  Many Hubble type S galaxies have a flat rotation curve, V extending into the
outer disk. The surface brightness there is too low to create V from stars of a
reasonable mass-to-light ratio. To maintain a stable disk, a massive dark
matter halo is usually assumed to dominate in creating V. Gravitational arm
amplification is used here to estimate whether V is created by a disk mixture,
or by a 100 percent halo. The disk surface density is not estimated from the
disk surface brightness and an assumed mass to light ratio. The observed arm
pitch angle is presumed to be long term maximally amplified. This permits
calculation of the disk surface mass density with radius relative to a 100
percent disk which would fully create V. NGC 7217's tightly wound 4.8 degree
arm pitch angle indicates a disk component contributing 43 percent of the
observed V for the range over the spiral arms (important but not dominant). It
has too low a disk density to create V with the rest presumably arising from a
halo. V is proportional to the square root of surface density. By contrast a
loose arm galaxy disk can dominate in creating V. For example NGC 3198's arm
pitch angle of 30 degrees implies a disk surface density sufficient to account
for 88 percent of V which is a dominant fraction. Little dark matter halo is
needed for galaxies like this. Observationally, the dark outer disk may be
molecular hydrogen. NGC 3198's dominant disk also provides a counter example to
the need for any universal (MOND) force law modification over the outer disk
spiral arms.
