Involved people at OAB: Cacciari, Federici.
It is currently in progress [in collaboration with Held (OAPd) and Testa (OAR)] the spectroscopic study of the globular cluster system of NGC 5128 (Cen A), which is the closest largest elliptical galaxy (distance = 3.5Mpc), and the nearest example of an elliptical galaxy that has recently accreted a late-type satellite, representing so an ideal place where to test current ideas about globular cluster formation. A preliminary analysis, started in 1996 on the basis of spectrophotometric data of a sample of 31 halo gc obtained with MEFOS at the ESO-3.6m telescope showed that the globular clusters of NGC5128 are on average more metal rich than those of our Galaxy, with no super metal-rich cluster. The most intriguing result, however, is the double peak in the metallicity distribution which points to a composite cluster population. In order to give statistical significance to this result and to be able to address the question of a metallicity gradient in the GC system of Cen A, this study has been extended to the inner regions of the galaxy. Spectroscopic observations of a sample of 45 confirmed clusters were taken with EMMI/MOS at NTT in 1998 and the analysis is being carried out.
In the same framework, deep MOS spectroscopy of globular clusters in the Sombrero galaxy (NGC 4594) has been obtained in 1999 using FORS1 at the VLT. NGC 4594 is an early-type spiral with a rich globular cluster system and a dominant bulge; photometric studies suggests the presence of a population of disk clusters more metal-rich than the halo globular clusters. The program aims to verify the existence of cluster sub-populations by estimating their ages and metallicities via measurements of absorption line indices. The cluster kinematics will also be employed to probe the mass of the galaxy.
A similar analysis has been undertaken on the elliptical galaxies NGC4125, NGC5831 and NGC6173, whose characteristics suggests them to be the product of recent mergers, and therefore to be good candidates for elliptical galaxies with a population of intermediate-age globular clusters. In order to study the globular clusters around these galaxies and to derive their metallicity distributions using a wide color baseline, deep CCD images were obtained with the TNG/OIG in 1999, and the reductions are in progress.
Spectroscopy of globular cluster systems is essential to investigate the existence of abundance/kinematics subsystems beyond the Local Group, to estimate galaxy masses and to probe dark matter halos, but efficient spectroscopic observations require that a pure sample of bona-fide globular cluster candidates be previously identified. In this scenario, and in preparation for a systematic spectroscopic study at the VLT, we have undertaken a wide-field multicolor imaging of galaxies out to the Virgo cluster distance. Five galaxies of all morphological types have been observed in March 2000 using the Wide Field Imager at the 2.2m ESO telescope; the analysis of these data will lead to the selection by color and shape criteria of large-area catalogs of globular clusters candidates, that will form a database where the deepness of CCD photometry with an area coverage comparable with that of photographic plates are combined.