No redshift evolution in the rest-frame UV emission line properties of quasars from z=1.5 to z=4.0

TL;DR

This study finds no significant evolution in the UV emission line properties of quasars from redshift 1.5 to 4.0, suggesting stable spectral energy distributions and accretion characteristics over this period.

Contribution

It provides a comprehensive analysis of UV emission line properties across a wide redshift range, improving systemic redshift estimates and clarifying the dependence on quasar luminosity and black hole mass.

Findings

C IV blueshifts increase with luminosity and are explained by higher quasar luminosities at high redshift.

He II EW and C IV blueshift trends are consistent across redshifts, indicating no evolution in quasar SEDs.

High-redshift quasars (>6) are less massive with higher Eddington ratios, affecting UV line properties.

Abstract

We analyse the rest-frame UV spectra of 2,531 high-redshift (3.5<z<4.0) quasars from the Sloan Digital Sky Survey DR16Q catalogue. In combination with previous work, we study the redshift evolution of the rest-frame UV line properties across the entire redshift range, 1.5<z<4.0. We improve the systemic redshift estimates at z>3.5 using a cross-correlation algorithm that employs high signal-to-noise template spectra spanning the full range in UV emission line properties. We then quantify the evolution of C IV and He II emission line properties with redshift. The increase in C IV blueshifts with cosmological redshift can be fully explained by the higher luminosities of quasars observed at high redshifts. We recover broadly similar trends between the He II EW and C IV blueshift at both 1.5<z<2.65 and 3.5<z<4.0 suggesting that the blueshift depends systematically on the spectral energy density (SED) of the quasar and there is no evolution in the SED over the redshift range 1.5<z<4.0. C IV blueshifts are highest when L/LEdd > 0.2 and Mbh > 10^9 Mo for the entire 1.5<z<4.0 sample. We find that luminosity matching samples as a means to explore the evolution of their rest-frame UV emission line properties is only viable if the samples are also matched in the Mbh - L/LEdd plane. Quasars at z>6 are on average less massive and have higher Eddington-scaled accretion rates than their luminosity-matched counterparts at 1.5<z<4.0, which could explain the observed evolution in their UV line properties.