Superfluid Dark Matter Faces Challenges Fitting Galaxy Rotation Curves and Explaining Gravitational Lensing

Introduction

The nature of dark matter remains one of the most profound mysteries in modern physics. While the standard cold dark matter (CDM) model has been successful in explaining large-scale cosmic structures, it faces challenges when describing the dynamics of individual galaxies. Alternative models, such as the superfluid dark matter (SFDM) model, have been proposed to address these discrepancies by combining aspects of dark matter with modified Newtonian dynamics (MOND). SFDM posits that dark matter is composed of a light scalar field that can condense into a superfluid, mediating a force similar to MOND's. This study investigates the SFDM model by fitting its predictions to the rotation curves of 169 galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample. The researchers aimed to assess how well SFDM fits the observational data and whether it aligns with expectations regarding stellar mass-to-light ratios and MOND-like behavior.

Testing the Superfluid Dark Matter Model

The researchers fitted the SFDM model to the rotation curves of 169 galaxies from the SPARC sample, treating the stellar mass-to-light ratio as a parameter to be determined. This approach allowed them to evaluate the model's performance in reproducing the observed rotational velocities of galaxies. The SFDM model itself is complex, involving four parameters that were kept fixed at fiducial values derived from previous research. The total acceleration within a galaxy's superfluid core is a combination of forces arising from the phonon field, the superfluid's own gravity, and the mass of the baryons. A key aspect of SFDM is its potential to mimic MOND, particularly in the "MOND limit" where a specific quantity, ε*, is much less than one. In this limit, the phonon force closely resembles the modified gravitational force proposed by MOND.

Findings on Stellar Mass-to-Light Ratios

A primary finding of the study concerns the stellar mass-to-light ratios (M/L*) derived from the SFDM fits. While the SFDM model generally produced acceptable M/L* values, a significant issue emerged: these best-fit M/L* values showed an unnatural dependence on galaxy size. Giant galaxies consistently yielded lower M/L* ratios than dwarf galaxies. This trend is problematic because stellar population synthesis (SPS) models, which predict M/L* based on stellar composition and age, generally expect M/L* to increase with galaxy mass, not decrease. The study also found that SFDM fits often occurred in a regime where the model's force deviates from MOND unless a specific boundary condition is adjusted for each galaxy. If this adjustment is made, SFDM loses one of its key advantages over standard dark matter models. Conversely, if the fits are forced to approximate MOND well, the total mass of the superfluid dark matter comes into tension with data from gravitational lensing.

Conflict with Gravitational Lensing Data

The research also highlighted a significant tension between the SFDM model and gravitational lensing observations, particularly when the model is constrained to behave like MOND. Gravitational lensing probes the total mass distribution in galaxies, including dark matter. When SFDM is forced into its MOND-like regime (i.e., the proper MOND limit), the resulting total dark matter mass is found to be considerably smaller than what is required to explain strong lensing data. This discrepancy is stark: for large galaxies, the ratio of dark matter mass to baryonic mass in the MOND limit of SFDM is less than 10, whereas strong lensing analyses suggest ratios of 1000 or higher. Even when allowing for a "pseudo-MOND limit," where the model's force deviates slightly from MOND, the dark matter masses remain insufficient to reconcile with lensing data. The two-field SFDM model, an alternative formulation, also faces challenges, although it offers a better agreement with MOND-like behavior. While it can accommodate larger dark matter masses in some cases, it still struggles to fully satisfy both rotation curve and lensing constraints simultaneously.

Conclusion

In conclusion, this comprehensive analysis of the superfluid dark matter model applied to galaxy rotation curves reveals significant challenges. The model's predictions for stellar mass-to-light ratios exhibit an unnatural trend with galaxy size, conflicting with expectations from stellar population synthesis. Furthermore, when SFDM is constrained to mimic MOND's behavior, it fails to provide sufficient dark matter mass to explain strong gravitational lensing observations. These findings suggest that while SFDM offers an intriguing alternative to standard dark matter models, it requires further refinement to resolve these critical discrepancies and fully align with observational data across different astrophysical scales.

Original source: "https://www.aanda.org/articles/aa/full_html/2022/08/aa43216-22/aa43216-22.html"