The complex nature of the nuclear star cluster in FCC 277

TL;DR

This study investigates the complex dynamical and chemical properties of the nuclear star cluster in FCC 277, revealing evidence of rotation, a nuclear disk, and diverse stellar populations, suggesting a formation history involving gas dissipation and merging.

Contribution

It provides a detailed analysis of the NSC in an elliptical galaxy using multi-modal observations, highlighting the presence of a nuclear disk and diverse stellar populations, which are less studied in elliptical hosts.

Findings

Detection of rotation at larger radii with disky isophotes

Presence of a central velocity dispersion drop indicating a cold sub-system

Evidence of younger, metal-rich stars in the NSC and nuclear disk

Abstract

Recent observations have shown that compact nuclear star clusters (NSCs) are present in up to 80% of galaxies. However, detailed studies of their dynamical and chemical properties are confined mainly to spiral galaxy hosts, where they are more easily observed. In this paper we present our study of the NSC in FCC 277, a nucleated elliptical galaxy in the Fornax cluster. We use a combination of adaptive optics assisted near-infrared integral field spectroscopy, Hubble Space Telescope imaging, and literature long slit data. We show that while the NSC does not appear to rotate within our detection limit of ~6 km/s, rotation is detected at larger radii, where the isophotes appear to be disky, suggesting the presence of a nuclear disk. We also observe a distinct central velocity dispersion drop that is indicative of a dynamically cold rotating sub-system. Following the results of orbit-based dynamical modelling, co-rotating as well as counter-rotating stellar orbits are simultaneously needed to reproduce the observed kinematics. We find evidence for varying stellar populations, with the NSC and nuclear disk hosting younger and more metal rich stars than the main body of the galaxy. We argue that gas dissipation and some level of merging have likely played an important role in the formation of the nucleus of this intermediate-mass galaxy. This is in contrast to NSCs in low-mass early- type galaxies, which may have been formed primarily through the infall of star clusters.