Hubble Space Telescope FUV observations of M31's globular clusters suggest a spatially homogeneous helium-enriched sub population

TL;DR

This study uses Hubble's FUV observations to analyze the spatial distribution of hot stellar populations in M31's globular clusters, suggesting helium-enriched subpopulations are common and correlated with cluster mass.

Contribution

It provides evidence that helium-enriched subpopulations are spatially homogeneous within clusters and links cluster mass to FUV brightness, advancing understanding of stellar populations in extragalactic clusters.

Findings

Some clusters show excess FUV emission indicating hot populations.

No spatial segregation of hot populations within clusters.

FUV brightness correlates with cluster mass and density.

Abstract

We present high spatial resolution, far ultraviolet (FUV) F140LP observations of 12 massive globular clusters in M 31 obtained using the ACS/SBC on the Hubble Space Telescope (HST). These observations resolve the cluster profiles to scales similar to their core radii and enable the study of the spatial distribution of blue and extreme horizontal branch (HB) stars, which dominate the emission in the F140LP images. We confirm that some of these clusters have excess FUV emission, suggesting additional hot populations beyond those expected by canonical single stellar populations models. We find no evidence that the hot populations are spatially distinct from the majority populations in these clusters, as would be expected if the excess FUV emission is a result of a dynamically enhanced population of extreme-HB stars. We conclude that a second population of stars with significantly enhanced helium abundance is a viable explanation for the observed FUV emission which is both bright and distributed similarly to the rest of the cluster light. Our results support the use of FUV observations as a path to characterising helium enhanced sub populations in extragalactic clusters. These M31 clusters also show a correlation such that more massive and denser clusters are relatively FUV bright. Similar to extant Milky Way results, this may indicate the degree of helium enrichment, or second population fraction increases with cluster mass.