The Effect of Subgrid Physics Models on the Pattern Speed of Bars in Cosmological Simulations

Abstract

The amount of dark matter in the central region of galaxies is intimately linked to the slowdown of galactic bars. Recent work has revealed a tension between bars that are observed in the local universe and those produced in cosmological hydrodynamical simulations at z = 0. Observed bars are found to be `fast', i.e. to have a small ratio between the corotation radius and bar length, while those in the simulations are `slow', i.e. the corotation radius much larger than the bar length. Recent work has been carried out in an attempt to find the root cause of this discrepancy, and indeed to explore whether fast bars can exist within a CDM universe. The ratio of stars to dark matter, along with other properties such as gas fraction and velocity dispersion, has been linked to the evolution of bars. The resolution of simulations is often cited as the underlying cause of differences between simulations. In this work, I explore the slowdown of bars in two sets of cosmological zoom-in simulations which are identical, apart from their galaxy formation model (i.e. the subgrid physics). I then study how the slowdown of bars in these two models is related to parameters such as the stellar-to-dark matter ratio, the gas fraction and velocity dispersion, all of which are determined by the subgrid physics itself. Using halos from the Auriga suite of zoom-in cosmological simulations, I rerun them with the subgrid physics model from IllustrisTNG. I find that the bars in Auriga are faster than those run with the TNG model, i.e. Auriga have a smaller ratio of the corotation radius to bar length. The bars in TNG are shorter and stronger than in the Auriga model. In terms of global halo properties, Auriga galaxies have a greater stellar mass in their disc, are more baryon dominated at 30kpc, have a greater gas fraction in the disc. They also have a lower stellar velocity dispersion within a disc of radius 6kpc and height 1kpc from the centre. All of these differences lead to the conclusion that the subgrid physics model has a profound effect on the overall properties of a galaxy, include the speed of the bar. We therefore show that the changes in subgrid physics can have a significant effect on the dynamical properties of barred spiral galaxies and, as such, the dynamical properties of bars can be used to constrain models of galaxy formation and evolution.