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 (W 20) and 50 per cent (W 50) 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. W 20 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. W 50 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.