State of the system at t = 196 Myr for our set of simulations. Top and middle: face-on gas surface density for all the simulations listed in Table 1. Bottom: projected mass-weighted magnetic field $\left|{\langle B\rangle }_z\right|={\left({\langle B_x\rangle }_z^2+{\langle B_y\rangle }_z^2+{\langle B_z\rangle }_z^2\right)}^{1/2}$ for the three magnetised simulations in Table 1, where 〈X〉_ɀ = (∫ ρX dɀ)/(∫ ρ dɀ) denotes the vertically integrated mass-weighted density of a quantity X. Contours denote the streamlines of the projected field ${\langle B_x\rangle }_z{\widehat{e}}_x+{\langle B_y\rangle }_z{\widehat{e}}_y$. The dashed circles indicate the region with higher numerical resolution at R < 500 pc (Sect. 2.2). Rotation is clockwise. Movies are available online.