The baryonic Tully-Fisher relation in the Simba simulation
  We investigate the Baryonic Tully-Fisher Relation (BTFR) in the
$(100\,h^{-1}{\rm Mpc})^3$ Simba hydrodynamical galaxy formation simulation
together with a higher-resolution $(25\,h^{-1}{\rm Mpc})^3$ Simba run, for over
$10,000$ disk-dominated, HI-rich galaxies. We generate simulated galaxy
rotation curves from the mass distribution, which we show yields similar
results to using the gas rotational velocities. From this we measure the galaxy
rotation velocity $V_{\rm circ}$ using four metrics: $V_{\rm max}, V_{\rm
flat}, V_{2R_e},$ and $V_{\rm polyex}$. We compare the predicted BTFR to the
SPARC observational sample and find broad agreement. In detail, however, Simba
is biased towards higher $V_{\rm circ}$ by up to 0.1 dex. We find evidence for
the flattening of the BTFR in $V_{\rm circ}>300$ km s$^{-1}$ galaxies, in
agreement with recent observational findings. Simba's rotation curves are more
peaked for lower mass galaxies, in contrast with observations, suggesting
overly bulge-dominated dwarf galaxies in our sample. We investigate for
residuals around the BTFR versus HI mass, stellar mass, gas fraction, and
specific star formation rate, which provide testable predictions for upcoming
BTFR surveys. Simba's BTFR shows sub-optimal resolution convergence, with the
higher-resolution run lowering $V$ in better agreement with data.
