  The relaxed motion of stars and gas in galactic discs is well approximated by
a rotational velocity that is a function of radial position only, implying that
individual components have lost any information about their prior states.
Thermodynamically, such an equilibrium state is a microcanonical ensemble with
maximum entropy, characterised by a lognormal probability distribution.
Assuming this for the surface density distribution yields rotation curves that
closely match observational data across a wide range of disc masses and galaxy
types, and provides a useful tool for modelling the theoretical density
distribution in the disc. A universal disc spin parameter emerges from the
model, giving a tight virial mass estimator with strong correlation between
angular momentum and disc mass, suggesting a mechanism by which the proto-disc
developed by dumping excess mass to the core, or excess angular momentum to a
satellite galaxy. The baryonic-to-dynamic mass ratio for the model approaches
unity for high mass galaxies, but is generally $<1$ for low mass discs, and
this discrepancy appears to follow a similar relationship to that shown in
recent work on the radial acceleration relation (RAR). Although this may
support Modified Newtonian Dynamics (MOND) in preference to a dark matter (DM)
halo, it does not exclude undetected baryonic mass or a gravitational DM
component in the disc.
