Tidally induced bars in Illustris galaxies

Abstract

We study barred galaxies selected from the Illustris cosmological simulation, focusing on tidally induced bars formed from flyby interactions. To guarantee high enough resolution we focus on high mass disc galaxies ($M_*>8.3 \times 10^{10} M_{\odot}$). We find that the fraction of barred galaxies among those (21% at redshift $z=0$) is lower in Illustris than observed in the local Universe, and the fraction grows slightly with redshift. The bar fraction also increases with the stellar mass and decreases with the amount of gas in the disc. Only very few bars at redshift $z=0$ are formed in secular evolution ($\sim$ 7%) and most of them are triggered by external perturbers in mergers or flybys. Many of these bars disappear over time, mostly during secular evolution, which leads to a lower fraction of bars at redshift $z=0$. We then focus on the effect of flyby interactions on the disc and look at tidally induced bars created by a flyby, or pre-existing bars influenced by the passage of a perturber. In the latter case, the interaction can enhance or weaken the bar. During the interaction, the change in the bar strength occurs right after the pericentre passage. The resulting tidally induced bars tend to be stronger than the overall bar sample in Illustris. The preferred scenario to create or enhance a bar seems to be with a strong interaction involving a perturber on a prograde orbit. Furthermore, the strength of the created bar grows with the strength of the interaction.