Ultra-diffuse cluster galaxies as key to the MOND cluster conundrum
  MOND reduces greatly the mass discrepancy in clusters of galaxies, but does
leave a consistent global discrepancy of about a factor of two. It has been
proposed, within the minimalist and purist MOND, that clusters harbor some
indigenous, yet-undetected, cluster baryonic (dark) matter (CBDM). Its total
amount has to be comparable with that of the observed hot gas. Following an
initial discovery by van Dokkum & al. (2015a), Koda & al. (2015) have recently
identified more than a thousand ultra-diffuse galaxy-like objects (UDGs) in the
Coma cluster. Robustness of the UDGs to tidal disruption seems to require,
within Newtonian dynamics, that they are much more massive than their observed
stellar component. Here, I propound that a considerable fraction of the CBDM is
internal to UDGs, which endows them with robustness. The rest of the CBDM
objects formed in now-disrupted kin of the UDGs, and is dispersed in the
intracluster medium. While the discovery of cluster UDGs is not in itself a
resolution of the MOND cluster conundrum, it lends greater qualitative
plausibility to CBDM as its resolution, for reasons I discuss. Alternatively,
if the UDGs are only now falling into Coma, their large size and very low
surface brightness could result from the adiabatic inflation due to the MOND
external-field effect, as described in Brada & Milgrom (2000). I also consider
briefly solutions to the conundrum that invoke more elaborate extensions of
purist MOND, e.g., that in clusters, the MOND constant takes up
larger-than-canonical values of the MOND constant.
