Cosmic evolution of bars in simulations of galaxy formation

We investigate the evolution of two bars formed in fully self-consistent hydrodynamic simulations of the formation of Milky Way-mass galaxies. One galaxy shows higher central mass concentration and has a longer and stronger bar than the other at z = 0. The stronger bar evolves by transferring its angular momentum mainly to the dark halo. Consequently the rotation speed of the bar decreases with time, while the amplitude of the bar increases with time. These features qualitatively agree with the results obtained by idealized simulations. The pattern speed of the stronger bar largely goes up and down within a half revolution in its early evolutionary stage. These oscillations occur when the bar is misaligned with the m = 4 Fourier component. These oscillations correlate with the oscillations in the triaxiality of the dark matter halo, but differently from the way identified by idealized simulations. The amplitude of the weaker bar does not increase despite the fact that its rotation slows down with time. This result contradicts what is expected from idealized simulations and is caused by the decline of the central density associated with the mass loss and feedback from the stellar populations. The amplitude of the weaker bar is further weakened by the angular momentum injection from interactions with stellar clumps in the disk. In the both galaxies, the bars are terminated around the 4:1 resonance.