Dwarf Galaxies with Central Cores in Modified Newtonian Dynamics Gravity

Some dwarf galaxies are within the Mondian regime at all radii, i.e., the gravitational acceleration provided by the observed baryons is always below the threshold of g _† ≃ 1.2 × 10^-10 m s^-2. These dwarf galaxies often show cores, in the sense that, assuming Newton's gravity to explain their rotation curves, the total density profile ρ(r) presents a central plateau or core ( $d\mathrm{log}\rho /d\mathrm{log}r\to 0$ when r → 0). Here we show that under modified Newtonian dynamics (MOND) gravity, the existence of this core implies a baryon content whose density ρ _bar must decrease toward the center of the gravitational potential (ρ _bar → 0 when r → 0). Such a drop of baryons toward the central region is neither observed nor appears in numerical simulations of galaxy formation following MOND gravity. We analyze the problem posed for MOND as well as possible work-arounds.