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

Dolfi, A, Gómez, FA, Bieri, R, Fragkoudi, F, Grand, RJJ ORCID: 0000-0001-9667-1340, Monachesi, A, Pakmor, R and van de Voort, F (2026) Coupling between stellar and HI lopsidedness in Milky Way--type galaxies from the Auriga Superstars cosmological simulations. Astronomy & Astrophysics, 715. pp. 1-20. ISSN 0004-6361

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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 Ropt, where Ropt 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.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
Date of acceptance:	13 September 2026
Date of first compliant Open Access:	5 November 2026
Date Deposited:	05 Nov 2026 10:53
Last Modified:	05 Nov 2026 10:53
DOI or ID number:	10.1051/0004-6361/202660650
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29823

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