MOND in galaxy groups: a superior sample

Mordehai Milgrom

doi:10.1103/PhysRevD.99.044041

paper

MOND in galaxy groups: a superior sample

arXiv:1811.12233 · doi:10.1103/PhysRevD.99.044041

Abstract

Intermediate-richness galaxy groups are an important test ground for MOND. First, they constitute a distinct type of galactic systems, with their own evolution histories and underlying physical processes; secondly, they probe little-chartered regions of parameter space, as they have baryonic masses similar to massive galaxies, and similar velocity dispersions, but much larger sizes -- similar to cluster cores (or even to clusters), but much lower dispersions. Importantly in the context of MOND, they have the lowest internal accelerations reachable inside galactic systems. I analyze a sample of 56 medium-richness groups having a large number ( $\geq 15$ ) of members with measured velocities. The groups obey the deep-MOND, baryonic-mass-velocity-dispersion relation, $M_{M} G a_{0} = (81/4) σ^{4}$ , with individual, MOND $M_{M} / L_{K}$ ratios of order $1$ solar unit, with $(M_{M} / L_{K})_{m e d ian} = 0.7$ s.u. compared with the much larger Newtonian $M_{d} / L_{K}$ -- several tens s.u., and $(M_{d} / L_{K})_{m e d ian} = 37$ s.u. The same MOND relation describes dwarf spheroidals -- 2-3 orders smaller in size, and 7-8 orders lower in mass. The groups conformation to the MOND relation is equivalent to their lying on the deep-MOND branch of the `acceleration-discrepancy relation', $g \approx (g_{N} a_{0})^{1/2}$ , for $g$ as low as a few percents of $a_{0}$ ( $g_{N}$ is the baryonic, Newtonian, gravitational acceleration, and $g$ the actual one). This argues against a breakdown of MOND at extremely low accelerations. This conformation also argues against the hypothesis that the remaining MOND conundrum in cluster cores bespeaks a breakdown of MOND on large-distance scales; our groups are as large as cluster cores, but do not show obvious disagreement with MOND. I also discuss the possible presence of the idiosyncratic, MOND external-field effect.

13 pages, 14 figures, 1 table. Version that will be published in Phys. Rev. D

MOND in galaxy groups: a superior sample

Abstract

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