Fast galaxy bars continue to challenge standard cosmology

Abstract: Many observed disc galaxies harbour a central bar. In the standard cosmological paradigm, galactic bars should be slowed down by dynamical friction from the dark matter halo. This friction depends on the galaxy's physical properties in a complex way, making it impossible to formulate analytically. Fortunately, cosmological hydrodynamical simulations provide an excellent statistical population of galaxies, letting us quantify how simulated galactic bars evolve within dark matter haloes. We measure bar strengths, lengths, and pattern speeds in barred galaxies in state-of-the-art cosmological hydrodynamical simulations of the IllustrisTNG and EAGLE projects, using techniques similar to those used observationally. We then compare our results with the largest available observational sample at redshift $z=0$. We show that the tension between these simulations and observations in the ratio of corotation radius to bar length is $12.62\sigma$ (TNG50), $13.56\sigma$ (TNG100), $2.94\sigma$ (EAGLE50), and $9.69\sigma$ (EAGLE100), revealing for the first time that the significant tension reported previously persists in the recently released TNG50. The lower statistical tension in EAGLE50 is actually caused by it only having 5 galaxies suitable for our analysis, but all four simulations give similar statistics for the bar pattern speed distribution. In addition, the fraction of disc galaxies with bars is similar between TNG50 and TNG100, though somewhat above EAGLE100. The simulated bar fraction and its trend with stellar mass both differ greatly from observations. These dramatic disagreements cast serious doubt on whether galaxies actually have massive cold dark matter haloes, with their associated dynamical friction acting on galactic bars.

• The paper reports a statistically significant tension (over 12 extsigma) between observed 'fast' galaxy bars and the slower bars predicted by standard ΛCDM cosmological simulations like IllustrisTNG and EAGLE.
• This discrepancy challenges the current understanding of galaxy internal dynamics within the ΛCDM framework, particularly regarding the impact of dynamical friction from dark matter halos on bar pattern speeds.
• The findings suggest that reconciling observations with simulations may require exploring alternative dark matter models, modified gravity theories, or refining baryonic physics in simulations.

Analysis of Bar Pattern Speeds in Disc Galaxies within ΛCDM Cosmological Simulations

The paper "Fast galaxy bars continue to challenge standard cosmology" presents a thorough investigation into the dynamics of barred galaxies, particularly focusing on the pattern speeds of these bars within the context of Λ Cold Dark Matter (ΛCDM) cosmological simulations. Utilizing advanced cosmological hydrodynamical simulations from the IllustrisTNG and EAGLE projects, the study explores the evolution of galactic bars and compares simulated data against observed patterns at redshift $z = 0$.

Key Findings

1. Pattern Speed Tension: The study reports a significant tension between simulations and observations concerning the bar pattern speed, particularly the ratio of the corotation radius to bar length ($\mathcal{R}$). The observed galaxies tend to have bars that are 'fast' ($1.0 < \mathcal{R} < 1.4$), contrary to the slower bars predicted by simulations (e.g., $\mathcal{R} = 2.77$ in TNG100).
2. Statistical Significance: The tension between observation and simulation results is quantified at $12.62\sigma$ for TNG50 and $13.56\sigma$ for TNG100, indicating a statistically significant discrepancy.
3. Impact of Resolution: The discrepancy in bar speed is evident across different resolutions, with TNG50 (having higher resolution) still showing significant tension, suggesting that resolution alone does not reconcile the differences.
4. Methodology Consistency: The pattern speed measurements in simulated data employed the Tremaine-Weinberg method, a reliable observational technique, ensuring a consistent comparative framework with observed galaxies.
5. Bar Fraction and Morphology: The fraction of barred galaxies and the trend with stellar mass differ notably between simulations and observations, with simulations showing a larger fraction in higher mass galaxies, irrespective of resolution improvements.

Implications and Speculations

The results emphasize inherent challenges in the ΛCDM model related to dynamical friction from dark matter haloes that traditionally slow down bar pattern speeds more than observed. This discrepancy suggests potential deficiencies in the model when applied to the internal dynamics of disc galaxies.

• Alternative Dark Matter Models: The study suggests that a reconsideration of dark matter dynamics might be necessary. Models such as those involving superfluid dark matter could potentially suppress dynamical friction effects on galactic bars, aligning better with observations.
• Gravity Modification: The fast bars observed may also imply modifications to gravity. The study hints at the need for alternative theories such as modified Newtonian dynamics (MOND), which naturally predict decreased friction and thus fast bars.

Future Developments

The pursuit of reconciling simulation data with observations may involve:

• Further refinement of baryonic physics in simulations.
• Exploration of alternative dark matter properties or modified gravity theories.
• Enhanced observational campaigns to refine the measurement of bar properties and their evolution over cosmic time scales.

Ultimately, this research underscores a critical aspect of galaxy formation that tests the boundaries of our current cosmological models, prompting both theoretical and observational advancements to resolve the existing disparities.