Dynamical mirages: How bar-induced resonant trapping can mimic substructure clustering in dynamical parameter spaces

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Piero Gobetti 93/2, Bologna 40129, Italy
² Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, INAF, Via Piero Gobetti 93/3, Bologna 40129, Italy
³ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ Instituto Milenio de Astrofísica MAS, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile

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

Received: 2 August 2025
Accepted: 6 November 2025

Abstract

Context. The complex task of unraveling the assembly history of the Milky Way evolves constantly, and new substructures are identified continuously. To properly validate and characterise the family of galactic progenitors, it is important to take all the effects into account that can shape the distribution of tracers in the Galaxy. First among the often overlooked actors of galactic dynamics is the rotating bar of the Milky Way, which can affect orbital tracers in multiple ways.

Aims. We wish to fully characterise the effect of the rotating bar of the Milky Way on the distribution of galactic tracers, provide diagnostics that can help to identify its effect and explore the implications for the search and identification of substructures.

Methods. We used the in-house code Orbital Integration Tool (ORBIT), built to include the full effect of the bar and exploit its multidimensional output to perform a complete dynamical characterisation of a large sample of carefully selected Milky Way stars with very precise astrometry.

Results. We identified conspicuous overdensities in several orbital parameter spaces and verified that they are caused by the bar-induced resonances. We also show that contamination by trapped tracers provides local density enhancements that mimic the clumping usually attributed to genuine substructures.

Conclusions. We provide a new and fast way of identifying resonant loci and consequently, of estimating the contribution of stars trapped into orbital resonances to phase-space overdensities that were previously identified as candidate relics of past merging events. Among those analysed here, we found that the detections of Cluster 3 and Shakti seem to have gained a non-negligible boost from resonance-trapped stars. Nyx is the most extreme case, with ~70% of the assigned member stars lying on resonant orbits. This strongly suggests that it is not a genuine merger relic but is instead an overdensity caused by bar-induced resonances.