Context.We present an analysis of UV spectra of quasars at intermediate redshifts believed to belong to the extreme Population A (xA), aimed to estimate the chemical abundances of the broad line emitting gas. We follow the approach described in a previous work, extending the sample to 42 sources.Aim.Our aim is to test the robustness of the analysis carried out previously, as well as to confirm the two most intriguing results of this investigation: evidence of very high solar metallicities and a deviation of the relative abundance of elements with respect to solar values.Methods.The basis of our analysis are multicomponent fits in three regions of the spectra centered at 1900, 1550, and 1400 Å in order to deblend the broad components of AlIIIλ1860, C III]λ1909, CIVλ1549, He IIλ1640, and SiIVλ1397+OIV]λ1402 and their blue excess.Results.By comparing the observed flux ratios of these components with the same ratios predicted by the photoionization codeCLOUDY, we found that the virialized gas (broad components) presents a metallicity (Z) higher than 10Z_⊙. For nonvirialized clouds, we derived a lower limit to the metallicity around ∼5Z_⊙under the assumption of chemical composition proportional to the solar one, confirming the previous results. We especially relied on the ratios between metal lines and He IIλ1640. This allowed us to confirm systematic differences in the solar-scaled metallicity derived from the lines of Aluminum and Silicon, and of Carbon, with the first being a factor ≈2 higher.Conclusions.For luminous quasars accreting at high rates, highZvalues are likely, butZ-scaled values are affected by the possible pollution due to highly-enriched gas associated with the circumnuclear star formation. The high-Zvalues suggest a complex process involving nuclear and circumnuclear star formation and an interaction between nuclear compact objects and an accretion disk, possibly with the formation of accretion-modified stars.

The 4D (four-dimensional) eigenvector 1 (E1) sequence has proven to be a highly effective tool for organizing observational and physical properties of type-1 active galactic nuclei (AGNs). In this paper, we present multiple measurements of metallicity for the broad line region gas, from new and previously-published data. We demonstrate a consistent trend along the optical plane of the E1 (also known as the quasar main sequence), defined by the line width of Balmer hydrogen H$\beta$ profile and by a parameter measuring the prominence of singly-ionized iron emission. The trend involves an increase from sub-solar metallicity in correspondence with extreme Population B (weak Feii emission, large H$\beta$ FWHM (full width at half maximum)) to metallicity several tens the solar value in correspondence with extreme Population A (strongest Feii optical emission, narrower H$\beta$ profiles). The data establish the metallicity as a correlate of the 4DE1/main sequence. If the considerably high metallicity ($Z\gtrsim 10Z_{\odot }$, solar metallicity) gas is expelled from the sphere of influence of the central black hole, as indicated by the widespread evidence of nuclear outflows and disk wind in the case of sources radiating at a high Eddington ratio, then it is possible that the outflows from quasars played a role in chemically enriching the host galaxy.

This review provides an observational perspective on the fundamental properties of super-Eddington accretion onto supermassive black holes in quasars. It begins by outlining the selection criteria, particularly focusing on optical and UV broad-line intensity ratios, used to identify a population of unobscured super-Eddington candidates. Several defining features place these candidates at the extreme end of the Population A in main sequence of quasars: among them are the highest observed singly-ionized iron emission, extreme outflow velocities in UV resonance lines, and unusually high metal abundances. These key properties reflect the coexistence of a virialized sub-system within the broad-line region alongside powerful outflows, with the observed gas enrichment likely driven by nuclear or circumnuclear star formation. The most compelling evidence for the occurrence of super-Eddington accretion onto supermassive black holes comes from recent observations of massive black holes at early cosmic epochs. These black holes require rapid growth rates that are only achievable through radiatively inefficient super-Eddington accretion. Furthermore, extreme Eddington ratios, close to or slightly exceeding unity, are consistent with the saturation of radiative output per unit mass predicted by accretion disk theory for super-Eddington accretion rates. The extreme properties of super-Eddington candidates suggest that these quasars could make them stable and well-defined cosmological distance indicators, leveraging the correlation between broad-line width and luminosity expected in virialized systems. Finally, several analogies with accretion processes around stellar-mass black holes, particularly in the high/soft state, are explored to provide additional insight into the mechanisms driving super-Eddington accretion.

Knowledge of the broad-band active galactic nuclei (AGNs) spectral energy distribution (SED) that ionizes the gas-rich broad emission line region is key to understanding the various radiative processes at play and their importance that eventually leads to the emission line formation. We modelled a SED for highly accreting quasars, also known as extreme population A sources, based mainly on observational data available in astronomical data bases, and on accretion disc models for the unobservable far-ultraviolet (UV) domain. Our selection criterion is the $R_\mathrm{{\rm{ Fe\,\small II}}}$ parameter – the ratio of the optical Fe ii emission between 4434 Å and 4684 Å to the h$\beta$ $\lambda$4861 Å intensity, $R_\mathrm{{\rm{ Fe\,\small II}}}$$\ge$ 1. This criterion is satisfied by highly accreting, possibly super-Eddington, black holes. We analysed 155 sources up to a redshift of approximately 1, previously reported in the literature, to construct a median radio-quiet SED spanning from radio to X-ray wavelengths. We find that the SED of quasars exhibits distinct features compared to lower accreting AGN, including a pronounced big blue bump and strong optical/UV emission along with a steep X-ray continuum. We classify the sources into radio-quiet, radio-intermediate, and radio-loud categories, observing that radio-intermediate and a subsample of radio-quiet AGN show a significant far-infrared (IR) excess over the radio-quiet SED and the far-IR excess appears to be related to the prominence of Fe ii emission. There is an overall consistency between the new SED and the one obtained for high Eddington ratio quasars in previous work. We provide the SEDs in digital format for eventual applications.

Eddington ratio is a paramount parameter governing the accretion history and life cycles of Active Galactic Nuclei (AGNs). This short review presents a multi-faceted view of the importance of the Eddington ratio spanning varied AGN studies. We find that the Eddington ratio is crucial for standardizing the Radius-Luminosity (R-L) relation - a necessary step for employing quasars (QSOs) as standardizable cosmological probes to help clarify the standing of the Hubble tension. In this data-driven era, we consolidated disparate aspects by developing novel relations borne out of large datasets, such as the robust, nearly universal anti-correlation between fractional variability and Eddington ratio derived from Zwicky Transient Facility (ZTF) data, which is vital for interpreting forthcoming high-cadence surveys like Rubin Observatory's LSST. Addressing the conundrum where JWST results suggest an overabundance of massive high-redshift black holes, we demonstrate that local AGNs offer clarification: Changing-Look AGNs (CLAGNs), driven by rapid Eddington ratio shifts, cluster in the low-accretion regime, a rate independently confirmed by our integral field spectroscopy and photoionization modeling of a well-known Seyfert 2 galaxy, rich in high-ionization, forbidden, coronal lines. Conversely, for the high-redshift, high-luminosity population where traditional reverberation mapping (RM) is highly impractical, photometric reverberation mapping (PRM) offers a rapid alternative to constrain accretion disk sizes, enabling efficient estimates of black hole masses and Eddington ratios. Finally, we developed tailored semi-empirical spectral energy distributions (SEDs) for extremely high-accretion quasars, successfully validating their characteristic extreme physical conditions.