The Baldwin Effect in Quasars from Observational Data and Cosmological Simulations

This thesis explores the origin and evolution of the C IV Baldwin effect: an observed inverse relation between emission-line strength and continuum luminosity using both observational quasar data and the SIMBA cosmological hydrodynamic simulation. The goal is to understand whether the Baldwin effect reflects a fundamental dependence on black hole accretion physics or merely a Malmquist-driven correlation. Using black-hole catalogs extracted from SIMBA snapshots spanning z ∼5.2 to 1.6, we derived black hole masses, accretion rates, bolometric luminosities, and Eddington ratios, con- structing flux-limited mock quasar samples at rest-frame 1450 ˚ A. These simulations were ana- lyzed in parallel with observational references, including the SDSS DR7 value-added catalog (Shen et al., 2011), the Eddington-ratio distribution of luminous quasars from (Kelly et al., 2010), and the C IV blueshift and disk–wind framework of (Richards et al., 2011). The anal- ysis employed orthogonal regressions and bootstrap resampling to quantify both the classical Baldwin relation (EW–L1450) and the intrinsic Baldwin relation (EW–λEdd). The results show that SIMBA reproduces the qualitative behavior of observed quasar sam- ples: while the classical Baldwin relation remains shallow, the intrinsic relation with Eddington ratio is significantly stronger, confirming that accretion state is the primary driver of C IV equiv- alent width. The Eddington-ratio distribution evolves markedly with redshift, its median value decreasing from near-Eddington at z ≳ 4 to∼0.05 by z ∼2 and becomes narrower at higher black hole masses, consistent with the expected transition from radiatively efficient growth to maintenance-mode accretion. Together, these trends reproduce the empirical downsizing pat- tern seen in observations and naturally connect to the broader Eigenvector 1 framework, where λEdd governs both spectral energy distribution hardness and the strength of disk winds. This work demonstrates that cosmological simulations can capture the demographic con- ditions that give rise to the Baldwin effect, linking black hole growth histories to observable quasar spectra. However, because line formation is not yet modeled from first principles, cur- rent simulations explain the statistics of the effect rather than the detailed emission line profiles themselves. Future efforts should integrate SIMBA outputs with photoionization and spectral- ii synthesis modeling, explore orientation-dependent effects, and extend to multi-line diagnostics such as the ones exploiting Hβ , Mg II, and Fe II. Such developments will enable a fully predic- tive framework that would connect accretion physics, feedback, and quasar spectral diversity across cosmic time.