[Paper Review] Galaxy interactions: dark matter vs. Modified Newtonian dynamics (MOND)

Michal Bílek|arXiv (Cornell University)|Jan 6, 2016

Galaxies: Formation, Evolution, Phenomena119 references3 citations

TL;DR

This doctoral thesis develops novel methods to test Modified Newtonian Dynamics (MOND) in elliptical galaxies using stellar shells—structures formed by galactic mergers. By analyzing shell radii and kinematics, the author demonstrates MOND's consistency with observations in NGC 3923 down to ultra-low accelerations and predicts a previously undiscovered, massive shell, which remains undetected despite deep imaging, highlighting the need for improved data and future high-resolution spectroscopy to test MOND in radial-orbit systems.

ABSTRACT

(doctoral thesis of Michal Bilek, finished on June 19, 2015) MOND is an observational rule for predicting the acceleration of stars and galaxies from the distribution of the visible matter. It possibly stems from a new law of physics. I list the theoretical aspects of MOND, its achievements and problems. MOND has been tested mainly in disc galaxies so far. Its tests in elliptical galaxies are rare because the MOND effects are small for them in the parts observable by the conventional methods. In the thesis, I explain the methods and ideas I developed for testing MOND in the ellipticals using stellar shells. Moreover, the shells enable us to test MOND for stars in radial orbits for the first time. The shells are results of galactic interactions. I discuss the shell formation mechanisms and summarize the findings from shell observations and simulations. The thesis contains as yet unpublished results mainly in: 1) the introduction of Sect. 3 (the expected differences in the shell morphology in the Newtonian dynamics with dark matter and in MOND), and 2) Sect. 2.4.1 (formulas for modeling the evolution of shell radii).

Motivation & Objective

• To develop methods for testing MOND in elliptical galaxies, where MOND effects are weak and conventional tests are limited.
• To utilize stellar shells—formed by minor mergers—as probes of gravitational dynamics in low-acceleration regimes.
• To test whether MOND predicts consistent shell radii and kinematics in the elliptical galaxy NGC 3923.
• To predict a new, large shell in NGC 3923 based on MOND and search for it using ultra-deep imaging.
• To explore the potential of shell spectra to test MOND predictions for circular and expansion velocities in radial-orbit systems.

Proposed method

• Developed a method for identifying and modeling shell systems from radial surface brightness profiles to constrain gravitational potentials.
• Applied analytic models to predict shell radii in axially symmetric systems, including precise formulas not previously published.
• Used the phase-wrapping minor merger model as the dominant formation mechanism, validated by simulations and observations.
• Calculated MOND-predicted shell radii based on the total baryonic mass and the MOND interpolating function.
• Modeled velocity profiles from shell spectra to derive expansion and circular velocities under MOND assumptions.
• Conducted ultra-deep imaging at the Canada-France-Hawaii Telescope (CFHT) to search for a predicted shell at surface brightness limit of 29 mag arcsec⁻².

Experimental results

Research questions

• RQ1 Can MOND explain the radial distribution of stellar shells in the elliptical galaxy NGC 3923 at accelerations below 1.2×10⁻¹⁰ m s⁻²?
• RQ2 Does MOND predict the existence of a previously undetected, large shell in NGC 3923, and can it be observed with deep imaging?
• RQ3 How do shell kinematics—specifically expansion and circular velocities—differ between MOND and Newtonian dynamics with dark matter?
• RQ4 Can the observed high number of shells (42) in NGC 3923 help discriminate between MOND and dark matter models?
• RQ5 To what extent do dynamical friction effects in MOND differ from Newtonian dynamics, and can they be constrained by shell morphology?

Key findings

• The radial distribution of shells in NGC 3923 is consistent with MOND predictions down to accelerations of approximately 1.2×10⁻¹⁰ m s⁻².
• A new, large shell was predicted in NGC 3923 based on MOND, which remains undetected despite ultra-deep imaging reaching 29 mag arcsec⁻² surface brightness.
• The failure to detect the predicted shell suggests that earlier data used in the analysis were of poor quality, necessitating a redetermination of the shell system's properties.
• The high number of 42 detected shells in NGC 3923 indicates a complex merger history, with at least two progenitors, one still observable.
• Shell spectra in MOND are expected to yield expansion and circular velocities determined by the total baryonic mass for very large shells, offering a new testable prediction.
• The shell system's morphology and kinematics provide a unique probe of dynamical friction differences in MOND versus Newtonian dynamics with dark matter, though this requires self-consistent simulations for full validation.