Chemical abundances along the Quasar Main Sequence

This thesis aims to show a clear pattern of metal content in the central regions of Active Galactic Nuclei (AGNs) along the Quasar Main Sequence (MS). The Quasar MS, the well-established anti-correlation between the Full Width at Half Maximum (FWHM) of the H$\beta$ emission line and the R$_{\rm FeII}$ quantity (the ratio between the intensity of the FeII line and the intensity of the H$\beta$ emission), holds significant importance akin to the iconic Hertzsprung-Russell diagram in stellar studies. The identification of a metallicity trend holds pivotal implications for our comprehension of the inner workings of AGNs and ultimately of the galaxy formation process. By meticulously analyzing the optical and UV spectra of 14 low-$z$ AGNs, selected as representative specimens from various types along the Quasar MS, we derived diagnostic ratios from optical and UV emission feature in Quasar spectra obtained from ground- and space-based (HST FOS and COS) observations. These ratios were then compared with values obtained from {\tt CLOUDY} photoionization simulations. Through this analysis, we aim to illustrate a method able to constrain the physical conditions within the Broad Line Region (BLR) of each active galaxy and to provide a robust estimation of the metallicity. The metallicities derived from diagnostic ratios are solar or sub-solar for the Population B of Quasars, radiating at modest Eddtington ratio. In contrast, higher Eddington-ratio Population A sources display a noteworthy upward trajectory in metallicity. Within this class, metallicity values can surpass solar levels, even by a factor of 5 or more, in Super-Eddington candidates. The initial and robust measurement of the metallicity trend along the Quasar MS substantially contributes to defining the characteristics of the nuclear environment, with implication on the star formation activity within it. It is a starting point to delineate the relevance of feedback effects for quasars in different accretion modes.