Spiral Galaxies' Disk Dominance and Testing MOND

Abstract

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.