High resolution imaging of the early-type galaxy NGC 1380: an insight into the nature of extended extragalactic star clusters

Abstract

NGC 1380 is a lenticular galaxy located near the centre of the Fornax Cluster northeast of NGC 1399. The globular cluster system of this galaxy was previously studied only from the ground. Recent studies of similar early-type galaxies, specially lenticular ones, reveal the existence of star clusters that apparently break up the traditional open/globular cluster dichotomy. With higher quality photometry from HST/WFPC2 we study the star clusters in NGC 1380, measuring their magnitudes, colours, sizes and projected distances from the centre of the galaxy. We used deep archival HST/WFPC2 in the B and V bands. We built colour magnitude diagrams from which we selected a sample of cluster candidates. We also analysed their colour distribution and measured their sizes. Based on their location in the luminosity-size diagram we estimated probabilities of them being typical globular clusters as those found in the Galaxy. A total of about 570 cluster candidates were found down to V=26.5. We measured sizes for approximately 200 of them. The observed colour distribution has three apparent peaks. Likewise for the size distribution. We identified the smaller population as being mainly typical globular clusters, while the more extended objects have small probabilities of being such objects. Different correlations between absolute magnitudes, sizes, colours and location were inferred for these cluster sub-populations. Most extended clusters (Reff > 4 pc) share similar properties to the diffuse star clusters reported to inhabit luminous early-type galaxies in the Virgo galaxy cluster such as being of low surface brightness and fainter than MV ~ -8. We also report on a small group of (Reff ~ 10 pc), -8< MV < -6, red clusters located near the centre of NGC 1380, which may be interpreted as faint fuzzies.