Rotation curves and scaling relations of extremely massive spiral
  galaxies
  We study the kinematics and scaling relations of a sample of 43 giant spiral
galaxies that have stellar masses exceeding $10^{11}$ $M_\odot$ and optical
discs up to 80 kpc in radius. We use a hybrid 3D-1D approach to fit 3D
kinematic models to long-slit observations of the H$\alpha$-[NII] emission
lines and we obtain robust rotation curves of these massive systems. We find
that all galaxies in our sample seem to reach a flat part of the rotation curve
within the outermost optical radius. We use the derived kinematics to study the
high-mass end of the two most important scaling relations for spiral galaxies:
the stellar/baryonic mass Tully-Fisher relation and the Fall (mass-angular
momentum) relation. All galaxies in our sample, with the possible exception of
the two fastest rotators, lie comfortably on both these scaling relations
determined at lower masses, without any evident break or bend at the high-mass
regime. When we combine our high-mass sample with lower-mass data from the
Spitzer Photometry & Accurate Rotation Curves catalog, we find a slope of
$\alpha=4.25\pm0.19$ for the stellar Tully-Fisher relation and a slope of
$\gamma=0.64\pm0.11$ for the Fall relation. Our results indicate that most, if
not all, of these rare, giant spiral galaxies are scaled up versions of less
massive discs and that spiral galaxies are a self-similar population of objects
up to the very high-mass end.
