The unexpected diversity of dwarf galaxy rotation curves (1504.01437v2)

Abstract: We examine the circular velocity profiles of galaxies in {\Lambda}CDM cosmological hydrodynamical simulations from the EAGLE and LOCAL GROUPS projects and compare them with a compilation of observed rotation curves of galaxies spanning a wide range in mass. The shape of the circular velocity profiles of simulated galaxies varies systematically as a function of galaxy mass, but shows remarkably little variation at fixed maximum circular velocity. This is especially true for low-mass dark matter-dominated systems, reflecting the expected similarity of the underlying cold dark matter haloes. This is at odds with observed dwarf galaxies, which show a large diversity of rotation curve shapes, even at fixed maximum rotation speed. Some dwarfs have rotation curves that agree well with simulations, others do not. The latter are systems where the inferred mass enclosed in the inner regions is much lower than expected for cold dark matter haloes and include many galaxies where previous work claims the presence of a constant density "core". The "cusp vs core" issue is thus better characterized as an "inner mass deficit" problem than as a density slope mismatch. For several galaxies the magnitude of this inner mass deficit is well in excess of that reported in recent simulations where cores result from baryon-induced fluctuations in the gravitational potential. We conclude that one or more of the following statements must be true: (i) the dark matter is more complex than envisaged by any current model; (ii) current simulations fail to reproduce the effects of baryons on the inner regions of dwarf galaxies; and/or (iii) the mass profiles of "inner mass deficit" galaxies inferred from kinematic data are incorrect.

• The paper demonstrates that dwarf galaxy rotation curves exhibit unexpected diversity compared to predictions from ΛCDM simulations.
• It employs cosmological hydrodynamical simulations and observational data to highlight the inner mass deficit and the 'cusp vs core' problem.
• The findings challenge current dark matter models and call for refined simulation techniques and reevaluation of baryonic effects in galaxy formation.

Analyzing the Diversity in Dwarf Galaxy Rotation Curves

The paper "The Unexpected Diversity of Dwarf Galaxy Rotation Curves" by Oman et al. investigates the discrepancies between observed rotation curves of dwarf galaxies and the predictions of $\Lambda$CDM cosmological simulations. This work focuses on the comparison of observational data with results from the EAGLE and LOCAL GROUPS simulation projects to assess the known "cusp vs core" problem in dwarf galaxy dynamics.

Key Insights from the Simulations

The authors utilize cosmological hydrodynamical simulations to generate circular velocity profiles of galaxies across a mass spectrum. Notably, the paper emphasizes the relative uniformity of these profiles within the simulations at a given maximum circular velocity (denoted as $V_{\rm max}$). For low-mass, dark matter-dominated systems, the expectation of homogeneous cold dark matter (CDM) halos suggests a consistent velocity profile shape. However, this is at odds with observational data, where dwarf galaxies present a diversity of rotation curve shapes even when $V_{\rm max}$ remains constant.

Observational Findings and Challenges

The challenge arises when observed rotation curves suggest an "inner mass deficit" compared to what CDM models predict. This discrepancy is termed the "cusp vs core" issue and highlights a fundamental misunderstanding, suggesting a need to reevaluate the baryonic effects or dark matter assumptions. The authors argue that baryon-induced fluctuations during galaxy formation, proposed as a solution, do not sufficiently account for the cases where rotation curves are inconsistent with predictions. Instead, observed galaxies often imply a missing mass problem within their inner regions.

Implications and Theoretical Considerations

The research calls into question the accuracy of current $\Lambda$CDM models and baryonic influence during galaxy formation. It posits three possible explanations for the discrepancies: a more complex nature of dark matter than currently assumed, inadequacies in simulating baryonic effects on galaxy formation, or inaccuracies in the kinematic data interpretation leading to incorrect mass inferences. These findings suggest a potential need to investigate alternative dark matter models, although the diversity in the observed data does not easily reconcile with predictions from self-interacting dark matter (SIDM) or warm dark matter (WDM) models.

Strong Numerical Results

The numerical simulations in this paper, particularly those from the EAGLE project, achieve significant alignment with observational baryonic Tully-Fisher relations and anticipated structural properties in simulated galaxies. However, the inner regions, notably within 2 kpc radiate from the nucleus stabilize the disk against non-axisymmetric perturbations, thereby hindering bar formation.

The controversial nature of the results highlights multiple alternative explanations for the observed rotation curves. Some theories suggest modifications to the nature of dark matter, proposing particles with properties distinct from traditional weakly interacting massive particles (WIMPs) or axions, such as warm dark matter (WDM) or self-interacting dark matter (SIDM). These scenarios can potentially lead to the formation of a core in dark matter halos. However, further limitations of particle mass or velocity profiles suggest these solutions still fall short of explaining all observed discrepancies.

Future Directions and Conclusions

The paper concludes by emphasizing the unresolved nature of the rotation curve discrepancies in dwarf galaxies. The observed diversity presents a challenge to current models of galaxy formation in $\Lambda$CDM frameworks and suggests that further exploration into the nature of dark matter, improved simulation methodologies, or refined observational techniques are needed to resolve these inconsistencies. Future studies might focus on the interplay between baryonic matter fluctuations and dark matter, which could reshape these profiles during galaxy formation. Moreover, other physical processes, non-circular motions, and observational uncertainties could also play critical roles in the observed rotation curves.

The paper sets a foundation for refining dark matter profiles, cosmological simulations, and observational strategies in future astrophysical inquiries, underscoring the dynamic nature of our understanding of dark matter and galaxy formation.