Lighting up a new Standard Candle: calibration of AGNs

The discovery of the Universe's expansion gave birth to modern cosmology, culminating in the flat Lambda Cold Dark Matter (ΛCDM) model as the prevailing description of cosmic evolution. This framework is supported by many independent probes. However, with improvements in observational precision, several significant discrepancies between the local and cosmologically inferred measurements have emerged, casting doubt on the completeness of the ΛCDM framework. Resolving these tensions requires new probes capable of tracing the expansion history across the vast, poorly explored redshift range between the local Universe and the epoch of recombination. Quasars, the most luminous class of Active Galactic Nuclei (AGNs), offer a compelling path to this goal, as they are detectable as early as 360 million years after the Big Bang. One of the strongest quasar properties to utilise as a cosmological probe is their non-linear empirical relation between X-ray and ultraviolet (UV) luminosities. However, the cosmological implementation of quasars still faces limitations, requiring further calibration and a more detailed exploration of their physics. This thesis investigates several open questions in the calibration of quasars as cosmological probes. Firstly, I present a new, highly homogeneous sample of quasars optimised for cosmological applications and demonstrate that the X-ray-to-UV relation is redshift-invariant within this sample. The residual non-intrinsic dispersion in the relation appears to be primarily driven by the X-ray photon index, with its more precise estimation leading to a notable improvement in homogeneity compared to prior cosmological quasar samples. The Hubble–Lemaître diagram derived from the new sample aligns remarkably well with that of Type Ia supernovae in the overlapping redshift range, thereby reinforcing the reliability of suitably selected quasars as standardisable candles. Furthermore, I explore the connection between X-ray emission and powerful outflows in the most luminous quasar phase. Most objects in the studied sample are found to have X-ray emission of expected strength, and the fraction of relatively X-ray 'weak' sources is fully consistent with that of typical quasar populations. I find a tentative correlation between X-ray weakness and the presence of outflows with the highest velocities, underscoring the need for a larger sample of UV-selected quasars to unravel the intricate interplay between winds and X-ray emission. Finally, I examine how the optical variability properties of AGNs influence the calibration of the X-ray-to-UV luminosity relation. I find a distinct bimodality in quasar variability properties that overshadows physical variability-based signals, making it challenging to assess the feasibility of applying optical variability parameters to further improve the dispersion in the X-ray-to-UV luminosity relation. A further key finding is an apparent decoupling between the outer observed-frame optical-emitting regions of the accretion disc and the X-ray corona seen even after removing the bimodality. At the same time, I confirm that the newly assembled homogeneous sample is a representative subset of the general quasar population in terms of observed-frame optical variability, thereby validating the cosmological selection criteria from this novel perspective. The results presented in this thesis contribute to the body of evidence on the complexity of quasar physical processes, which ultimately set the limits of their precision as distance indicators.