Work

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

Year: 2026

Type: article

Abstract: Context. Lopsidedness is a common feature in disk galaxies, but its origin and evolution remain poorly understood. Previous works have generally focused on stellar or gas lopsidedness separately. However, a combined analysis can help identify the mechanisms driving lopsidedness and constrain recent galactic evolution and environmental interactions. Aims. We study lopsidedness in the density and kinematics of the stellar and atomic hydrogen (HI) components of nine Milky Way-type galaxies from the Auriga Superstars cosmological zoom-in simulations. The high stellar mass resolution (m * = 800 M ⊙ ) of the simulations improves the visibility of features in the stellar disk and halo, reduces the effect of noise on the measurement of lopsidedness, and provides a refined sampling of the gravitational potential. Methods. We quantified morphological and kinematical lopsidedness through the amplitude of the first Fourier mode ( m = 1) of the face-on projected mass distribution and radial velocity maps consistently for stars and gas and over 0.5–1 R opt , where R opt is the stellar optical radius. Results. At z = 0, the morphological lopsidedness of the old stellar component (> 0.5 Gyr old) strongly correlates with that of HI, tracing distortions in the global gravitational potential, while the young stellar population (< 0.5 Gyr old) primarily traces asymmetric star formation along strong spiral arms. Morphological and kinematical lopsidedness are strongly correlated in stars but weakly correlated in HI, which typically exhibits stronger kinematical asymmetries. We also find that stellar disk lopsidedness is anticorrelated with bar strength, with strongly barred galaxies hosting more symmetric disks and higher central stellar mass densities. Based on our simulations of lopsidedness evolution over the past 7 Gyr, we observed that tidal interactions with massive satellites (mass ratio > 1:50) produce coherent lopsided responses in stars and HI. In contrast, smooth gas accretion primarily drives lopsidedness in HI and young stars, while the total stellar component remains largely symmetric. Individual galaxies often experience multiple mechanisms, producing complex coupling and decoupling among distinct tracers.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202660650