The ultra-diffuse dwarf galaxies NGC 1052-DF2 and 1052-DF4 are in conflict with standard cosmology (1909.04663v1)

Abstract: Recently van Dokkum et al. (2018b) reported that the galaxy NGC 1052-DF2 (DF2) lacks dark matter if located at $20$ Mpc from Earth. In contrast, DF2 is a dark-matter-dominated dwarf galaxy with a normal globular cluster population if it has a much shorter distance near $10$ Mpc. However, DF2 then has a high peculiar velocity wrt. the cosmic microwave background of $886$ $\rm{km\,s^{-1}}$, which differs from that of the Local Group (LG) velocity vector by $1298$ $\rm{km\,s^{-1}}$ with an angle of $117 \, ^{\circ}$. Taking into account the dynamical $M/L$ ratio, the stellar mass, half-light radius, peculiar velocity, motion relative to the LG, and the luminosities of the globular clusters, we show that the probability of finding DF2-like galaxies in the lambda cold dark matter ($\Lambda$CDM) TNG100-1 simulation is at most $1.0\times10^{-4}$ at $11.5$ Mpc and is $4.8\times10^{-7}$ at $20.0$ Mpc. At $11.5$ Mpc, the peculiar velocity is in significant tension with the TNG100-1, TNG300-1, and Millennium simulations, but occurs naturally in a Milgromian cosmology. At $20.0$ Mpc, the unusual globular cluster population would challenge any cosmological model. Estimating that precise measurements of the internal velocity dispersion, stellar mass, and distance exist for $100$ galaxies, DF2 is in $2.6\sigma$ ($11.5$ Mpc) and $4.1\sigma$ ($20.0$ Mpc) tension with standard cosmology. Adopting the former distance for DF2 and assuming that NGC 1052-DF4 is at $20.0$ Mpc, the existence of both is in tension at $\geq4.8\sigma$ with the $\Lambda$CDM model. If both galaxies are at $20.0$ Mpc the $\Lambda$CDM cosmology has to be rejected by $\geq5.8\sigma$.

• The paper reveals that DF2 and DF4 exhibit unusual properties, such as high peculiar velocities and dark matter deficits, occurring with probabilities as low as 1.0×10⁻⁴ in ΛCDM simulations.
• The study utilizes advanced simulations (Illustris, TNG100-1, TNG300-1) along with statistical tests to rigorously compare observed galactic features against theoretical predictions.
• These findings prompt a reexamination of ΛCDM cosmology and suggest that alternative theories like MOND may better account for the anomalous dynamics in these ultra-diffuse galaxies.

An Analysis of the Ultra-Diffuse Dwarf Galaxies NGC 1052-DF2 and NGC 1052-DF4 in the Context of Lambda Cold Dark Matter Cosmology

The paper presented by Haslbauer et al. offers a comprehensive examination of the ultra-diffuse dwarf galaxies NGC 1052-DF2 (DF2) and NGC 1052-DF4 (DF4) within the framework of the standard lambda cold dark matter (ΛCDM) cosmological model. The paper casts doubt on the compatibility of these galaxies' observed properties with the predictions of ΛCDM, suggesting significant tensions that warrant thorough analysis.

Key Numerical Results and Statistical Analysis

The authors employ simulations such as Illustris-1, TNG100-1, and TNG300-1 to probe the occurrence probability of analogous galaxies within a ΛCDM universe. The observed properties of DF2, particularly its apparent lack of dark matter and high peculiar velocity, are analyzed against these simulations, yielding probabilities of occurrence as low as $1.0 \times 10^{-4}$ at the optimal distance of 11.5 Mpc. Such results indicate a significant tension with ΛCDM, especially when considering that 100 galaxies might provide a similar level of observational constraint.

Notably, the peculiar velocity of DF2 presents a substantial anomaly when considered within the context of the typical expectations of ΛCDM. At a proposed distance of approximately 11.5 to 13 Mpc, the peculiar velocity of DF2 exceeds 676 km/s relative to the cosmic microwave background (CMB), diverging sharply from typical expectations even in TNG100-1 and TNG300-1 simulation runs. In the same cosmological context, DF4 exhibits similar inconsistencies.

Analysis of Globular Cluster Populations

The authenticity of DF2's dark matter deficiency report relies, in part, on the brightness and distribution of its globular clusters (GCs). At the presumed distance of 20 Mpc, the GCs exhibit anomalously high luminosity compared to typical Galactic clusters, raising questions about their inherent nature and the assumed distance. Statistical calculations including the Mann-Whitney U test and Kolmogorov-Smirnov test underline the improbability of such GC populations in a standard cosmological context, further emboldening the suspicion of DF2's true distance.

Theoretical and Practical Implications

This paper prompts reconsideration of the applicability of ΛCDM to such ultra-diffuse galaxies, underscoring the potential need for alternative gravitational theories, such as Milgromian dynamics (MOND), which might offer a coherent explanation for the high-velocity fields observed. Within MOND frameworks, the higher velocity dispersions become plausible, narrowing the gap between observed discrepancies and theoretical predictions.

The emergence of tension between DF2, DF4, and the tenets of ΛCDM emphasizes the urgency for further observational and theoretical inquiry. Potential future developments in this field could include more refined simulations reflective of alternative gravitational paradigms or deeper observational campaigns that might better constrain these galaxies' distance and intrinsic properties.

Conclusion

Haslbauer et al.'s investigation into DF2 and DF4 presents a compelling case for re-evaluating aspects of ΛCDM cosmology. Their findings highlight critical areas where current models falter, particularly in explaining the nature and behavior of select dwarf galaxies. The continued investigation into galactic properties close to the edges of current cosmological understanding holds promise for significant advances in our understanding of dark matter and the fabric of the cosmos.