Rings and shells of "dark matter" as MOND artifacts

TL;DR

This paper explores how MOND predicts the formation of shell or ring-like features in the apparent distribution of dark matter, which may explain observed rings in galaxy clusters and features in galaxy rotation curves, providing potential evidence for MOND.

Contribution

It demonstrates that MOND can naturally produce observable ring features without underlying dark matter, offering a possible explanation for recent astronomical observations.

Findings

Observed ring in galaxy cluster Cl 0024+17 can be explained by MOND predictions.

Ring properties depend on the MOND interpolating function and source distribution.

High surface brightness galaxies may show features marking the MOND transition.

Abstract

MOND predicts that a mass, M, contained within its transition radius r_t=(MG/a0)^{1/2}, may exhibit a feature at about that radius in the form of a shell, or projected ring, in the deduced distribution of its phantom dark matter. This occurs despite the absence of any underlying feature in the true (``baryon'') source distribution itself. The phenomenon is similar to the appearance of an event horizon and other unusual physics ``in the middle of nothing'' near the transition radius of General Relativity $MG/c^2$. We consider the possibility that this pure MOND phenomenon is in the basis of the recent finding of such a ring in the galaxy cluster Cl 0024+17 by Jee et al. We find that the parameters of the observed ring can be naturally explained in this way; this feature may therefore turn out to be a direct evidence for MOND. We study this phenomenon in simple, axisymmetric configurations aligned with the line of sight: spherical masses, a dumbbell of spherical masses, and an elongated, thin structure. The properties of the apparent ring: its radius, surface density, and contrast, depend on the form of the MOND interpolating function and on the exact three dimensional distribution of the sources (the thin-lens approximation is quite invalid in MOND). We also comment on the possible appearance of orphan features, marking the Newtonian-to-MOND transition, in high surface brightness galaxies. In particular, we find that previously unexplained structure in the rotation curves of some galaxies may be evidence for such features.