Explaining the Broad-line Region through photoionisation modelling

TL;DR

This paper models the broad-line region of active galaxies using photoionization to relate FeII emission strength with ionization and density, incorporating dispersion due to accretion rate variations.

Contribution

It introduces a photoionization model that accounts for dispersion in the radius-luminosity relation by including accretion rate and virial factors.

Findings

Constrained ionization parameter and density based on FeII emission.

Incorporated dispersion factors related to accretion rate and Eddington ratio.

Linked FeII strength to BLR properties through a fundamental plane relation.

Abstract

Broad-line regions (BLR) are one of the main components that constitute the phenomenological picture of active galaxies near the vicinity of the accreting supermassive black holes. Both theoretical and observational studies have shown that the BLR is made of dense, ionized gas clumps that have a strong virialized distribution at parsec-scale distances from the nuclei. Using a theoretically motivated photoionized gas model, I constrain the ionisation parameter (U) and cloud density (n$_{\rm{H}}$) as a function of the strength of the FeII emission. Recent observations in the reverberation mapping studies have contested the standard radius-luminosity relation showing increased dispersion in the relation, in particular, after the inclusion of highly accreting quasars. I incorporate the departure coefficient that accounts for this dispersion. This departure term in terms of the dimensionless accretion rate ($\dot{\mathcal{M}}$) and Eddington ratio ($L\mathrm{_{bol}}/L\mathrm{_{Edd}}$), also includes the virial factor that accounts for the BLR geometry. I then combine the fundamental plane relation for the BLR to connect the FeII strength (R$_{\rm{FeII}}$) in terms of U and n$_{\rm{H}}$ using selected values for the shape of the broad H$\beta$ profile.