Extreme Quasars as Distance Indicators in Cosmology

Abstract

Quasars accreting matter at very high rates (known as extreme Population A [xA] quasars, possibly associated with super-Eddington accreting massive black holes) may provide a new class of distance indicators covering cosmic epochs from present day up to less than 1 Gyr from the Big Bang. At a more fundamental level, xA quasars are of special interest in studies of the physics of AGNs and host galaxy evolution. However, their observational properties are largely unknown. xA quasars can be identified in relatively large numbers from major optical surveys over a broad range of redshifts, and efficiently separated from other type-1 quasars thanks to selection criteria defined from the systematically-changing properties along the quasars main sequence. It has been possible to build a sample of ∼ 250 quasars at low and intermediate redshift, and larger samples can be easily selected from the latest data releases of the Sloan Digital Sky Survey. A large sample can clarify the main properties of xA quasars which are expectedunlike the general population of quasars -to radiate at an extreme, well defined Eddington ratio with small scatter. As a result of the small scatter in Eddington ratio shown by xA quasars, we propose a method to derive the main cosmological parameters based on redshift-independent "virial luminosity" estimates from measurements of emission line widths, roughly equivalent to the luminosity estimates based from line width in early and late type galaxies. A major issue related to the cosmological application of the xA quasar luminosity estimates from line widths is the identification of proper emission lines whose broadening is predominantly virial over a wide range of redshift and luminosity. We report on preliminary developments using the AlIIIλ1860 intermediate ionization line and the Hydrogen Balmer line Hβ as virial broadening estimators, and we briefly discuss the perspective of the method based on xA quasars. Keywords: quasar main sequence -line profiles -emission lines -supermassive black holes -black hole physics -dark energycosmological paremeters -cosmology 1 arXiv:2001.10368v1 [astro-ph.CO]

Broad UV Emission Lines in Type-1 Active Galactic Nuclei: A Note on Spectral Diagnostics and the Excitation Mechanism

“…Broad absorption lines in AlIII are detected, but much less frequently than for CIV [49]. The privileged emission in the low ionization, virialized part of the BLR is consistent with the possibility of using the AlIII width as a viral broadening estimator for high redshift quasars [50].…”

The quasar main sequence and its potential for cosmology

“…References Bischetti, M., Piconcelli, E., Vietri, G., et al 2017, AAp, 598, A122 Figure 2. The Hubble diagram distance modulus µ vs z for a sample of quasars (Dultzin et al 2020). Individual measurements have been averaged over redshift bins.…”

Dark energy constraints from quasar observations

“…The previous xA-QSO sample included ∼ 200 objects at redshift z < 2.7 [30]. In order to increase the redshift range, we considered the most recent edition of the The Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) catalog [31].…”