INFRARED PHOTOMETRY OF 487 SOURCES IN THE INNER REGIONS OF NGC-5128 (CENTAURUS-A)

We study the sources present in the inner 3 kpc region of NGC 5128 (Ce n A), most of which are star clusters of different ages. Photometry of archival Hubble Space Telescope WFPC images (F675W filter) is complem ented with IR photometry (JHK' filters) obtained with the IRAC2B infra red array camera at the ESO/MPI2.2 m telescope. From IR color maps we divide the field into two regions: a clear region outside the dust lan e, and an obscured region well inside the dust lane of NGC 5128. In th e unreddened region there is a great variety of sources such as globul ar clusters, star associations, and H II regions. These sources are no t individual stars, which would be too faint to be resolved from groun d-based telescopes. The vast majority of IR sources in the reddened re gion, where the dust lane dominates, are not seen at all in the deep H ST images. The presence of large amounts of differential extinction ma kes it difficult to evaluate them. In total, there are 372 objects det ected in the inner region of NGC 5128. From them, 125 objects are dete cted both in IR and HST frames. There are 247 IR sources without optic al counterparts (47 in the clear region and 200 in the dust lane). Acc ounting for the small volume sampled, there must be a total of similar to 500 sources with K < 18 in the dust lane region. The distribution of these sources is rather uniform and not particularly centrally conc entrated. This fact suggests that the majority of them are located in a disk, as would be expected if they are young associations or cluster s. The degree of background and foreground contamination is evaluated using observations of a nearby field. We found 115 IR sources in this field. The nucleus itself is invisible in deep optical images, but it is clearly identified in the IR. In the region just south of the nucle us the extinction must be larger than A(K) = 3. In the clear region, w here the effect of the dust lane is negligible, we have identified som e objects as intermediate-age clusters containing carbon stars. From c olor-magnitude diagrams we do not find evidence of very young clusters in this region. Such clusters might be fainter than our detection lim it in JHK'. We measure metallicities for 42 globular clusters, confirm ing the presence of a metallicity gradient with Delta[Fe/H]/Delta R = -0.06 dex kpc(-1).