The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter

Abstract: Due to their low surface brightness, dwarf galaxies are particularly susceptible to tidal forces. The expected degree of disturbance depends on the assumed gravity law and whether they have a dominant dark halo. This makes dwarf galaxies useful for testing different gravity models. In this project, we use the Fornax Deep Survey (FDS) dwarf galaxy catalogue to compare the properties of dwarf galaxies in the Fornax Cluster with those predicted by the Lambda cold dark matter ($\Lambda$CDM) standard model of cosmology and Milgromian dynamics (MOND). We construct a test particle simulation of the Fornax system. We then use the MCMC method to fit this to the FDS distribution of tidal susceptibility $\eta$ (half-mass radius divided by theoretical tidal radius), the fraction of dwarfs that visually appear disturbed as a function of $\eta$, and the distribution of projected separation from the cluster centre. This allows us to constrain the $\eta$ value at which dwarfs should get destroyed by tides. Accounting for an $r'$-band surface brightness limit of 27.8 magnitudes per square arcsec, the required stability threshold is $\eta_{\textrm{destr}} = 0.25^{+0.07}_{-0.03}$ in $\Lambda$CDM and $ 1.88^{+0.85}_{-0.53}$ in MOND. The $\Lambda$CDM value is in tension with previous $\textit{N}$-body dwarf galaxy simulations, which indicate that $\eta_{\textrm{destr}} \approx 1$. Our MOND $\textit{N}$-body simulations indicate that $\eta_{\textrm{destr}} = 1.70 \pm 0.30$, which agrees well with our MCMC analysis of the FDS. We therefore conclude that the observed deformations of dwarf galaxies in the Fornax Cluster and the lack of low surface brightness dwarfs towards its centre are incompatible with $\Lambda$CDM expectations but well consistent with MOND.

• The paper finds that tidal analysis of Fornax Cluster dwarf galaxies challenges the notion of dominant dark matter halos.
• It employs tidal susceptibility (η) measurements to directly compare predictions from ΛCDM and MOND frameworks.
• The findings imply that MOND better accounts for observed tidal disturbances, questioning standard dark matter interpretations.

Analyzing the Tidal Stability of Fornax Cluster Dwarf Galaxies

The paper "The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter" by E. Asencio et al. investigates the tidal stability of dwarf galaxies in the Fornax Cluster with a critical eye on their dark matter content. The study extensively uses the Fornax Deep Survey (FDS) and compares the empirical data with predictions from two cosmological models: the standard Lambda Cold Dark Matter ($\Lambda$CDM) paradigm and Modified Newtonian Dynamics (MOND). The research highlights significant tensions in the interpretation of dwarf galaxy properties within $\Lambda$CDM, potentially suggesting a reevaluation of the presence and influence of dark matter in these systems.

Methodology and Findings

The study starts by exploring the gravitational vulnerability of dwarf galaxies in the Fornax Cluster. Due to low surface brightness and mass, these galaxies are thought to be ideal candidates for testing dark matter presence through their reaction to tidal forces. The authors simulate the orbital dynamics of the Fornax dwarfs and calculate their tidal susceptibility ($\eta$), defined as the ratio of the half-mass radius to the theoretical tidal radius.

Remarkably, while $\Lambda$CDM posits that dwarf galaxies are ensconced in dominant dark matter haloes, this research concludes otherwise. Under $\Lambda$CDM, the dwarf galaxies exhibit an $\eta$ considerably lower than one might expect for a system that would be morphologically disturbed by tidal forces—indicating that they should be untouched by these forces. Contrarily, observed deformations and the spatial distribution of the dwarfs suggest that they have indeed undergone significant tidal interaction.

In sharp contrast, the application of MOND to the same data offers conforming results; MOND predicts higher susceptibility values coherent with dwarf galaxy disturbances. The paper posits an $\eta$ threshold of 1.88 in MOND for tidal disruption, compared to a strikingly lower $\eta$ threshold of just 0.25 in $\Lambda$CDM. Notably, empirical observation aligns well with MOND's recalibration of gravitational influences absent dark matter.

Implications

The findings provocatively suggest that the observed tidal effects align more with expectations from MOND rather than $\Lambda$CDM. This could imply that dark matter may not play as significant a role in the dynamics of these systems as previously believed. If confirmed through additional observations and simulations, this could fundamentally alter our understanding of galaxy formation and the role of dark matter at smaller scales.

MOND, typically disregarded in favor of dark matter-centric models, emerges as a compelling explanation where conventional models fall short. Future work could focus on reconciling these revelations with the large-scale successes of $\Lambda$CDM or on developing hybrid models that incorporate additional physics.

Conclusion

This study suggests inconsistencies with the $\Lambda$CDM paradigm based on the morphological characteristics and distribution of Fornax Cluster dwarf galaxies. The research raises questions about dark matter's influence at small scales, with implications that echo broadly into galactic dynamics and evolution theories. MOND presents a viable alternative as it successfully explains the observations without invoking extensive and otherwise undetected dark matter components. The collaboration between observations and modified gravity theories may become increasingly crucial as we probe the cosmos at smaller, more intricate scales.