2024MNRAS.528.6425G

We investigate possible factors that drive fast quasar outflows using a sample of 39 249 quasars at median redshift 〈z〉 ≈ 2.17. Unique to this study, the quasar redshifts are re-measured based on the Mg II emission line, and explore unprecedented outflow velocities (>6000 km s^–1), while maintaining statistical significance. We measure reliable C IV blueshifts for 1178 quasars with velocities >2500 km s^–1. From those, 255(13) quasars have blueshifts above 4000(6000) km s^–1, with the highest C IV velocity ≈7000 km s^–1. Several correlations are observed, where higher C IV blueshifts in general are in quasars with broader, weaker C IV emission profiles, weak He II emission, larger Eddington ratios, and bluer ultraviolet (UV) continuum slope across the rest-frame UV to near-infrared. Analysis reveals two primary factors contributing to faster outflows: higher Eddington ratios, and softer far-UV continuum (hν >24.6 eV). We find supporting evidence that radiative line driving may generate extreme outflow velocities, influenced by multiple factors as suggested by the aforementioned correlations. This evidence highlights the importance of considering a multidimensional parameter space in future studies when analyzing large C IV blueshifts to determine the fundamental causes of outflows.