Compact stellar systems in the Fornax cluster: a UV perspective

Abstract

Context. There is increasing evidence for chemical complexity and multiple stellar populations in massive globular clusters (GCs), including extreme horizontal branches (EHBs) and UV excess. Aims. We aim to improve our understanding of the UV excess in compact stellar systems, covering the regime of both ultra-compact dwarf galaxies (UCDs) and massive GCs. Methods. We use deep archival GALEX data of the central Fornax cluster to measure NUV and FUV magnitudes of UCDs and massive GCs. Results. We obtain NUV photometry for a sample of 35 compact objects that cover a range -13.5 < M v < -10 mag. Of those, 21 objects also have FUV photometry. Roughly half of the sources fall into the UCD luminosity regime (M v ≤ - 11 mag). We find that seven out of 17 massive Fomax GCs exhibit a NUV excess with respect to expectations from stellar population models, both for models with canonical and enhanced Helium abundance. This suggests that not only He-enrichment has contributed to forming the EHB population of these GCs. The GCs extend to stronger UV excess than GCs in M 31 and massive GCs in M 87, at the 97% confidence level. Most of the UCDs with FUV photometry also show evidence for UV excess, but their UV colours can be matched by isochrones with enhanced Helium abundances and old ages 12-14 Gyr. We find that Fornax compact objects with X-ray emission detected from Chandra images are almost disjunct in colour from compact objects with GALEX UV detection, with only one X-ray source among the 35 compact objects. However, since this source is one of the three most UV bright GCs, we cannot exclude that the physical processes causing X-ray emission also contribute to some of the observed UV excess.