Column density of the disc galaxies projected onto the z-plane. The columns represent the initial plasma β (ratio of thermal to magnetic pressure) of the runs, and the rows from top to bottom show the evolution of the galaxy. The panels roughly depict the times when the spiral arms first form, when the arms fragment into over-densities, and when they separate out into clouds. The disc becomes more diffuse with decreasing plasma β in the first row. The second and the third row highlight the magnetic field effects on the stability of the spiral arms. The β = 10 run is stable to fragmentation, while the hydro and β = 50 cases fragment and yet still have noticeable morphological differences.