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 HI gas, V-last, and using 20 per cent (W-20) and 50 per cent (W-50) of the width of HI line profiles. We also compare with the maximum circular velocity of the parent halo, V-max(DM), 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, V-max(DM) bends towards high velocities for low-mass galaxies, as cold gas does not extend out to the radius at which haloes reach V-max(DM). 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.