Gas-rich "ultra-diffuse" galaxies are consistent with the baryonic Tully-Fisher relation and with Milgromian dynamics

Some gas-rich "ultra-diffuse" galaxies (UDGs), which are extreme examples of low surface brightness (LSB) dwarf galaxies, have been reported to lack dark matter and to be offset from the baryonic Tully-Fisher relation (BTFR). If confirmed, these UDGs would represent a serious challenge for both LCDM galaxy-formation models and Milgromian dynamics. Here I demonstrate that these conclusions are very dubious due to underestimated uncertainties on inclinations and/or distances. First, I show that UDGs are offset from the BTFR in the same way as usual face-on LSB dwarfs due to systematic biases at low inclinations. Next, I analyze the two UDGs with the best available rotation-curve data. The first UDG (AGC 242019) is ideally inclined for kinematic studies; MOND can fit the observed rotation curve with a distance of 12.5 +/- 0.6 Mpc, which is consistent with Virgocentric flow models. The second UDG (AGC 114905) is close to face-on, so not ideal for kinematic studies; MOND can fit the observed rotation curve with a distance of 68 +/- 13 Mpc and inclination of 15 +/- 2 degrees, which are consistent with existing data. In particular, I show that the disk inclination is more uncertain than previously estimated due to significant asymmetries (lopsidedness) in the stellar distribution. In conclusion, there is no strong evidence that gas-rich UDGs and gas-rich LSB dwarfs are distinct galaxy populations with different dynamical properties; instead, UDGs seem to be a subset of LSB dwarf galaxies biased toward face-on systems.