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

Abstract

Context: Using Gaia DR3 data, Jim\'enez-Arranz et al. (2024a) computed the LMC bar pattern speed using three different methods. One of them suggested that the LMC might be hosting a bar that barely rotates, and is slightly counter-rotating with respect to the LMC disc, with a pattern speed of $\Omega_p = -1.0 \pm 0.5$ km/s/kpc. Aims: To confirm that tidal interactions can trigger the LMC hosting a non-rotating bar due to its interaction with the SMC, which could cause the LMC bar to slow down significantly until it (momentarily) stops. Methods: We analyse a subset of models (K9 and K21) from the KRATOS suite (Jim\'enez-Arranz et al. 2024b) where we detected non-rotating bars. We make use of two different methods to track the evolution of the bar pattern speed: the program patternSpeed.py (Dehnen et al. 2023), and temporal finite-differences of the change rate the bar major axis' phase angle. Results: In the second LMC-SMC-like pericenter passage of K9, the bar of the LMC-like galaxy weakens to almost disappear and regenerates with a pattern speed that suffers a slowdown from $\Omega_p \sim 20$ km/s/kpc to $\Omega_p \sim 0$ km/s/kpc in less than 75 Myr. Then, the bar rotates at less than $\Omega_p \sim 3-5$ km/s/kpc for around 100 Myr, until it recovers the initial (before interaction) pattern speed of $\Omega_p \sim 10$ km/s/kpc. The results for the K21 simulation are comparable. Conclusions: This work is the first direct evidence that galactic bars can be slowed down or even stopped by tidal interaction, which strengthens the possibility of the LMC hosting a non-rotating bar, and can add an alternative formation scenario for observed slow-rotating bars.