Radial and azimuthal gradients of the moving groups in Gaia DR3: The slow and fast bar degeneracy problem

¹ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), C Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: mbernet@fqa.ub.edu
² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), C Martí i Franquès, 1, 08028 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
⁴ National Astronomical Observatory of Japan, Mitaka-shi, Tokyo 181-8588, Japan
⁵ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, 11 rue de l’Université, 67000 Strasbourg, France

Received: 27 October 2023
Accepted: 1 March 2024

Abstract

Context. The structure and dynamics of the central bar of the Milky Way (MW) are still under debate whilst being fundamental ingredients for the evolution of our Galaxy. The recent Gaia DR3 offers an unprecedented detailed view of the 6D phase space of the MW, allowing for a better understanding of the complex imprints of the bar on the phase space.

Aims. We aim to identify and characterise the dynamical moving groups across the MW disc, and use their large-scale distribution to help constrain the properties of the Galactic bar.

Methods. We used 1D wavelet transforms of the azimuthal velocity (V_ϕ) distribution in bins of radial velocity to robustly detect the kinematic substructure in the Gaia DR3 catalogue. We then connected these structures across the disc to measure the azimuthal (ϕ) and radial (R) gradients of V_ϕ of the moving groups. We simulated thousands of perturbed distribution functions using backward integration, sweeping a large portion of parameter space of feasible Galaxy models that include a bar, in order to compare them with the data and to explore and quantify the degeneracies.

Results. The radial gradient of the Hercules moving group (∂V_ϕ/∂R = 28.1 ± 2.8 km s⁻¹ kpc⁻¹) cannot be reproduced by our simple models of the Galaxy that show much larger slopes both for a fast and a slow bar. This suggests the need for more complex dynamics (e.g. a different bar potential, spiral arms, a slowing bar, a complex circular velocity curve, external perturbations, etc.). We measured an azimuthal gradient for Hercules of ∂V_ϕ/∂ϕ = −0.63 ± 0.13 km s⁻¹ deg⁻¹ and find that it is compatible with both the slow and fast bar models. Our analysis points out that in using this type of analysis, at least two moving groups are needed to start breaking the degeneracies.

Conclusions. We conclude that it is not sufficient for a model to replicate the local velocity distribution; it must also capture its larger-scale variations. The accurate quantification of the gradients, especially in the azimuthal direction, will be key for the understanding of the dynamics governing the disc.