Apparent optical depth and column density ratio toward J0019−0137. Top: Apparent optical depth of the N Vλλ1338 (green) and Si IVλλ1393 (black) absorption lines toward J0019−0137. The velocity extent of the low-ionization species (Δv₉₀) is marked by the blue shaded region and the horizontal segment, with the tick marks corresponding to the 5, 50, and 95 percentiles of the cumulative optical depth (see Prochaska & Wolfe 1997). Bottom: Apparent column density ratio per unit velocity bin (red dots) and the ratio obtained for individual components used in the Voigt-profile modeling (stars).