Extended Data Fig. 6: Comparison between observation and model calculations for selected metal absorption lines.

a, Recovered spectra of C iv, corrected for redshifted absorption, assuming C³⁺ ion column densities of \({N}_{{\rm{col}}}\left({{\rm{C}}}_{{\rm{ground}}}^{3+}\right)\) = 10¹⁵, 10¹⁶ and 10¹⁷ cm⁻² in the quasar’s rest frame. The error bars on the observed flux denote 1σ uncertainty. Compared with the best-fit SDSS composite spectrum (blue dashed line), the recovered flux is much too weak for the absorption when \({N}_{{\rm{col}}}\left({{\rm{C}}}_{{\rm{ground}}}^{3+}\right)\) = 10¹⁵ cm⁻², while it is too high (showing two extra deceptive peaks at around 1,550 and 1,570 Å) for the absorption when \({N}_{{\rm{col}}}\left({{\rm{C}}}_{{\rm{ground}}}^{3+}\right)\)  = 10¹⁷ cm⁻². The absorption model with \({N}_{{\rm{col}}}\left({{\rm{C}}}_{{\rm{ground}}}^{3+}\right)\) ≈ 10¹⁶ cm⁻² predicts unabsorbed flux that is reasonably consistent with the composite spectrum, and is thus adopted. b, Absorption-corrected UV Fe ii spectra between 2,000 and 2,750 Å for the post-C³⁺ inflow models with n_H = 10⁷  cm⁻³ (red), 10^7.5 cm⁻³ (yellow), 10⁹ cm⁻³ (green), and 10^9.5 cm⁻³ (violet) in the high ‘probability’ zone of Extended Data Fig. 5b. The error bars on observed flux are 1σ uncertainties. Compared with the best-fit composite (blue dashed line), the models with higher densities can be clearly ruled out.