2019ApJ...884L..25H

The observed line-of-sight velocity dispersion σ_los of the ultra-diffuse galaxy Dragonfly 44 (DF44) requires a Newtonian dynamical mass-to-light ratio of M_dyn/L_I=26_–6⁺⁷ solar units. This is well outside the acceptable limits of our stellar population synthesis (SPS) models, which we construct using the integrated galactic initial mass function (IGIMF) theory. Assuming DF44 is in isolation and using Jeans analysis, we calculate σ_los profiles of DF44 in Milgromian dynamics (MOND) and modified gravity (MOG) theories without invoking dark matter. Comparing with the observed kinematics, the best-fitting MOND model has M_dyn/L_I=3.6_–1.2^+1.6 and a constant orbital anisotropy of β=-0.5_–1.6^+0.4. In MOG, we first fix its two theoretical parameters α and µ based on previous fits to the observed rotation curve data of The HI Nearby Galaxy Survey (THINGS). The DF44 σ_los profile is best fit with M_dyn/L_I=7.4_–1.4^+1.5, larger than plausible SPS values. MOG produces a σ_los profile for DF44 with acceptable M_dyn/L_I and isotropic orbits if α and µ are allowed to vary. MOND with the canonical a₀ can explain DF44 at the 2.40σ confidence level (1.66%) if considering both its observed kinematics and typical star formation histories in an IGIMF context. However, MOG is ruled out at 5.49σ (P-value of 4.07 x 10^–8) if its free parameters are fixed at the highest values consistent with THINGS data.