A neutral-hydrogen column-density fit to the damped Lyα line at the host-galaxy redshift, shown with the red solid curves, provides log N(H i) = 18.7  ±  0.2 at epoch II (top panel) and log N(H i) = 18.7  ±  0.1 at epoch IV (bottom panel). The redshift was fixed to the mean redshift of the low-ionization metal components “b” and “c” (z = 2.427315, taken here as the zero-point of the velocity scale), and the velocity broadening was fixed to b = 20 km s^-1, corresponding to the difference in velocities of these two components or alternatively the quadratic sum of their b-values. The 1σ errors are shown with dashed lines. To show that our measurement is robust, we also performed a fit – shown by the green curve – in which we allowed both the redshift and velocity broadening to vary as free parameters, finding z = 2.42670, b = 46 km s^-1, and log N(H i) = 18.8 for both epochs. The small vertical lines mark the central redshifts of the two independent fits.