Letter to the Editor

Tidal interaction can stop galactic bars: On the LMC non-rotating bar

Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 43 SE-22100 Lund, Sweden

^⋆ Corresponding author.

Received: 3 April 2025
Accepted: 5 May 2025

Abstract

Context. The bar pattern speed of the LMC has been computed based on data from Gaia DR3 using three different methods. One method suggested that the LMC might be hosting a bar that barely rotates, and is slightly counter-rotating with respect to the disc of the LMC, with a pattern speed of Ω_p = −1.0 ± 0.5 km s⁻¹ kpc⁻¹.

Aims. We aim to confirm that tidal interactions might prevent the bar hosted by the LMC from rotating as a result of its interaction with the SMC. This might cause the bar in the LMC to slow down significantly until it (momentarily) stops.

Methods. We analysed a subset of models (K9 and K21) from the KRATOS suite in which we detected non-rotating bars. We used two different methods to track the evolution of the bar pattern speed: the program patternSpeed.py and temporal finite-differences of the change in the rate in the major axis of the phase angle of the bar.

Results. In the second LMC–SMC-like pericenter passage of K9, the bar of the LMC-like galaxy weakens so much as to almost disappear and regenerates with a pattern speed that slows down from Ω_p ∼ 20 km s⁻¹ kpc⁻¹ to Ω_p ∼ 0 km s⁻¹ kpc⁻¹ in less than 75 Myr. Then, the bar rotates at less than Ω_p ∼ 3 − 5 km s⁻¹ kpc⁻¹ for about 100 Myr, until it recovers the initial (before the interaction) pattern speed of Ω_p ∼ 10 km s⁻¹ kpc⁻¹. The results for the K21 simulation are similar.

Conclusions. We show that galactic bars can be slowed down or even stopped by tidal interaction. This strengthens the hypothesis that the LMC hosts a non-rotating bar, and it can be an alternative formation scenario for other observed slowly rotating bars.