Intermediate-line Emission in AGNs: The Effect of Prescription of the Gas Density

TL;DR

This study investigates the intermediate line region in AGNs through photoionization simulations, revealing how gas density, dust, and cloud structure influence emission lines and align with observational data.

Contribution

It introduces a detailed model of the intermediate line region considering dust, cloud density profiles, and compares results with reverberation mapping observations.

Findings

Fe II line is sensitive to dust content and can trace dust in line regions.

Cloud density profile does not affect the existence of ILR.

Model reproduces observed Hβ emission region distances.

Abstract

The requirement of intermediate line component in the recently observed spectra of several AGNs points to possibility of the existence of a physically separate region between broad line region (BLR) and narrow line region (NLR). In this paper we explore the emission from intermediate line region (ILR) by using the photoionization simulations of the gas clouds distributed radially from the AGN center. The gas clouds span distances typical for BLR, ILR and NLR, and the appearance of dust at the sublimation radius is fully taken into account in our model. Single cloud structure is calculated under the assumption of the constant pressure. We show that the slope of the power law cloud density radial profile does not affect the existence of ILR in major types of AGN. We found that the low ionization iron line, Fe~II, appears to be highly sensitive for the presence of dust and therefore becomes potential tracer of dust content in line emitting regions. We show that the use of disk-like cloud density profile computed at the upper part of the accretion disc atmosphere reproduces the observed properties of the line emissivities. In particular, the distance of H${\beta}$ line inferred from our model agrees with that obtained from the reverberation mapping studies in Sy1 galaxy NGC 5548.