-magnitude diagrams of the shell to study the ages and metallicities of the stars belonging to the cannibalized galaxy.