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

TL;DR

The paper investigates how the Milky Way's rotating bar reshapes tracer dynamics and can mimic substructures in dynamical spaces. Using the Orbital Integration Tool (OrbIT) with a time-varying bar potential, it analyzes a large Gaia-based giant sample to identify bar-induced overdensities in $R_{peri}$, $R_{apo}$, and $ecc$, and in $L_{z,cha}-E_{cha}$, guarded by resonance criteria. It shows that resonant trapping accounts for a substantial fraction of members in several alleged substructures (notably Nyx) and quantifies contamination in Cluster 3, Shakti, and Thamnos, emphasizing the need for bar-aware diagnostics in substructure studies. The results provide a practical framework to estimate the bar's contribution to phase-space overdensities, improving reliability in the search for relics of past mergers and informing interpretations of dynamical clustering in the Milky Way.

Abstract

The complex task of unraveling the assembly history of the Milky Way is in constant evolution with new substructures identified continuously. To properly validate and characterise the family of galactic progenitors, it is important to take into account all the effects 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 that can affect orbital tracers in multiple ways. We want to fully characterise the effect of the rotating bar of the Milky Way on the distribution of galactic tracers, provide diagnostics helpful in identifying its effect and explore the implications for the search and identification of substructures. We use the in-house 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. We identify conspicuous overdensities in several orbital parameter spaces and verify that they are caused by the bar-induced resonances. We also show how contamination by trapped tracers provides local density enhancements that mimic the clumping usually attributed to genuine substructures. We provide a new and expedite way of identifying resonant loci and, consequently, to estimate the contribution of stars trapped into orbital resonances to phase-space overdensities 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 assigned member stars lying on resonant orbit, strongly suggesting that it is not the genuine relic of a merger event but an overdensity caused by bar-induced resonances

Figures (21)

• Figure 1: Density distribution of the sample stars in $R_{peri}-ecc$ plane, higher density is indicated by a lighter colour.
• Figure 2: Density distribution of the sample stars in $R_{peri}-ecc$ plane for an integration with a static axisymmetric potential (without the rotating bar). There is no trace of the overdensities seen in Fig. \ref{['fig:periecc']}.
• Figure 3: Density distribution of the full sample in the $R_{peri}-ecc$ plane computed from orbits generated with AGAMA. Left panel: the orbital parameters have been recovered using the AGAMA built-in routine agama.potential.Rperiapo. Right panel: the orbital parameters have been recovered directly from the orbital history of each tracer.
• Figure 4: Density distribution of the sample stars in $L_{z,cha}-E_{z,cha}$ plane. The diagonal overdensities seen in Fig. \ref{['fig:periecc']} appear as almost straight horizontal lines in prograde space ($L_{z,cha} < 0$) and turn at an angle when entering the retrograde region of space ($L_{z,cha} > 0$).
• Figure 5: The density distribution is made of the stars from our full sample on the $J_{\phi}-J_R$ plane, the red points are the stars on the overdensities in Fig. \ref{['fig:periecc']} and Fig. \ref{['fig:lechar']}, the coloured solid lines are the theoretical predictions for the loci of the different resonances at specific values of the $l/m$ ratio (see the legend).