Baryonic Tully-Fisher relation
= Baryonic Tully-Fisher relation
{title2=$M_b\propto v_f^4$}
An empirical scaling between a disk <galaxy>'s total baryonic <mass> and its characteristic flat rotation speed. The ideal isolated deep-<MOND> limit predicts $v_f^4=GM_ba_0$. Observational tests must specify the speed estimator, distances, gas masses and stellar <mass-to-light ratios>; the empirical relation alone does not determine a unique theory of gravity.