Density distribution and the radial velocity field in the spiral arms of M31

Abstract

The density distribution and the radial velocity field in the Andromeda galaxy, M 31, have been studied on the basis of the 21-cm radio-line data from Jodrell Bank and Green Bank. The true density has been obtained from the observed one by solving a two-dimensional integral equation. As the resolving power of the radio telescopes is too low to locate all spiral arms separately, optical data on the distribution of ionized hydrogen clouds have been also used. The mean radial velocities have been derived by solving a two-dimensional non-linear integral equation with the help of hydrogen densities, and a model radial velocity field.The inner concentrations of hydrogen form two patchy ringlike structures with mean radii 30′ and 50′, the outer concentrations can be represented as fragments of two leading spiral arms.The rotational velocity, derived from the radial velocity field, in the central region differs considerably from the velocity curves obtained by earlier authors. The difference can be explained by the fact that in this region the correction for the antenna beam width is much greater than adopted by previous investigators.