ABSTRACT of paper by Keith Serra

Less Dark Matter Than Expected is Needed to Explain the Rotational Rates of Spiral Galaxies

Mass estimates for spiral galaxies may be high by a factor of two due to the assumptions made about the mass distribution. In the plane of a spiral galaxy, the gravitational potential is different (usually greater) than the gravitational potential due to the same mass concentrated at the center of a galaxy. The gravitational potential of a disk shaped mass distribution depends on the density gradient as well as the mass and radius. This is unlike a spherical distribution where the potential depends only on the enclosed mass and radius. The gravitational potential of a disk with an exponentially decaying density may be as much as twice the potential of a spherical distribution for the same total mass. Near the center, a disk may have a lower potential than a sphere.

The dark matter is generally assumed to be in the halo. If it is in the disk instead, then far less dark matter is required to explain the observed rotational rates. There may be no need to assume that any exotic types of dark matter are involved. A rising rotational velocity curve exists for a disk shaped mass density that decays exponentially. The shape of the curve depends on the mass distribution. Non-Keplerian velocities are predicted on the inner edges of spiral arms and these effects match observed data.