Globular clusters in the inner regions of NGC 5128 (Centaurus A)

We have identified 26 new globular cluster candidates in the inner 3 kpc of NGC 5128 (Centaurus A), the nearest known large galaxy that is the probable product of a merger. The clusters are selected on the basis of their structural parameters (observed core diameters and ellipticities), as measured from archival Wide Field Planetary Camera (WFPC) Hubble Space Telescope (HST) images. IR photometry obtained with IRAC2B at the ESO/MPI 2.2 m telescope is combined with the optical HST photometry. Most of these clusters have normal colors typical of old globular clusters like those found in the Milky Way and M31. We estimate their metal abundances based on the R-K-0 color, confirming the existence of a metallicity gradient in the inner regions of NGC 5128. The presence of metal-rich globular clusters suggests that one of the colliding galaxies was a bulge-dominated galaxy (E or early S). A few clusters have colors and magnitudes similar to intermediate-age clusters containing carbon stars in the Magellanic Clouds. If the intermediate-age clusters were formed during a merger, then this episode must have occurred a few gigayears ago. Alternatively, we are looking at the cluster members of one of the colliding galaxies, which would then have been a late-type disk galaxy.