# Quasars: from the Physics of Line Formation to Cosmology

## TL;DR

This paper explores extreme Population A quasars with high accretion rates as stable, reliable distance indicators across cosmic history, analyzing their spectral properties and physical conditions for cosmological applications.

## Contribution

It identifies spectral features of extreme Population A quasars that enable their use as redshift-independent distance estimators based on virial broadening.

## Key findings

- Spectral properties allow easy identification of xA quasars in large surveys.
- Physical conditions support the use of low-ionization line widths as virial estimators.
- xA quasars can serve as stable cosmological distance indicators.

## Abstract

Quasars accreting matter at very high rates (known as extreme Population A [xA] or super-Eddington accreting massive black holes) provide a new class of distance indicators covering cosmic epochs from the present-day Universe up to less than 1 Gyr from the Big Bang. The very high accretion rate makes it possible that massive black holes hosted in xA quasars radiate at a stable, extreme luminosity-to-mass ratio. This in turns translates into stable physical and dynamical conditions of the mildly ionized gas in the quasar low-ionization line emitting region. In this contribution, we analyze the main optical and UV spectral properties of extreme Population A quasars that make them easily identifiable in large spectroscopic surveys at low-z (z < 1) and intermediate-z (2 < z < 2.6), and the physical conditions that are derived for the formation of their emission lines. Ultimately, the analysis supports the possibility of identifying a virial broadening estimator from low-ionization line widths, and the conceptual validity of the redshift-independent luminosity estimates based on virial broadening for a known luminosity-to-mass ratio.