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