The star cluster system of the 3-Gyr-old merger remnant NGC 1316: clues from optical and near-infrared photometry

The giant merger remnant galaxy NGC 1316 (Fornax A) is an ideal probe for studying the long-term effects of a past major merger on star cluster systems, given its spectroscopically derived merger age of similar to3 Gyr which we reported in a recent paper. Here we present new ground-based, large-area optical and near-IR imaging of star clusters in NGC 1316. The ground-based photometry is complemented with deep Hubble Space Telescope WFPC2 imaging, constituting an excellent combination for studying globular cluster systems. We find that the optical-near-IR colours and luminosities of the brightest similar to 10 clusters in NGC 1316 are consistent with those of intermediate-age (2-3 Gyr) populations. In particular, the near-IR data preclude ages less than or similar to1.5 Gyr and greater than or similar to4 Gyr for those clusters. Unlike 'normal' giant ellipticals, the B - I colour distribution of clusters in NGC 1316 is not clearly bimodal. However, the luminosity functions (LFs) of the blue and red parts of the cluster colour distribution are different. The red cluster LF is well represented by a power law, phi (L) dL proportional to L-1.2+/-0.3 dL, extending to about 1.5 mag brighter (in B) than those of typical giant ellipticals. In contrast, the shape of the blue cluster LF is consistent with that of 'normal' spiral and elliptical galaxies. We conclude that the star cluster system of NGC 1316 is a combination of a population of age similar to3 Gyr having roughly solar metallicity and a population of old, metal-poor clusters that probably belonged to the pre-merger galaxies. After the 3-Gyr-old, metal-rich clusters fade to an age of 10 Gyr, they will form a red 'peak' in a bimodal cluster colour distribution. This 'red peak' will have a colour consistent with that found in 'normal, old' giant ellipticals of the same galaxy luminosity (taking age dimming into account). The surface density profile of clusters in the innermost regions is lower than that of the integrated light of the galaxy, presumably owing to the collective effect of extended star formation in the inner regions of NGC 1316 and tidal shocking of the inner clusters. Outside the core, the surface density profile of clusters is consistent with that of the underlying starlight, suggesting that the cluster system originally experienced the same violent relaxation as did the main body of the merger remnant. The specific cluster frequency is presently S-N = 1.7 +/- 0.4 down to the 50 per cent completeness limit of the WFPC2 photometry, and will increase to S-N greater than or similar to 2.0 as the merger-induced stellar (and star cluster) population fades to an age of similar to 10 Gyr (barring further merger events), consistent with specific frequencies of typical giant ellipticals in the field and in poor groups. These features of the star cluster system of NGC 1316 are fully consistent with scenarios for forming 'normal' giant elliptical galaxies through gas-rich mergers at look-back times greater than or similar to 10 Gyr.