Enigmatic velocity dispersions of ultradiffuse galaxies in light of modified gravity theories and the radial acceleration relation

Recent observations of anomalous line-of-sight velocity dispersions of two ultradiffuse galaxies (UDGs) provide a stringent test for modified gravity theories. While NGC 1052-DF2 exhibits an extremely low dispersion value ( σ∼7.8+5.6−2.2 km/s), the reported dispersion value for NGC 1052-DF44 is quite high (σ∼41.0 ± 8 km/s). For DF2, the dynamical mass is almost equal to the luminous mass suggesting the galaxy have little to no "dark matter" in Λ CDM whereas DF4 requires a dynamical mass-to-light ratio of ∼ 30 making it to be almost entirely consists of dark matter. It has been claimed that both these galaxies, marking the extreme points in terms of the estimated dynamical mass-to-light ratio among known galaxies, would be difficult to explain in modified gravity scenarios. Extending the analysis presented in [T. Islam and K. Dutta, Phys. Rev. D 100, 104049 (2019)], we explore the dynamics of DF2 and DF44 within the context of three popular alternative theories of gravity [modified Newtonian dynamics (MOND), Weyl conformal gravity and modified gravity (MOG)] and examine their viability against the dispersion data of DF2 and DF44. We further show that the galactic "radial acceleration relation" (RAR) is consistent with DF44 dispersion data but not with DF2.