Abstract
The tight radial acceleration relation (RAR) obeyed by rotationally supported disk galaxies is one of the most successful a priori predictions of the modified Newtonian dynamics (MOND) paradigm on galaxy scales. Another important consequence of MOND as a classical modification of gravity is that the strong equivalence principle (SEP) – which requires the dynamics of a small, free-falling, self-gravitating system not to depend on the external gravitational field in which it is embedded – should be broken. Multiple tentative detections of this so-called external field effect (EFE) of MOND have been made in the past, but the systems that should be most sensitive to it are galaxies with low internal gravitational accelerations residing in galaxy clusters within a strong external field. Here, we show that ultra-diffuse galaxies (UDGs) in the Coma cluster do lie on the RAR, and that their velocity dispersion profiles are in full agreement with isolated MOND predictions, especially when including some degree of radial anisotropy. However, including a breaking of the SEP via the EFE seriously deteriorates this agreement. We discuss various possibilities to explain this within the context of MOND, including a combination of tidal heating and higher baryonic masses. We also speculate that our results could mean that the EFE is screened in cluster UDGs. The fact that this would happen precisely within galaxy clusters, where classical MOND fails, could be especially relevant to the nature of the residual MOND missing mass in clusters of galaxies.
Highlights
Alternatives to general relativity (GR) have been the subject of active research over the past decades (e.g. Clifton et al 2012, and references therein), the reason being that GR alone cannot explain the observed dynamics on scales ranging from galaxies to the largest scales in the observable Universe without additional degrees of freedom
We show that ultra-diffuse galaxies (UDGs) in the Coma cluster do lie on the radial acceleration relation (RAR), and that their velocity dispersion profiles are in full agreement with isolated modified Newtonian dynamics (MOND) predictions, especially when including some degree of radial anisotropy
Since the radial acceleration relation (RAR; McGaugh et al 2016; Milgrom 2016; Lelli et al 2017; Li et al 2018) between the radial dynamical acceleration at the half-light radius, inferred from the velocity dispersion, and the radial gravitational acceleration predicted by the observed baryonic distribution within this radius, is a core prediction of MOND in isolation, we first check whether the Coma cluster UDGs do fall on this relation or not
Summary
Alternatives to general relativity (GR) have been the subject of active research over the past decades (e.g. Clifton et al 2012, and references therein), the reason being that GR alone cannot explain the observed dynamics on scales ranging from galaxies to the largest scales in the observable Universe without additional degrees of freedom. A great deal of research into modified gravity concentrates on the dark energy question, which is related to the cosmological constant problem; it has been suggested since the early 1980s (Milgrom 1983a,b,c, 1984; Bekenstein & Milgrom 1984) that the phenomena attributed to DM might be, at least partly, related to new gravitational degrees of freedom rather than to new particles in the matter sector as generally assumed This approach, known as modified Newtonian dynamics (MOND; see e.g. Sanders & McGaugh 2002; Famaey & McGaugh 2012; Milgrom 2014, for revie√ws) postulates that the gravitational acceleration g approaches gNa0 when the Newtonian gravitational acceleration gN falls below a characteristic acceleration scale a0 ≈ 10−10 m s−2 but remains Newtonian above this threshold.
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