The different baryonic Tully-Fisher relations at low masses

We compare the Baryonic Tully-Fisher relation (BTFR) of simulations and observations of galaxies ranging from dwarfs to spirals, using various measures of rotational velocity V_rot. We explore the BTFR when measuring V_rot at the flat part of the rotation curve, V_flat, at the extent of H i gas, V_last, and using 20 per cent (W₂₀) and 50 per cent (W₅₀) of the width of H i line profiles. We also compare with the maximum circular velocity of the parent halo, [Formula: see text], within dark matter only simulations. The different BTFRs increasingly diverge as galaxy mass decreases. Using V_last one obtains a power law over four orders of magnitude in baryonic mass, with slope similar to the observed BTFR. Measuring V_flat gives similar results as V_last when galaxies with rising rotation curves are excluded. However, higher rotation velocities would be found for low-mass galaxies if the cold gas extended far enough for V_rot to reach a maximum. W₂₀ gives a similar slope as V_last but with slightly lower values of V_rot for low-mass galaxies, although this may depend on the extent of the gas in your galaxy sample. W₅₀ bends away from these other relations towards low velocities at low masses. By contrast, [Formula: see text] bends towards high velocities for low-mass galaxies, as cold gas does not extend out to the radius at which haloes reach [Formula: see text]. Our study highlights the need for careful comparisons between observations and models: one needs to be consistent about the particular method of measuring V_rot, and precise about the radius at which velocities are measured.