Photoionisation model constraints on the UV absorbers in I Zw 1. The measured column densities and the associated uncertainties of each ion are represented by the coloured bands, with the mean value depicted with the solid line. 3σ upper limits for undetected lines are shown as dashed lines. The dash-dotted lines, representing the measured column densities of H I and N V (in the upper left panel), are regarded as lower limits. For these limits, the permitted portion of the parameter space is indicated with an arrow. In each panel, the black dot depicts the minimal χ² value, surrounded by a 1σ contour. In the upper right panel (absorber B), the location of the X-ray warm absorber seen at the same radial velocity is represented by a diamond marker, with its size covering the associated 1σ uncertainties.