Stacked reverberation lags at high redshift

TL;DR

This paper demonstrates a stacking technique for reverberation mapping at high redshift, revealing emission line lags and their relation to luminosity, thus extending black hole environment studies to more distant quasars.

Contribution

It introduces a novel stacking method for high-redshift reverberation mapping, enabling efficient measurement of emission line lags in distant quasars.

Findings

Detected clear peaks in stacked cross-correlation functions for CIV and MgII lines.

MgII lags are longer than CIV, consistent with low-redshift results.

CIV lag increases with luminosity, supporting the radius-luminosity relation.

Abstract

Over the past 20years reverberation mapping has proved one of the most successful techniques for studying the local (<1pc) environment of super-massive black holes that drive active galactic nuclei. Key successes of reverberation mapping have been direct black hole mass estimates, the radius-luminosity relation for the Hbeta line and the calibration of single-epoch mass estimators commonly employed up to z~7. However, observing constraints mean that few studies have been successful at z>0.1, or for the more-luminous quasars that make up the majority of current spectroscopic samples, or for the rest-frame ultra-violet emission lines available in optical spectra of z>0.5 objects. Previously we described a technique for stacking cross correlations to obtain reverberation mapping results at high z. Here we present the first results from a campaign designed for this purpose. We construct stacked cross-correlation functions for the CIV and MgII lines and find a clear peak in both. We find the peak in the MgII correlation is at longer lags than CIV consistent with previous results at low redshift. For the CIV sample we are able to bin by luminosity and find evidence for increasing lags for more-luminous objects. This CIV radius-luminosity relation is consistent with previous studies but with a fraction of the observational cost.