Dissecting the origin of obscuration in AGN via comprehensive semi-analytic and semi-empirical orientation and evolutionary models

Abstract

The origin of obscuration in Active Galactic Nuclei (AGN)remains a topic of debate, particularly regarding the relative contributionsfrom galaxy-scale and torus-linked obscuration. While torus-linked obscuration iscentral to Orientation models, galaxy-scale obscuration is considered relevantin both Orientation and Evolutionary models. However, it remains unclearwhether AGN obscuration is primarily driven by line-of-sight effects, atemporary dust-enshrouded phase in galaxy evolution, or a combination of both. Thisthesis investigates the effects of Orientation and Evolutionary models on AGNobscuration, using cosmological semi-analytic models, semi-empiricalprescriptions, and hydrodynamical simulations. By modelling AGN evolution atthe object level, considering different light curves, gas density profiles, andAGN feedback, I explore how factors such as host galaxy properties, torus-likecomponents, gas fractions, mergers, starbursts, and clustering influence AGNobscuration. Both unobscured and obscured AGN, including Compton-thick sources,are analysed to assess the role of environment and galaxy characteristics.
Irrespective of the assumptions on specificAGNlight curve (accretion rate orbolometric luminosity evolution with time) or galaxy gas fractions, I findthat, on the strict assumption of an exponential profile for the gas component,galaxy-scale obscuration alone can hardly reproduce the fraction oflog(NH/cm−2) ≥ 24 sources at least at z ≲ 3. This requires an additionaltorus component with a thickness that decreases with luminosity to match thedata. The torus should be present in all evolutionary stages of a visible AGNto be effective, although galaxy-scale gas obscuration may be sufficient to reproducethe obscured fraction with 22 < log(NH/cm−2) < 24 (Compton-thin, CTN) ifI assume extremely compact gas disc components. The claimed drop of CTNfractions with increasing luminosity does not appear to be a consequence of AGNfeedback, but rather of gas reservoirs becoming more compact. Traditional(pure) Evolutionary models with obscuration pre-peak and optically/UV visiblepost-peak struggle to reproduce the AGN obscured fractions at z ≲3 inferred from X-ray surveys. However, incorporating a central source withComptonthick (CTK) column densities between 24 < log(NH/cm−2) < 26, suchas a torus-like component, or a fine-tuned luminosity dependency, successfullyreproduces observations. My results indicate that traditional Evolutionarymodels are less effective at reproducing the high fractions of obscured AGNobserved at z ≲ 3,while models with sharp post-peak declines orpersistent or multiple obscuration phases are more successful. Incorporating atorus component, particularly one that decreases in thickness with increasing AGNpower,effectively aligns with observed luminosity-dependent obscuration fractions,suggesting that orientation effects may still represent a key component even inEvolutionary models. In addition, I explore the clustering properties of AGNusing the two-point correlation function, showing that stellar mass and obscurationhave minimal influence on clustering strength. Environmental effects, however,introduce significant variability. These findings provide new insights into thecomplex relation between orientation, evolution, and environmental factors inAGN obscuration.  

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