BOSS Quasar Outflows Traced by C IV

TL;DR

This study analyzes a large quasar sample to identify key factors driving high-velocity outflows, emphasizing the roles of Eddington ratios and UV continuum softness, and suggests radiative line-driving as a primary mechanism.

Contribution

It introduces a uniform re-measurement method for quasar redshifts based on Mg II, enabling exploration of unprecedented outflow velocities and correlations with physical quasar properties.

Findings

Higher C IV blueshifts correlate with broader, weaker emission profiles.

Faster outflows are associated with higher Eddington ratios and softer UV spectra.

Evidence supports radiative line-driving as a key mechanism for extreme outflows.

Abstract

We investigate possible factors that drive fast quasar outflows using a sample of 39,249 quasars at median redshift $\langle z \rangle \approx$ 2.17. Unique to this study, the quasar redshifts are re-measured based on the Mg II emission line, allowing for exploration of unprecedented outflow velocities (>6000 km/s) while maintaining statistical significance and uniformity. We measure reliable C IV blueshifts for 1178 quasars with velocities >2500 km/s. From those, 255(13) quasars have blueshifts above 4000(6000) km/s, with the highest C IV velocity $\approx$ 7000 km/s. 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 UV continuum slope across the rest-frame UV to Near-IR. Analysis reveals two primary factors contributing to faster outflows: higher Eddington ratios, and softer far-UV continuum (h$\nu$ >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 multi-dimensional parameter space in future studies when analyzing large C IV blueshifts to determine the fundamental causes of outflows.