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

TL;DR

This study investigates the orbital structure of the unusually large NSC in FCC 47 (NGC 1336) by performing a dynamical decomposition with Schwarzschild orbit-superposition modelling (DYNAMITE) guided by non-parametric Bayes-LOSVD kinematics from VLT/MUSE data. The analysis identifies a hot, dispersion-dominated component and embedded counter-rotating disks (cold and warm CR) plus a counter-rotating core, pointing to a hybrid NSC formation combining early star cluster accretion and in situ star formation. The stellar orbits are quantified via the circularity parameter $\lambda_z = L_z / L_{\rm circ}(E)$, revealing a dominant hot component ($\sim$43%), substantial CR structure ($\sim$27%), and smaller prograde disks ($\sim$26%), with a central rotation opposite to the host. The best-fit model yields a central BH mass of $M_{\rm BH} = 6.2^{+2.6}_{-4.5} \times 10^{7} M_\odot$ and a stellar mass-to-light ratio $M/L = 0.64^{+0.11}_{-0.04}$, illustrating how resolving non-Gaussian LOSVDs with Bayes-LOSVD can influence mass budgets and formation inferences. Overall, the work supports a composite NSC formation scenario and demonstrates the value of orbit-based dynamical decomposition for understanding nucleus assembly in early-type galaxies.

Abstract

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. 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. 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. 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. 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.

Figures (10)

• Figure 1: HST/ACS image of FCC 47 overlaid with surface brightness contours from the collapsed MUSE data cube. The white rectangle indicates a $\sim15" \times 15"$ region centred on the galaxy nucleus, which is used to extract the stellar kinematics.
• Figure 2: Spectrum fitted with Bayes-LOSVD. The x-axis represents the rest-frame wavelength in Å. The y-axis shows the normalized flux. The modelled spectrum (red) closely matches the observed spectrum (black) with minimal residuals (green). Dashed lines show the continuum level of the spectrum and residual, and dotted vertical lines enclose the fitted wavelength region.
• Figure 3: LOSVDs extracted with Bayes-LOSVD. Solid lines denote the median LOSVD, the dotted lines the 68% confidence intervals. Each panel shows a different spatial bin of the MUSE data. Bins 0, 1, 4 and 10 are likely dominated by the NSC based on their central location and kinematic features. The position of each bin in the field is shown in Fig. \ref{['fig:chi2_map']}.
• Figure 4: Reduced $\chi^2$ map of the explored parameter space. The colour scale indicates the relative goodness of the fit, with darker shades corresponding to lower $\chi^2$ values. Bottom: Reduced $\chi^2$ map over the entire $\sim 15" \times 15"$ field. Masked bins are coloured grey. We highlight the position of the spatial bins 50 and 175, whose LOSVD histograms are shown in Fig. \ref{['fig:bayes_kinematics']}. Top: Zoom-in into the nuclear region. We highlight the position of the spatial bins 0,1,4 and 10, whose LOSVD histograms are shown in Fig. \ref{['fig:bayes_kinematics']}.
• Figure 5: Orbital distributions around the minor $z$-axis at two radial scales. Top:$r = 2"$: substantial counter-rotating motion towards the centre, the red line marks $1\,R_{\mathrm{eff,NSC}}$, the green line marks the spatial resolution at FWHM. Bottom:$r = 60"$: extended view, the red line marks $1\,R_{\mathrm{eff}}$ of the galaxy.