Tracing the origins of galaxy lopsidedness across cosmic time

¹ Departamento de Astronomía, Universidad de La Serena, Av. Raúl Bitrán 1305, La Serena, Chile
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
³ Centro de Astro-Ingenieria, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
⁴ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile

^⋆ Corresponding author: arianna.dolfi@userena.cl

Received: 28 November 2024
Accepted: 29 April 2025

Abstract

Context. Current studies of large-scale asymmetries (i.e., lopsidedness) in the stellar density distribution of disk galaxies have mainly focused on the Local Universe. However, recent observations have found a significant fraction (over 60%) of lopsided galaxies at high redshift (i.e., 1.5≲z≲3), which is significantly larger than the fraction (∼30%) observed in the nearby Universe.

Aims. We aim to understand whether simulations can reproduce the observed fraction of lopsided galaxies at high redshift. We also consider whether the more widespread lopsidedness at high redshift (rather than low-redshift) could be associated with environmental mechanisms being more effective in producing lopsided perturbations at high redshift.

Methods. At each redshift between 0<z<2, we independently selected a sample of disk-like galaxies from the IllustrisTNG simulations. We then characterized lopsidedness in the disks of galaxies at each redshift and studied the relevant mechanisms generating lopsidedness, as well as the correlation between such perturbations, the local environment, and the galaxy internal properties as a function of redshift.

Results. In line with previous and new observational results, we find that: (1) simulations predict a significant fraction (∼60%) of lopsided galaxies at a high redshift, namely, 1.5<z<2; (2) the fraction of lopsided galaxies, as well as the lopsided amplitude, decreases from high-to-low redshift, meaning that galaxies become more symmetric toward low redshift; and (3) there is no significant dependence of lopsidedness on the local environment. However, there is a strong correlation between the lopsided amplitude and basic galactic structural properties at all redshifts between 0<z<2. This means that independently of the mechanisms behind lopsidedness, galaxies with a low central stellar mass density and more extended disks are more susceptible of developing significant lopsidedness. We find that both recent interactions with mass-ratio >1:10 and gas accretion with subsequent star formation can produce lopsided perturbations at all redshifts, but they are both significantly more effective at high redshift.

Conclusions. These results suggest that the mechanisms behind lopsidedness vary across cosmic time, with a greater influence from environmental interactions and gas accretion at higher redshift.