  Galactic bars are found in the majority of disc galaxies. They rotate nearly
rigidly with an angular frequency called pattern speed. Previous idealised
simulations have shown that bar pattern speed decreases with time due to
dynamical friction exerted by the dark matter halo, while cold gas can reduce
or even reverse this trend. We want to understand how different galaxy
properties affect the evolution of the bar pattern speed in more realistic
situations, including ongoing star formation, mass infall, AGN feedback and
galaxy interactions. We used the high-resolution run TNG50-1 of the
magnetohydrodynamical cosmological simulations suite IllustrisTNG to trace the
pattern speed of simulated bars and see how it depends on various galaxy
properties. Simulated bars with initially high pattern speed and a subsequent
rapid slowdown are more likely found in more massive galaxies. Lower mass
galaxies, on the other hand, preferentially host bars that start at relatively
low pattern speeds and retain the same value until the end of the simulation.
More massive barred galaxies are also more affected by the AGN feedback, which
removes (or heats up) the cold gas that could have prevented the slowdown. We
find that bars grow and strengthen with slowdown, in agreement with higher
resolution simulations. We find that strong correlations between the bar
slowdown rate and galaxy mass weaken considerably when we use dimensionless
measures to quantify the slowdown. In TNG50, the AGN feedback prescription
amplifies the mass dependence. Turned around, this provides an interesting
statistic to constrain subgrid physics by bar growth/slowing.
