Presentation #400.02 in the session Galactic Bars.
Despite decades of research, the mechanism by which bars form in galaxies is still a subject of debate. Studies using isolated galaxy evolution simulations have explored different formation mechanisms in controlled simulations with fine-tuned initial conditions. However, galaxies in the Universe evolve in a much more complex environment: they undergo multiple satellite interactions, mergers, gas accretion events, and star formation while simultaneously interacting with their dark matter halo. Past studies have also shown that bars can direct material into the central supermassive black hole in a galaxy, coupling bar formation to black hole feedback. Thus, investigating bar formation and reproducing their observed properties in cosmological simulations is an important step in understanding galaxy formation.
In this work, we look at the 13 high resolution Milky Way mass galaxies from the zoomed-in cosmo-hydro simulation FIRE2 (Feedback in Realistic Environments) suite to first ask: How do bars form and when does bar formation fail? We find examples where bars form due to satellite interactions and others where bars form secularly in the disk. We observe that gas infall and outflow, catalyzed by starburst events, make the disk less likely to host a bar. Particularly, the stellar feedback prescription in FIRE2, and the lack of a prescription for black hole feedback to mitigate the amount of gas in the galaxy centre, together make the disk kinematically hot, such that bars that do form are relatively short and rotate slowly relative to a fast bar having similar length. We verify predictions from controlled simulations for the influence of the disk mass fraction on bar length and shape: when a disk is dominated by baryons up to a large radius, the bar is weak and elliptical in its face-on view. As the disk becomes more dark matter dominated towards the centre, the bar morphology transforms from weak elliptical to a more rectangular and stronger bar.
Our work helps understand the complex mechanism of bar formation in Milky Way type galaxies in a cosmological environment. In contrast with the Auriga and Illustris simulations, which use a different feedback model, FIRE2 galaxies tend to be more disk-dominated, which gives rise to shorter bars.