Asymmetries in the LMC velocity maps

¹ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), C Martí i Franquès, 1, 08028 Barcelona, Spain
² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), C Gran Capità, 2-4, 08034 Barcelona, Spain
⁴ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 43, 22100 Lund, Sweden

^★ Corresponding author: marie.schoelch@fqa.ub.edu

Received: 8 July 2025
Accepted: 31 July 2025

Abstract

Context. The analysis of precise Gaia DR3 astrometry in the LMC region has revealed asymmetric patterns in the bar quadrupole and the disc outskirts of the LMC in-plane velocity maps.

Aims. We aim to quantify the asymmetries detected in the LMC radial and residual tangential velocity maps, and determine whether they are generated naturally due to the LMC’s interaction with the SMC.

Methods. We analyse the velocity maps of different simulations from the KRATOS suite of N-body simulations of the LMC–SMC– MW system, proposing a new methodology to quantify the kinematic asymmetry in the bar and the outskirts of the disc. We also transform the KRATOS simulations into mock catalogues with G magnitudes and Gaia observational errors, to confirm that the asymmetric signature in the LMC is not an effect of observational uncertainties. In addition, we investigate the possibility of a classification bias in the neural network classifier of the Gaia optimal sample.

Results. In the KRATOS simulations of the LMC and SMC interaction, the dynamical effect of the SMC passages produces a displacement of the bar and asymmetries in the LMC velocity maps. Individual stars of the SMC do not have a substantial effect on the kinematics of the LMC. By comparing the velocity maps of mock catalogues of the future Gaia data releases DR4, DR5, and GaiaNIR, we find that the asymmetric signature in the bar quadrupole is independent of observational errors. We thereby confirm that it is a consequence of the interaction of the LMC with the SMC. We also find a classification bias in the neural network classifier, indicating that the outer disc asymmetry observed in the optimal sample is artificial.

Conclusions. The analysis of the KRATOS simulations reveals that the interaction of the LMC with the SMC can generate asymmetric patterns in the velocity field. In the case of the Gaia DR3 LMC velocity maps we conclude that the bar quadrupole asymmetry is directly correlated with the SMC interaction, while the outer disc asymmetry is an artefact of the classifier for the optimal sample.