The Third Law of Galactic Rotation
  I review the connection between dynamics and the baryonic mass distribution
in rotationally supported galaxies. The enclosed dynamical mass-to-light ratio
increases with decreasing galaxy luminosity and surface brightness. The
correlation with surface brightness appears to be the more fundamental, with
the dependence on luminosity following simply from the weaker correlation
between luminosity and surface brightness. In addition to this global relation,
there is also a local relation between the amplitude of the mass discrepancy
and the acceleration predicted by the observed distribution of baryons. I
provide an empirical calibration of this mass discrepancy-acceleration
relation. The data are consistent with the operation of a singe effective force
law in disk galaxies, making this relation tantamount to a natural law. I
further provide formulae by which the radial dark matter distribution can be
estimated from surface photometry. The form of the dark matter halo depends
uniquely on the distribution of baryons in each galaxy, and in general is
neither a cusp nor a core. It remains difficult to see how galaxy formation
models can reproduce the observed behavior, which is uniquely predicted by
MOND.
