Disk thickness and spiral arm structure of M31

► We obtain M31's disk thickness based on solving Poisson's equation for a disturbance. ► This method is suitable for non-edge-on spiral galaxies but not for edge-on ones. ► We fit a two-armed spiral structure of M31 to logarithmic spirals. ► We suggest that the 10 kpc ring of M31 might come from the spiral structure.