With arms wide open: A VLT/MUSE view of the mechanisms driving unwinding spiral arms in cluster galaxies

¹ INAF-Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, I-35122 Padova, Italy
² Instituto de Radioastronomía y Astrofísica, UNAM Campus Morelia, A.P. 3–72, C.P. 58089, Mexico
³ Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680 Valparaíso, Chile
⁴ Flatiron Institute, Center for Computational Astrophysics, 162 5th Avenue, New York, NY 10010, USA
⁵ Department of Physics, Faculty of Science, University of Zagreb, Bijenička Cesta 32, 10000 Zagreb, Croatia
⁶ Universitäts-Sternwarte München, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstraße 1, 81679 München, Germany
⁷ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Via Piero Gobetti 93/2, I-40129 Bologna, Italy

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

Received: 3 March 2026
Accepted: 25 April 2026

Abstract

Context. The environmental mechanisms driving unwinding spiral arms in cluster galaxies remain debated. While earlier studies attributed this phenomenon primarily to gravitational interactions, more recent works suggest that ram-pressure stripping (RPS) alone can induce unwinding arms.

Aims. Using VLT/MUSE observations, we present a spatially resolved investigation of the external mechanisms responsible for spiral-arm unwinding. We focused on two galaxies, UG101 and UG103, selected from a larger sample of unwinding systems. They are selected as tidal- and RPS-driven candidates, respectively, based on the presence or absence of close neighbors.

Methods. We estimated the galactocentric radius at which tidal forces, either from a companion or the cluster potential, become relevant (R_tid). We examined gas and stellar kinematics, exploiting how these two components respond differently to gravitational and hydrodynamical perturbations. The spectrophotometric code SINOPSIS was used to map stellar populations in different age bins and constrain the unwinding timescale for each galaxy.

Results. For UG101, we find R_tid ≈ 1.5 R_e, while the unwound features extend beyond this radius. Additionally, UG101 shows irregular stellar and gas kinematics; its rotation curve indicates similar stellar and gas motions, although the gas appears truncated on one side and extended on the other. For UG103, neither the closest companion (r_proj ∼ 117 kpc) nor the cluster appears capable of triggering the unwinding arms through tidal forces. UG103 displays regular stellar kinematics but disturbed gas kinematics, with a truncation on the disk side that likely faces the intracluster medium wind and gas extended in the opposite direction. Stellar population maps show the emergence and subsequent unwinding of the spiral arms in UG103 on timescales consistent with its inferred cluster infall time (∼1.6 Gyr).

Conclusions. We conclude that spiral-arm unwinding in UG101 and UG103 is primarily driven by tidal interactions and RPS, respectively, although a combined effect cannot be excluded in the case of UG101. Our methodology provides a robust framework to disentangle the mechanisms driving spiral-arm unwinding in cluster galaxies from their spatially resolved properties.