Far Ultra-Violet Insights Into NGC 1399's Globular Cluster Population

TL;DR

This study uses Hubble Space Telescope data to explore ultraviolet properties of globular clusters in NGC 1399, revealing potential signs of multiple stellar populations and extreme horizontal branch stars across different metallicities.

Contribution

It provides the first detailed analysis of FUV emission in NGC 1399's globular clusters, suggesting the presence of He-enhanced populations independent of optical color.

Findings

FUV counterparts found in 58 out of 149 clusters

FUV emission observed in both metal-rich and metal-poor clusters

X-ray detected clusters are not unusually FUV bright

Abstract

We investigate archival Hubble Space Telescope ACS/SBC F140LP observations of NGC~1399 to search for evidence of multiple stellar populations in extragalactic globular clusters. Enhanced FUV populations are thought to be indicators of He-enhanced second generation populations in globular clusters, specifically extreme/blue horizontal branch stars. Out of 149 globular clusters in the field of view, 58 have far ultraviolet (FUV) counterparts with magnitudes brighter than 28.5. Six of these FUV-deteced globular clusters are also detected in X-rays, including one ultraluminous X-ray source ($L_X > 10^{39}$ erg/s). While optically bright clusters corresponded to brighter FUV counterparts, we observe FUV emission from both metal-rich and metal-poor clusters, which implies that the FUV excess is not dependent on optical colour. We also find no evidence that the cluster size influences the FUV emission. The clusters with X-ray emission are not unusually FUV bright, which suggests that even the ultraluminous X-ray source does not provide significant FUV contributions. NGC 1399 is only the fourth galaxy to have its globular cluster system probed for evidence of FUV-enhanced populations, and we compare these clusters to previous studies of the Milky Way, M31, M87, and the brightest cluster in M81. These sources indicate that many globular clusters likely host extreme HB stars and/or second generation stars, and highlight the need for more complete FUV observations of extragalactic globular cluster systems.