MEASURING SPIRAL ARM TORQUES - RESULTS FOR M100

Spiral arms, if they are massive, exert gravitational torques on stars that transport angular momentum radially within galactic disks. These torques depend not on the pattern speed or permanence of the arms but only on the nonaxisymmetric mass distribution. Hence the torques can be measured directly from photometry. We demonstrate this using gri CC D data for M100 (NGC 4321). Since we find consistency among the three bands, we believe that dust and young stars in the arms do not serious ly bias our results. if the present epoch is representative, the time scale for redistribution of angular momentum in M100 is 5-10 Gyr. (C) 1995 American Astronomical Society.