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

TL;DR

This paper argues that gas-rich ultra-diffuse galaxies are consistent with the baryonic Tully-Fisher relation and Milgromian dynamics when accounting for uncertainties, challenging previous claims of anomalies.

Contribution

It demonstrates that previous claims of UDGs lacking dark matter are due to underestimated uncertainties and shows UDGs align with established dynamical relations when properly analyzed.

Findings

UDGs are offset from BTFR due to inclination biases.

MOND fits rotation curves with plausible distances and inclinations.

UDGs are likely a subset of LSB dwarf galaxies, not a distinct population.

Abstract

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.