The wedding of modified dynamics and non-exotic dark matter in galaxy clusters

TL;DR

This paper reviews the status of Modified Newtonian Dynamics (MOND) in galaxy clusters, highlighting the need for some form of dark matter, such as neutrinos, to explain observed mass discrepancies, especially in central regions.

Contribution

It proposes that neutrinos with masses near current detection limits can account for missing mass in MOND, and discusses how lensing data supports this scenario.

Findings

Neutrinos can explain mass discrepancies in some clusters.

Lensing mass reconstructions do not challenge the neutrino hypothesis.

Mass discrepancies in groups suggest dark baryons may be needed.

Abstract

We summarize the status of Modified Newtonian Dynamics (MOND) in galaxy clusters. The observed acceleration is typically larger than the acceleration threshold of MOND in the central regions, implying that some dark matter is necessary to explain the mass discrepancy there. A plausible resolution of this issue is that the unseen mass in MOND is in the form of ordinary neutrinos with masses just below the experimentally detectable limit. In particular, we show that the lensing mass reconstructions of the clusters 1E0657-56 (the bullet cluster) and Cl0024+17 (the ring) do not pose a new challenge to this scenario. However, the mass discrepancy for cool X-ray emitting groups, in which neutrinos cannot cluster, pose a more serious problem, meaning that dark baryons could present a more satisfactory solution to the problem of unseen mass in MOND clusters.