Lopsided galactic bars

TL;DR

This paper identifies and analyzes asymmetric, lopsided galactic bars in simulations, exploring their formation, evolution, and possible origins through controlled models, revealing their persistence and links to disk asymmetries.

Contribution

It reports the discovery of lopsided bars in the IllustrisTNG simulation and investigates their formation mechanisms via controlled simulations, highlighting their evolution and connection to disk asymmetries.

Findings

Lopsided bars are present in the IllustrisTNG simulation and can persist for several Gyr.

Asymmetry in bars can develop during or after bar formation, often linked to disk asymmetries.

Controlled simulations suggest interactions and initial disk asymmetries can lead to lopsided bars.

Abstract

Most of the observed and simulated galactic bars are symmetric in the face-on view. However, there are indeed cases of bars that are off-center with respect to the disk or have an asymmetric shape. The only well-known example showing both these features is the Large Magellanic Cloud. We report on the identification of several lopsided galactic bars in the Illustris TNG100 simulation found among a sample of elongated bar-like galaxies studied in the past. The bars show a clear asymmetry in the face-on view, which is in the shape of a footprint. We measured the evolution of the different parameters of the bars' shape and asymmetry as a function of time and find that the asymmetry is preserved for a few Gyr. It can grow together with a bar or appear later, after bar formation. We considered two scenarios leading to the formation of lopsided bars using controlled simulations. In the first, a Milky Way-like galaxy interacts with a massive companion placed on a radial orbit in the plane of the disk and perpendicular to the orientation of the bar at the time of the first passage. In the second, the galaxy initially has an off-center disk and the growth of the bar and its asymmetry is more similar to the one found in IllustrisTNG galaxies, where it is also preceded by the presence of an asymmetric disk. It is thus possible that lopsided bars are born in lopsided disks, although in some cases, the time difference between the occurrence of the asymmetry in the two components is quite large.