The oldest tidally induced bar-like galaxy in the IllustrisTNG cluster

Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland

^⋆ Corresponding author: lokas@camk.edu.pl

Received: 25 May 2025
Accepted: 29 July 2025

Abstract

New JWST observations have revealed the presence of a significant number of high-redshift barred galaxies. The origin of these bars remains unclear, and their properties appear difficult to reconcile with the results of cosmological simulations of galaxy formation. I present an example of a tidally induced bar-like galaxy formed at z = 2.9 in the TNG100 suite of the IllustrisTNG simulations. The galaxy was identified among the sample of bar-like galaxies studied before and has the earliest bar formation time among the tidally induced subsample of those objects. Its disk transformed into a bar as a result of a close interaction with a massive progenitor of a brightest cluster galaxy (BCG). It remained on a tight orbit around the host and survived until the present, losing most of its initial mass and becoming red but preserving its prolate shape. Even before the interaction, at z = 3.5, the galaxy experienced a few mergers, which elongated its shape. This temporary distortion also made it look like a bar with spiral extensions of up to 6 kpc. The long-lived bar formed later was about 3 kpc long and grew over the next few gigayears. This example demonstrates that high-z bars should not be sought among the progenitors of present-day simulated barred galaxies but rather among the tidally interacting early population of galaxies in forming groups and clusters. Some of these galaxies may have survived as ellipticals, and some may have merged with their BCGs.