The last few years have seen the confirmation of several trends associated with the quasar main sequence. The idea of a main sequence for quasars is relatively recent, and its full potential for the observational classification and contextualization of quasars' properties has yet to be fully exploited. The main sequence drivers are discussed in terms of the properties of extreme objects. We briefly summarize developments that constrain the viewing angle of the accretion disk in a particular class of quasars (extreme Population B, radiating at low Eddington ratio), as well as inferences on the chemical composition of the broad line emitting gas, and on the nature of radio emission along the quasar main sequence.

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. Aims. 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, Ciii]λ1909, Civλ1549, Heiiλ1640, 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 code CLOUDY, 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 Heiiλ1640. 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, high Z values are likely, but Z-scaled values are affected by the possible pollution due to highly-enriched gas associated with the circumnuclear star formation. The high-Z values 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.

Highly accreting quasars show fairly distinctive properties in their optical, UV, and X spectra, and are easy to recognize because of their specific location in the quasar main sequence: they are the strongest optical FeII emitters. They show a surprisingly high rate of radio detections and, at variance with the classical radio-loud (jetted) sources, the origin of their radio emission is probably “thermal.” The chemical composition of the broad line emitting gas implies high metallicity values, above 10 times solar. A fraction of highly accreting quasars at intermediate and high redshift might therefore be in a particular evolutionary stage that is unobscured albeit still involving a contribution of nuclear and circum-nuclear star formation in their multifrequency properties.

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. Aims. 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 deviation of the relative abundance of elements with respect to solar values. Methods. The basis of our analysis are multi-component fits in three regions of the spectra centered at 1900, 1550 and 1400 Å in order to deblend the broad components of Aliiiλ1860, Ciii]λ1909, Civλ1549, Heiiλ1640, and Siivλ1397 + Oiv]λ1402 and their blue excess. Results. By comparing the observed flux ratios of these components with the same ratios predicted by photoionization code CLOUDY we found that the virialized gas (broad components) presents a metallicity (Z) higher than 10Z . For non-virialized clouds we derive 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 rely on the ratios between metal lines and Heiiλ1640. This allowed us to confirm systematic differences in the solar-scaled metallicity derived from the lines of Aluminium and Silicon, and of Carbon, with the first being a factor ≈ 2 higher. Conclusions. For luminous quasars accreting at high rates, high Z values are likely, but that Z scaled-values are affected by the possible pollution due to highly-enriched gas associated with the circumnuclear star formation. The high-Z values suggest a complex process involving nuclear and circumnuclear star formation, interaction between nuclear compact objects and accretion disk, possibly with the formation of accretion-modified stars.