The Challenge to MOND from Ultra-faint Dwarf Galaxies

Modified Newtonian Dynamics (MOND) at low acceleration has been astonishingly powerful explaining the flat rotation curve of galaxies and relation between baryonic content their observed circular velocity, known as Baryonic Tully-Fisher Relationship (BTFR). It is that MOND fails velocity dispersion ultra-faint dwarf (UFDs) with justification UFDs are more prone to tidal disruption in compared cold dark matter model. We show that: (i) ratio internal extremely low, (ii) there no correlation deviation from MOND's prediction a function susceptibility, (iii) recent constraints Gaia proper motion analysis on orbital parameters exacerbates challenge MOND. In particular, data indicates Ursa Major I experiencing infall into Milky Way's halo, its inconsistency 7-$\sigma$ level can not be attributed being an early satellite. Moreover, new DR2 shows Willman have least eccentric orbit all UFDs, 4-$\sigma$ highly previously suggested. Finally, given Tucana III only UFD features, Reticulum II Segue two other potentially they comparable galactocentric distances while showing features. Whether wide binaries inflated remains open question addressed future multi-epoch observations.