No need for dark matter: resolved kinematics of the ultra-diffuse galaxy AGC 114905

Abstract: We present new HI interferometric observations of the gas-rich ultra-diffuse galaxy AGC 114905, which previous work, based on low-resolution data, identified as an outlier of the baryonic Tully-Fisher relation. The new observations, at a spatial resolution $\sim 2.5$ times higher than before, reveal a regular HI disc rotating at about 23 km/s. Our kinematic parameters, recovered with a robust 3D kinematic modelling fitting technique, show that the flat part of the rotation curve is reached. Intriguingly, the rotation curve can be explained almost entirely by the baryonic mass distribution alone. We show that a standard cold dark matter halo that follows the concentration-halo mass relation fails to reproduce the amplitude of the rotation curve by a large margin. Only a halo with an extremely (and arguably unfeasible) low concentration reaches agreement with the data. We also find that the rotation curve of AGC 114905 deviates strongly from the predictions of Modified Newtonian dynamics. The inclination of the galaxy, which is measured independently from our modelling, remains the largest uncertainty in our analysis, but the associated errors are not large enough to reconcile the galaxy with the expectations of cold dark matter or Modified Newtonian dynamics.

• The paper presents high-resolution H I observations of the ultra-diffuse galaxy AGC 114905, finding its rotation curve can be explained almost entirely by baryonic mass.
• The study shows that standard CDM halo models cannot reproduce the observed kinematics of AGC 114905, suggesting minimal or absent dark matter within the measured radius.
• AGC 114905's dynamics also deviate significantly from MOND predictions, making it a potential test case for alternative theories of gravity and dark matter distribution.

Overview of "No Need for Dark Matter: Resolved Kinematics of the Ultra-Diffuse Galaxy AGC 114905"

The paper entitled "No Need for Dark Matter: Resolved Kinematics of the Ultra-Diffuse Galaxy AGC 114905" presents a detailed analysis of the gas-rich ultra-diffuse galaxy (UDG) AGC 114905 using high-resolution H\,I interferometric observations. This study, undertaken by Pavel E. Mancera Piña et al., provides significant insights into the kinematic behavior and dark matter content of this galaxy, which deviates notably from conventional expectations within the prevailing cold dark matter (CDM) framework and the Modified Newtonian Dynamics (MOND) paradigm.

Key Findings

1. Kinematic Observations: The research utilizes H\,I data obtained through the Karl G. Jansky Very Large Array at higher spatial resolution, revealing a regular H\,I disk for AGC 114905 rotating at approximately 23 km/s. This rotation curve reaches a flat part within the observational extent, challenging previous findings based on lower-resolution data that classified this galaxy as an outlier.
2. Dark Matter Analysis: Intriguingly, the rotation curve of AGC 114905 can be almost entirely explained by its baryonic mass distribution alone, suggesting a minimal or potentially nonexistent role for dark matter within the observed radius. This characteristic starkly contrasts typical dwarf galaxies that are generally dark matter-dominated.
3. Incompatibility with CDM Halo Models: The study tries to reconcile the rotation curve with the presence of a standard dark matter halo. However, no physically realistic NFW or coreNFW halo model, constrained by the CDM concentration-halo mass relation, can reproduce the observed kinematic parameters of AGC 114905. Particularly, the halo concentration parameter needed to fit the data is implausibly low, inconsistent with expectations from CDM simulations.
4. Deviation from MOND Predictions: The galaxy also deviates significantly from the predictions of MOND, which has been successful in describing rotation curves of many low-surface-brightness galaxies. This finding raises questions about the universality of MOND as an explanation for the dynamics of all galaxies.

Implications

The observed dynamics of AGC 114905 call into question our understanding of dark matter distribution, particularly in ultra-diffuse galaxies. The implications extend to cosmological models that rely heavily on dark matter as a universal component. A reinterpretation might be necessary if the dark matter is not as prevalent or influential in these galaxies as traditionally thought.

The apparent incongruity with both CDM and MOND indicates that AGC 114905, and potentially similar galaxies, could serve as critical test cases for alternative theories of gravity and dark matter. Observations such as these may necessitate revisions in theoretical models, potentially invoking interactions or feedback mechanisms not accounted for in current structures.

Future Directions

To resolve whether the unusual properties of AGC 114905 are unique or representative of a broader class of galaxies, further investigations into similar ultra-diffuse systems with high-resolution kinematic data are essential.

Continued surveys and observational campaigns should target larger samples of UDGs, assessing whether the baryon-dominated kinematics observed are commonplace. Additionally, refined simulations exploring the formation and evolution of UDGs could elucidate the role of baryonic feedback, environmental factors, and the intrinsic properties that might limit or eliminate the necessity for massive dark matter halos.

In summary, this study of AGC 114905 challenges existing frameworks by demonstrating the inadequacy of standard dark matter models in explaining its kinematics, urging the astrophysics community to reconsider the role of dark matter and alternative gravity theories in the context of ultra-diffuse galaxies.