Is Fornax 4 the nuclear star cluster of the Fornax dwarf spheroidal galaxy?

TL;DR

This study uses HST photometry to analyze Fornax 4, revealing complex stellar populations with iron and helium variations, supporting its identification as the galaxy's nuclear star cluster.

Contribution

It provides the first detailed stellar population analysis of Fornax 4, suggesting it is the nuclear star cluster of the Fornax dwarf galaxy based on chemical and population evidence.

Findings

Fornax 4 hosts multiple stellar populations with iron spread up to 0.4 dex.

Possible helium abundance variations of about 0.03 are indicated.

Evidence supports Fornax 4 being the galaxy's nuclear star cluster.

Abstract

Fornax 4 is the most distinctive globular cluster in the Fornax dwarf spheroidal. Located close to the centre of the galaxy, more metal-rich and potentially younger than its four companions (namely, Fornax clusters number 1, 2, 3 and 5), it has been suggested to have experienced a different formation than the other clusters in the galaxy. Here we use HST/WFC3 photometry to characterize the stellar population content of this system and shed new light on its nature. By means of a detailed comparison of synthetic horizontal branch and red giant branch with the observed colour-magnitude diagrams, we find that this system likely hosts stellar sub-populations characterized by a significant iron spread up to $\Delta$[Fe/H]$\sim$0.4 dex and possibly by also some degree of He abundance variations $\Delta$Y$\sim0.03$. We argue that this purely observational evidence, combined with the other peculiarities characterizing this system, supports the possibility that Fornax 4 is the nuclear star cluster of the Fornax dwarf spheroidal galaxy. A spectroscopic follow-up for a large number of resolved member stars is needed to confirm this interesting result and to study in detail the formation and early evolution of this system and more in general the process of galaxy nucleation.