Disc radius (solid lines) as a function of time categorized by the velocity of the ISM in the simulation. The colour coding indicates the ISM density: green indicates 1.9 × 10^-19, brown 1.9 × 10^-18, and purple 1.9 × 10^-17 g/cm³. The dashed lines in corresponding colours give our theoretical estimates of the disc radius (see Sect. 2.2). The simulations with v_ISM = 30 km s^-1 were computationally expensive and have not reached 10 000 yr.