# New Constraints on Quasar Evolution: Broad Line Velocity Shifts over $1.5\lesssim z\lesssim 7.5 $

## TL;DR

This study analyzes UV spectra of 394 quasars across redshifts 1.5 to 7.5, revealing that C IV line blueshifts remain stable at lower redshifts but increase significantly at z > 6, suggesting evolving quasar wind properties.

## Contribution

Introduces a model-independent method for measuring quasar emission line shifts and provides new insights into quasar wind evolution over cosmic time.

## Key findings

- C IV blueshift does not significantly evolve from z=1.5 to 6
- C IV blueshift increases by a factor of ~2.5 at z > 6
- Method yields unbiased velocity shifts insensitive to spectral resolution

## Abstract

We present the results of a model-independent investigation of the rest-frame UV spectra from a comprehensive sample of $394$ quasars in the redshift range $1.5\leq z \leq 7.5$. We fit the main Broad Emission Lines (BELs) in the rest-frame range $1280 \text{\AA} \leq \lambda \leq 3000 \text{\AA}$ (O I, C II, Si IV, C III, C IV and Mg II) with a lightly-supervised spline fitting technique. Redshifts are derived from the peaks of each fitted BEL and used to compute relative velocity shifts. We show that our method gives unbiased velocity shifts and is insensitive to spectral resolution and instrumental parameters. It is found that the average blueshift of the \cfour\, line with respect to several low-ionisation lines in luminosity-matched samples does not significantly evolve over $1.5\gtrsim z\gtrsim6$. However, the average blueshift increases significantly by a factor $\sim 2.5$ at $z\gtrsim 6$. We propose that this redshift evolution can be explained by \cfour\, winds launched perpendicularly to an accretion disk with increased torus opacity at high-redshift, coupled with a potential orientation-driven selection bias. Our results open new exciting avenues of investigation into young quasars in the reionisation epoch.