Does the galaxy NGC1052-DF2 falsify Milgromian dynamics? (1903.11612v1)

Abstract: A great challenge in present-day physics is to understand whether the observed internal dynamics of galaxies is due to dark matter matter or due to a modification of the law of gravity. Recently, van Dokkum et al. reported that the ultra-diffuse dwarf galaxy NGC1052-DF2 lacks dark matter, and they claimed that this would -- paradoxically -- be problematic for modified gravity theories like Milgromian dynamics (MOND). However, NGC1052-DF2 is not isolated, so that a valid prediction of its internal dynamics in MOND cannot be made without properly accounting for the external gravitational fields from neighbouring galaxies. Including this external field effect following Haghi et al. shows that NGC1052-DF2 is consistent with MOND.

• The paper reexamines NGC1052-DF2 using MOND, incorporating the external field effect to challenge previous claims of dark matter absence.
• The study employs a MOND N-body integrator with observed parameters to predict a velocity dispersion near 14 km/s that matches observations.
• The findings underscore that accounting for external gravitational influences preserves MOND's validity, encouraging further research on alternative gravity models.

Milgromian Dynamics and the Challenge of NGC1052-DF2

The paper "Does the galaxy NGC1052-DF2 falsify Milgromian dynamics?" authored by Pavel Kroupa and collaborators, addresses an important query within astrophysics: whether the kinematics and dynamics of NGC1052-DF2, an ultra-diffuse galaxy, are compatible with the Milgromian dynamics (MOND) framework. This investigation is rooted in the claim by van Dokkum et al. that NGC1052-DF2 lacks significant dark matter, which could pose challenges for MOND's explanatory power concerning galactic dynamics.

Context and Motivation

Milgromian dynamics provides an alternative to the dark matter hypothesis by suggesting modifications to the laws of gravity on cosmological scales. This theory postulates that the internal dynamics of galaxies can be explained without requiring dark matter, using a nonlinear modification of Newtonian gravity. The case of NGC1052-DF2, which reportedly lacks dark matter, presents a potential litmus test for MOND's viability, primarily because MOND predicts behaviors indistinguishable from galaxies with dark matter under certain conditions.

Methodology and Calculations

The researchers examine the claims made about NGC1052-DF2 within the MOND framework, specifically accounting for the external field effect (EFE). This effect is significant in MOND, as it posits that a galaxy's dynamics are influenced by external gravitational fields from nearby massive objects, impacting its internal gravitational dynamics.

The authors utilize parameters such as distance measurements, the mass of the host galaxy NGC1052, and the separation between NGC1052 and NGC1052-DF2. They employ a MOND N-body integrator to gauge the system's velocity dispersions (σ) while considering the external field's influence dictated by MOND's generalized Poisson equation. The equation used reflects how MOND diverges from standard gravity by affecting the strength and behavior of a baryonic object's gravitational potential.

Results

The paper determines that previous assessments claiming that MOND is falsified by NGC1052-DF2 did not correctly account for the external field from the nearby galaxy, NGC1052. With the external field considered, MOND predicts a velocity dispersion for NGC1052-DF2 that aligns within the observational error margins presented by van Dokkum et al. The team calculates a velocity dispersion of approximately 14 km/s, which fits the observed data within the 2σ confidence interval when assuming the external field's influence. Their results indicate that at separations of 80–113 kpc from NGC1052, MOND dynamics are preserved, explaining NGC1052-DF2's observed characteristics without invoking dark matter.

Implications and Future Directions

The implications of this work are twofold. Practically, it underscores the significance of correctly accounting for the external field effect in non-isolated systems when applying MOND to real-world astrophysical observations. Theoretically, it bolsters MOND's credibility, indicating that what might appear as discrepancies can be explained by considering more complex interactions within the framework.

Future research could be directed towards more precise distance measurements to further verify the applicability of MOND in such systems. Additionally, refining assumptions about galaxy separations and testing a broader array of galaxies with different morphologies could offer deeper insights into the regime where MOND successfully models galactic dynamics.

In summary, this paper provides evidence that NGC1052-DF2 does not falsify MOND when external dynamics are appropriately modeled. This reinforces MOND's position as a plausible alternative theory to dark matter, providing fertile ground for inquiry into the fundamental laws governing galactic dynamics.