Tracing the dynamical and structural complexity of spiral galaxy centres

¹ Dep. of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
² Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
³ Dep. de Astrofísica, Universidad de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, E-38206 La Laguna, Tenerife, Spain
⁴ Graduate Institute for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Subaru Telescope, National Astronomical Observatory of Japan, 650 N Aohoku Pl, Hilo HI96720, Japan
⁶ Dep. de Física e Astronomia, Faculdade de Ciências, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands

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

Received: 11 December 2025
Accepted: 23 March 2026

Abstract

Context. The formation and evolution of late-type galaxies have traditionally been described in terms of two pathways, one producing pressure-supported classical bulges and the other producing rotationally supported pseudo-bulges. Early studies supporting this view were primarily based on photometric decompositions that assumed an exponential disk, extrapolating it inwards. However, recent high-resolution observations have revealed a far more complex landscape in disk galaxy centres.

Aims. We investigated the morphology of central stellar components in a subset of intermediate-to-massive spiral galaxies at unprecedented detail, focusing on disentangling the contributions of their cold, warm, and hot orbital components. Our goal is to critically reassess the standard approach of extrapolating a disk’s exponential profile into the galaxy centre.

Methods. To this end, we developed the pipeline GLANCE (Galactic archaeoLogy via chronochemicAl & dyNamiCal modElling), a dedicated tool for photometric, chronochemical, and dynamical galaxy analysis. We applied GLANCE to eight high-resolution MUSE galaxies, deriving stellar population properties, and decomposing their orbits into cold, warm, hot, and counter-rotating components.

Results. We uncovered a remarkable structural diversity in the dynamically cold central component of our sample galaxies: While one galaxy displays a tentatively exponential profile throughout its full extent, the majority exhibit either a pronounced central drop resembling a doughnut-shaped structure or a compact inner disk that is significantly steeper than the outer parent disk. Surprisingly, most galaxies that host nuclear disks could be classified as classical bulges – being dynamically hot, old, and red and having a high bulge-to-total ratio – in contrast to the majority of galaxies that exhibit a central deficit in the cold component. On the other hand, the luminosity contribution of cold plus warm orbits beyond the bulge generally remains below the total, indicating that the parent disk contains a non-negligible fraction of hot or counter-rotating orbits, displaying radial profiles with varying Sérsic indexes consistently larger than unity.

Conclusions. Our analysis indicates that the centres of disk galaxies are often more complex than what is implied by a simple inwards continuation of the outer exponential disk profile. These results highlight the composite nature of central structures in disk galaxies and the need for decomposition techniques that do not indiscriminately rely on extrapolating the outer disk into the innermost regions. Development of such methods will require detailed studies of a statistically representative sample with high-quality integral field spectroscopy data spanning a broad mass range, ideally complemented with high-resolution simulations such as Illustris TNG50.