Revisiting the tension between fast bars and the $\Lambda$CDM paradigm

TL;DR

This study uses high-resolution cosmological simulations to show that galactic bars can remain fast in the $ m oldsymbol{ extLambda}$CDM framework, challenging previous findings of excessive slowdown and highlighting the importance of baryon dominance.

Contribution

It demonstrates that bars stay fast in $ m oldsymbol{ extLambda}$CDM simulations when galaxies are more baryon-rich than previously modeled, addressing the tension with observational data.

Findings

Bars remain fast down to z=0 in Auriga simulations.

Galaxies with higher stellar-to-dark matter ratios host faster bars.

Results suggest massive spirals are more baryon-dominated than standard models assume.

Abstract

The pattern speed with which galactic bars rotate is intimately linked to the amount of dark matter in the inner regions of their host galaxies. In particular, dark matter haloes act to slow down bars via torques exerted through dynamical friction. Observational studies of barred galaxies tend to find that bars rotate fast, while hydrodynamical cosmological simulations of galaxy formation and evolution in the $\Lambda$CDM framework have previously found that bars slow down excessively. This has led to a growing tension between fast bars and the $\Lambda$CDM cosmological paradigm. In this study we revisit this issue, using the Auriga suite of high resolution, magneto-hydrodynamical cosmological zoom-in simulations of galaxy formation and evolution in the $\Lambda$CDM framework, finding that bars remain fast down to $z=0$. In Auriga, bars form in galaxies that have higher stellar-to-dark matter ratios and are more baryon-dominated than in previous cosmological simulations; this suggests that in order for bars to remain fast, massive spiral galaxies must lie above the commonly used abundance matching relation. While this reduces the aforementioned tension between the rotation speed of bars and $\Lambda$CDM, it accentuates the recently reported discrepancy between the dynamically inferred stellar-to-dark matter ratios of massive spirals and those inferred from abundance matching. Our results highlight the potential of using bar dynamics to constrain models of galaxy formation and evolution.