IAUS379: Dynamical Masses of Local Group Galaxies

It is routinely assumed that galaxy rotation curves are equal to their circular velocity curves (modulo some corrections) such that they are good dynamical mass tracers. I will present the results of an unconventional, visualisation-driven analysis of 33 low-mass field galaxies from the APOSTLE suite of galaxy formation simulations exploring the limits of the validity of this assumption. Only 4/33 galaxies have HI rotation curves nearly equal to their circular velocity curves; the rest are undergoing a wide variety of dynamical perturbations of both secular and environmental origin. While some types of perturbations, such as ongoing mergers, have obvious observable signatures, others, such as wind from motion through the intergalactic medium, are much more subtle. Discrepancies between the rotation curves and circular velocity curves of low-mass galaxies have direct consequences for key results in near-field cosmology. They likely lead to an overestimation of the low-velocity end of the baryonic Tully-Fisher relation that is difficult to avoid (even by attempting to select 'equilibrium' galaxies), and could plausibly be the source of a significant portion of the observed diversity in low-mass galaxy rotation curve shapes.