MOND and the unique void galaxy KK246

Abstract

MOND predictions are compared with the mass discrepancy, Gamma (the dynamical-to-baryon mass ratio) deduced from the recently measured rotation curve, for the gas-rich, dwarf galaxy KK246, "the only galaxy observed in the local void". KK246 is special in at least two regards: a. It is, to my knowledge, the record holder for the largest mass discrepancy deduced from a rotation curve, Gamma= 15. b. It is very isolated, residing in a large, very empty void. I also discuss another extreme case: Andromeda IV, a dwarf considered here for the first time in light of MOND, with a very large mass discrepancy, Gamma =12, also conforming accurately to the MOND prediction. In both cases, MOND predicts Gamma, or the total dynamical mass at the last observed radius, from only the knowledge of the small mass of baryons. If MOND is accepted as the root of the mass discrepancy, these are just two more expected, albeit reassuring, conformities. However, in the framework of the dark-matter paradigm--where the mass discrepancy is strongly dependent on the buildup history of a galaxy--every new such conformity with a tight law is another difficult-to-understand surprise, and does carry a new import: What, in the LCDM paradigm, would prevent such galactic baryons from residing in a halo of half, or twice, the observed rotational velocities, instead of selecting exactly the velocities predicted by MOND? This conundrum is especially poignant for KK246, whose great isolation points to a relatively unique buildup history. This note underscores the individual importance of each galaxy as a new test, as opposed to the view of them all as a statistical ensemble.