The low-mass end of the baryonic Tully-Fisher relation

The scaling of disc galaxy rotation velocity with baryonic mass (the 'baryonic Tully-Fisher' relation, BTF) has long confounded galaxy formation models. It is steeper than the M proportional to V-3 scaling relating halo virial masses and circular velocities and its zero-point implies that galaxies comprise a very small fraction of available baryons. Such low galaxy formation efficiencies may, in principle, be explained by winds driven by evolving stars, but the tightness of the BTF relation argues against the substantial scatter expected from such a vigorous feedback mechanism. We use the APOSTLE/EAGLE simulations to show that the BTF relation is well reproduced in Lambda cold dark matter (CDM) simulations that match the size and number of galaxies as a function of stellar mass. In such models, galaxy rotation velocities are proportional to halo virial velocity and the steep velocity-mass dependence results from the decline in galaxy formation efficiency with decreasing halo mass needed to reconcile the CDM halo mass function with the galaxy luminosity function. The scatter in the simulated BTF is smaller than observed, even when considering all simulated galaxies and not just rotationally supported ones. The simulations predict that the BTF should become increasingly steep at the faint end, although the velocity scatter at fixed mass should remain small. Observed galaxies with rotation speeds below similar to 40 km s(-1) seem to deviate from this prediction. We discuss observational biases and modelling uncertainties that may help to explain this disagreement in the context of Lambda CDM models of dwarf galaxy formation.