M ar 2 00 8 Rings and shells of “ dark matter ” as MOND artifacts

MOND predicts that a mass, M , contained within its transition radius rt ≡ (MG/a0) , 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. 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 Newtonianto-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. Subject headings: dark matter galaxy clusters: kinematics and dynamics