  The scaling of disk galaxy rotation velocity with baryonic mass (the
"Baryonic Tully-Fisher" relation, BTF) has long confounded galaxy formation
models. It is steeper than the M ~ 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
vigorous feedback mechanism. We use the APOSTLE/EAGLE simulations to show that
the BTF relation is well reproduced in LCDM 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. Despite the strong feedback, 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 ~40 km/s seem to deviate from this
prediction. We discuss observational biases and modeling uncertainties that may
help to explain this disagreement in the context of LCDM models of dwarf galaxy
formation.
