On the globular cluster formation history of NGC 5128

Abstract

We deduce the globular cluster formation history of the nearby elliptical NGC 5128 by using a phenomenological chemical enrichment model to reproduce its observed metallicity distribution function (MDF). We derive the observed MDF using recently obtained U and B photometry of the NGC 5128 GC system, with (U-B) used as the metallicity indicator. We test whether the observed MDF can be reproduced using a single starburst at high redshift followed by passive evolution (monolithic scenario). Our results strongly indicate that this scenario is incapable of reproducing the observed MDF of the NGC 5128 GC system. At least one more major starburst is required. A persistent feature of our best-fit models is a strong GC formation episode in the first 1-2 Gyrs, followed by a second formation episode typically after 10-12 Gyrs. Roughly one-third of the stellar mass in the GC system is created in this late starburst, producing clusters that are potentially less than 1 Gyr old. Although we do not have adequate resolution in the model or in the observed MDF to make a reliable claim about the possibility of additional GC formation episodes, our results suggest that the GCs in this galaxy did not form solely in a monolithic collapse and seem to support observational evidence which indicates that NGC 5128 is a merger remnant.