A Heavy Baryonic Galactic Disc
  We investigate the possibility that the observed rotation of galaxies can be
accounted for by invoking a massive baryonic disc with no need for non-baryonic
dark matter or a massive halo. There are 5 primary reasons for suggesting this:
1. there are well known disc surface mass density distributions that naturally
produce the observed rotation curves of galaxies. 2. there are a number of
rotation curve `puzzles' that cannot be explained by a massive dark matter halo
i.e. the success of maximum disc fitting, HI gas scaling to the observed
rotation, the disc/halo conspiracy and the interpretation of the Tully-Fisher
relation. 3. recent 21cm observations show an almost constant HI surface
density and a distinct `cut-off' or edge to galactic discs. We explain this
constant surface density in terms of either an optical depth effect or the
onset of molecular gas formation and hence the possibility of considerably more
gas existing in galaxies. We suggest that the HI cut-off does indeed mark the
edge of the galactic disc. 4. there have been an increasing number of recent
observations that imply that X_CO may be ten times higher in the outer Galaxy.
This `dark' gas may provide adequate mass to account for galaxy rotation. 5. we
show that the additional baryonic mass required to account for the rotation of
galaxies is just that required to reconcile observed baryons with those
predicted by big bang nucleosynthesis. Mestel discs can be used to straight
forwardly explain the scaling laws of galaxies, particularly the observed
relation between rotation velocity and radius and the oft used Tully-Fisher
relation. We discuss observations of the baryonic content of galactic discs and
where sufficient `hidden' baryons might be found to account for the rotation.
