Observed Lyα profile of COLA1 (green) is well modelled by an expanding shell of neutral gas (black). The shell-model parameters indicate a low neutral hydrogen column density N_HI = 10^{17.0 ± 0.3} cm⁻² and low expansion velocity km s⁻¹. We note that we have assumed here that the systemic redshift lies at line-centre (between the red and blue peak, where the flux is consistent with zero), or z_Lyα = 6.591.