Coupling between stellar and HI lopsidedness in Milky Way-type galaxies from the Auriga Superstars cosmological simulations

TL;DR

This study uses cosmological simulations to analyze the coupling between stellar and HI lopsidedness in Milky Way-like galaxies, revealing how different processes influence galaxy asymmetries over time.

Contribution

It provides a detailed, combined analysis of stellar and gas lopsidedness in simulated galaxies, highlighting the roles of interactions, accretion, and internal dynamics.

Findings

Stellar lopsidedness correlates strongly with HI at z=0.

Tidal interactions induce coherent lopsidedness in stars and HI.

Gas accretion mainly affects HI and young stars, not the total stellar component.

Abstract

Lopsidedness is common in disk galaxies, yet its origin and evolution remain unclear. Previous studies typically examined stellar and gas asymmetries separately, but a combined analysis offers a stronger probe of the mechanisms driving lopsidedness, recent galaxy evolution, and environment. We analyze the density and kinematics of stellar and atomic hydrogen (HI) components in nine Milky Way type galaxies from the Auriga Superstars cosmological zoom-in simulations. The high stellar mass resolution improves the visibility of disk features while reducing noise, enabling a detailed study of dynamical processes in a cosmological context. Morphological and kinematical lopsidedness are quantified using the first Fourier mode (m=1) of the face-on mass distribution and radial velocity maps, measured consistently for stars and gas between 0.5 and 1 stellar optical radius. At z=0, morphological lopsidedness in old stars (>0.5Gyr) strongly correlates with HI, tracing distortions in the global gravitational potential. In contrast, young stars (<0.5Gyr) trace asymmetric star formation along spiral arms. Stellar morphological and kinematical lopsidedness are strongly correlated, whereas HI shows a weaker correlation, with kinematic asymmetries dominating. We also find an anti-correlation between stellar lopsidedness and bar strength. Strongly barred galaxies tend to host more symmetric disks and higher central stellar mass densities. Tracing lopsidedness evolution over time, tidal interactions with massive satellites (mass ratio >1:50) induce coherent lopsidedness in both stars and HI. In contrast, smooth gas accretion mainly affects HI and young stars, leaving the total stellar component largely symmetric. Overall, these results demonstrate that lopsidedness is a powerful diagnostic of internal disk evolution, gas accretion, and environmental interactions across cosmic time.