Revisiting the tension between fast bars and the ΛCDM 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 cold dark matter (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.

Automated summary

Research has shown that the rotation speed of bars in galaxies is closely related to the amount of dark matter in their inner regions. Recent cosmological simulations suggest that bars form fastest in galaxies with higher stellar-to-dark matter ratios and more baryon-dominated composition. This highlights a discrepancy between dynamically inferred ratios and those inferred from abundance matching.