8 The shell

In our deep V-band image of Field 1, the prominent NE shell is easily recognizable as the region with higher surface brightness, and also with higher density of stellar objects. All the stars that are spatially coincident with the shell on the V-band image are shown in Fig. 18. In the left two panels we plot the UV CMD of all stars coincident with the position of the diffuse shell (middle panel), while the corresponding VK CMD is on the right. The much smaller field of view of ISAAC is centered on the shell so that the shell stars cover most of the image. Note that projected on the shell is the stream of blue, young objects that are aligned with the jet direction. Excluding the stars that are coincident with the star-forming area (lower panels), the stellar population belonging to the shell is very similar to the one observed in the halo of NGC 5128, i.e., our Field 2 population (Figs. 9, 11). As in the rest of the galaxy there is a significant spread in metallicity.

The presence of stellar shells or ripples around early-type galaxies is usually explained as evidence of the accretion and later merging of a smaller companion galaxy. The simulations of the stellar component in the merging galaxies have shown that the shells are created as a result of phase-wrapping or of spatial-wrapping of the tidal debris (Quinn 1984;

Figure 18: The color-magnitude diagrams for the stars located at the position of the diffuse NE shell.

Hernquist & Quinn 1989). The companion galaxies may contain different stellar populations with respect to the giant elliptical (e.g. most dwarf galaxies have much lower metallicity than giants Mateo 1998).

Here for the first time we obtained the magnitudes and colors of stars belonging to a shell. They are surprisingly similar to the rest of the halo stars in NGC 5128. However, due to incompleteness of our V-band photometry, we cannot probe the most metal-rich part of the halo of NGC 5128. Very high resolution and deeper observations of the shell stars are possible with the Advanced Camera for Surveys at Hubble Space Telescope. In order to be able to better separate the stars that genuinely belong to shells from the "normal'' halo stars in NGC 5128 and confirm the present result a more quantitative comparison between the stellar populations in our two fields is necessary (Rejkuba et al., in preparation).