Orbital decomposition of the nuclear regions in the early-type galaxy FCC 47: Unveiling the nuclear cluster origin

¹ Department of Theoretical Physics and Astrophysics, Masaryk University Kotlářská 267/2 611 37 Brno, Czech Republic
² Department of Astrophysics, University of Vienna Türkenschanzstraße 17 1180 Vienna, Austria
³ European Southern Observatory (ESO) Karl-Schwarzschild-Straße 2 85748 Garching, Germany
⁴ Instituto de Astrofísica de Canarias Vía Láctea s/n E-38205 La Laguna Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna E-38200 La Laguna Tenerife, Spain
⁶ SDB Science-driven Business Ltd., Faneromenis Avenue 85, Ria Court 46 Suite 301 6025 Larnaca, Cyprus
⁷ CSIRO Space & Astronomy PO Box 1130 Bentley Western Australia 6102, Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 14 November 2025
Accepted: 24 January 2026

Abstract

Context. Nuclear star clusters (NSCs) are among the densest stellar systems in the Universe and often coexist with supermassive black holes (SMBHs) at galaxy centres. While SMBH formation histories are essentially lost, NSCs preserve evolutionary imprints through their stellar populations and stellar kinematics, reflecting the cumulative effects of mergers, accretion, and internal dynamical evolution.

Aims. We aim to investigate the orbital structure of the unusually large NSC in FCC 47 (NGC 1336) by decomposing its stellar orbits into dynamically distinct components.

Methods. We extract stellar kinematics, and in particular the line-of-sight velocity distributions (LOSVDs), from VLT/MUSE integral-field spectroscopy using the non-parametric BAYES-LOSVD approach, and apply triaxial Schwarzschild orbit-superposition modelling with the DYNAMITE software. We decompose the orbit library into hot, warm, cold, and counter-rotating components.

Results. We detect triple-peaked LOSVDs in the nucleus, indicating a complex orbital structure. The NSC forms a counter-rotating, kinematically decoupled component. A hot pressure-supported component, a warm counter-rotating structure and a counter-rotating cold disk in the centre suggest hierarchical assembly via early star cluster accretion and later in situ star formation.

Conclusions. Our orbital decomposition of FCC 47 supports a hybrid formation scenario for this NSC. Dynamically distinct substructures reflect the interplay of accretion and in situ star formation during galaxy evolution.