Computer Study of Galactic Spiral Arm Shapes

Preliminary results are presented for a computer study which used a galactic explosion model in an attempt to interpret the shapes and Doppler velocities observed in the spiral arms of M31 and M33. The adopted 'gradual explosion' model is described along with a 'deflating explosion' model that was found to be inadequate. The drastic difference in the arm shapes of the two galaxies is interpreted by ascribing culminating velocities higher than the escape velocity to M33 and similar galaxies ('hypervelocity spirals') and culminating velocities lower than the escape velocity to M31 and other galaxies with the same characteristics ('subvelocity spirals'). Theoretical shapes and calculated Doppler velocities of six spiral arms in M31 are given, and it is suggested that minor explosions may have been more frequent in this galaxy than the larger explosions leading to arms approaching the escape velocity. The total mass and the mass distribution in M31 are estimated. The shapes and radial velocities of two spiral arms in M33 are computed, the total galactic mass is estimated, and the age of the spiral arms is inferred. Objections to the explosion models and their predictions are discussed.