2025ApJ...980...28C

Quasar outflows often consist of two clouds with velocity separations matching the doublet spacings of common UV resonance transitions, a phenomenon known as line locking, which is commonly observed in quasar spectra. Multiple clouds can be locked together through multi-ion doublets, forming a “line-locking web.” In the second paper of the TOLL project, we present the discovery of one of the largest “line-locking webs” known to date from the Very Large Telescope UVES spectra of QSO J151352+085555. We identify 12 associated narrow absorption line systems through the C IV, N V, Si IV, O VI, and multiple Lyman lines (Lyα to Lyϵ) and find 10 out of the 12 absorbers are line-locked together by comparing velocity separations between different absorption systems. By conducting photoionization modeling with CLOUDY, we measure the total hydrogen column densities, metallicities, and ionization parameters of these absorbers, which suggests that the absorbers likely have subsolar metallicities. A preliminary statistical analysis demonstrates that the shadowed clouds tend to have similar ionization states compared to the shadowing ones. Identification of one of the largest line-locking webs implies that radiative acceleration plays an important role in sorting out cloud velocities in quasar outflows and highlights the need for more sophisticated theoretical models to explain its formation and evolution.