Towards Quasar Reverberation Mapping as a Cosmological Probe

Abstract

Quasars are the most luminous objects in the Universe and serve as powerful probes of black hole physics and cosmology. This thesis develops novel pipelines to measure quasar properties using large-scale survey data in order to enable new insight into supermassive black hole evolution and the expansion history of the Universe. The size of the broad line region (BLR) within a quasar is measured through reverberation mapping (RM), which estimates this physical scale from a time lag between continuum and broad emission line variability. Traditional RM requires intensive monitoring, severely limiting lag measurements of high redshift and high luminosity quasars.
Astronomy has entered an exciting era of large-scale surveys, bringing new possibilities to RM studies. The Dark Energy Spectroscopic Instrument (DESI) observed an un-precedented 2.5 million quasars in its first three years. Large-scale spectroscopic surveys typically lack the observational cadence required by traditional RM methods. To address this, we develop a stacked RM framework that combines variability information from large quasar ensembles to recover average BLR sizes. The main aim of the work undertaken in this thesis is to assess the ability to employ stacked RM techniques on data obtained by large scale spectroscopic surveys.
We validate this technique using mock light curve pairs that mimic DESI-like sparse spectroscopy combined with well-sampled photometry. The method successfully recovers time lags with high precision, even when a significant fraction of objects contribute only two spectroscopic epochs. To facilitate stacked RM applications in the high redshift regime, we construct the largest catalogue to date of C iv and Mg ii emission line properties for DESI quasars, including estimates of black hole masses and Eddington ratios. This dataset enables precise constraints on scaling relations such as the Baldwin effect, offering new insight into their physical origins. Important to this thesis, the catalogue informs the selection of quasars suitable for C iv RM.
Applying stacked RM to DESI Data Release 2 yields null lag detections due to strong seasonal aliasing, a limitation expected to be mitigated with extended observational base-lines and improved lag-recovery algorithms in the near future. Finally, we propose us-ing stacked RM measurements to build a quasar-based cosmological distance ladder from redshift 0.01 to 7, establishing quasars as a promising independent intermediate-redshift cosmological probe.

Date of Award	16 Jun 2026
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Becky Canning (Supervisor), Eva Mueller (Supervisor), David Bacon (Supervisor) & Daniel Whalen (Supervisor)