Exploring the evolution of active galactic nuclei with the next generation of astronomical surveys

Abstract

Active galactic nuclei (AGN) represent a phase of luminous accretion of matter onto supermassive black holes (SMBH) residing at the centers of galaxies. Current theoretical models and observational evidence, such as the M_BH-σ relation, point towards AGN playing a profound role in galaxy evolution. This realisation has taken AGN to the forefront of extragalactic astronomy in recent years, yet fundamental questions about their impact, duty cycles, and cosmic evolution remain unanswered. The Euclid Wide Survey (EWS) and Euclid Deep Survey (EDS) will provide extensive optical-NIR imaging, photometry, and spectroscopy across large areas of the extragalactic sky, generating a rich data set for AGN science exploitation. This thesis first presents detailed forecasts of the performance of Euclid's surveys in regard to the observation and analysis of AGN, before addressing some of the gaps in our understanding of AGN evolution through the scrutiny of optical spectra. All analysis of AGN spectroscopy in this thesis is performed using a consolidated spectral analysis code that the present author contributed to developing.

We simulated mock photometric catalogs of the AGN expected in the EWS and EDS. The simulations predict that approximately 40 million AGN will be detectable in the EWS and around 0.25 million in the EDS, with uncertainties of 6.7% and 12.5% respectively, driven by the adopted X-ray luminosity function (XLF). Using photometric colour cuts we expect to identify significant samples on the order of millions of AGN from the Euclid data, facilitating diverse studies of AGN populations and their cosmic evolution. We also forecast the performance of the Euclid NISP-S instrument for spectroscopic measurements of Type 1 (unobscured) and Type 2 (obscured) AGN. Our analyses suggest that prominent emission line features and black hole mass estimates can be reasonably extracted from AGN spectra obtained with the NISP-S instrument, despite its low spectral resolution.

We studied a sample of 45 Type 2 AGN with double-peaked narrow emission lines, confirming that all lines were due to illumination from an AGN ionizing continuum. Visual inspection of optical imaging shows the double-peaked AGN in our sample occur in merging systems at similar rates to luminosity and redshift matched single-peaked AGN, implying a kinematically disturbed narrow line region origin for the double-peaks rather than dual AGN, though a dual origin could not be completely ruled out.

Finally, we investigated the multiwavelength variability of the Seyfert 1 galaxy KUG 1141+371, which has experienced a decade-long flaring and dimming event. Analysis of optical spectroscopy at different epochs shows the luminosity changes correspond to variations in mass accretion rate. Inferences from emission line analysis suggest a fundamental change in the BLR structure after the peak of the outburst, with the apparent disappearance of emission from the innermost BLR clouds. Variations in Eigenvector 1 parameters suggest the Eddington ratio indeed drives quasar spectral diversity, however, we propose that the Eddington ratio governs these parameters through its associated impact on the innermost structure of the AGN central engine.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Sotiria Fotopoulou (Supervisor) & Malcolm N Bremer (Supervisor)