The first high-redshift changing-look quasars

TL;DR

This paper presents the discovery of the first high-redshift changing-look quasars with dramatic C IV emission line variability, revealing rapid high-ionisation line changes at z>2 and providing new insights into quasar state transitions.

Contribution

It reports the first detection of high-redshift changing-look quasars with high-ionisation line variability, expanding understanding of quasar evolution at early cosmic times.

Findings

First high-redshift CLQs with C IV line changes

Rapid emission line variability on 240-1640 day timescales

CLQs are near the edge of the population in the EW plane and show an intrinsic Baldwin effect.

Abstract

We report on three redshift $z>2$ quasars with dramatic changes in their C IV emission lines, the first sample of changing-look quasars (CLQs) at high redshift. This is also the first time the changing-look behaviour has been seen in a high-ionisation emission line. SDSS J1205+3422, J1638+2827, and J2228+2201 show interesting behaviour in their observed optical light curves, and subsequent spectroscopy shows significant changes in the C IV broad emission line, with both line collapse and emergence being displayed on rest-frame timescales of $\sim$240-1640 days. These are rapid changes, especially when considering virial black hole mass estimates of $M_{\rm BH} > 10^{9} M_{\odot}$ for all three quasars. Continuum and emission line measurements from the three quasars show changes in the continuum-equivalent width plane with the CLQs seen to be on the edge of the full population distribution, and showing indications of an intrinsic Baldwin effect. We put these observations in context with recent state-change models, and note that even in their observed low-state, the C IV CLQs are generally above $\sim$5\% in Eddington luminosity.