CLOUDY PI models of the high ions in the two PDLA components showing O vi: C1 (at  −57 km s^-1 relative to z_abs), and C2 (at  − 12 km s^-1). The colored lines show the predicted high-ion column densities versus ionization parameter log U, with the observations (large data points) plotted at the best-fit value of log U. The lower x-axis show the corresponding gas density n_H. In the C1 model (upper plot), a solution is found at log U =  −1.8 that explains the O vi, C iv, and upper limits on Si iv and N v. The dotted red line shows the O vi prediction from a UVB-only CLOUDY model (i.e., without the ionizing radiation from the QSO). In the C2 model (lower plot), no single-phase solution is found; a fit to the C iv and Si iv at log U =  −2.6 under-predicts the O vi column by 0.6 dex (the “O vi shortfall”). These models are described in Sect. 8.1.