On the Intermediate Line Region in AGNs

Tek P Adhikari,Krzysztof Hryniewicz,Gary J Ferland,Bozena Czerny,Agata Różańska

doi:10.3389/fspas.2017.00019

Abstract

In this paper we explore the intermediate line region (ILR) by using the photoionisation simulations of the gas clouds present at different radial distances from the center, corresponding to the locations from BLR out to NLR in four types of AGNs. We let for the presence of dust whenever conditions allow for dust existence. All spectral shapes are taken from the recent multi-wavelength campaigns. The cloud density decreases with distance as a power law. We found that the slope of the power law density profile does not affect the line emissivity radial profiles of major emission lines: H${\beta}$, He~II, Mg~II, C~III] ~and [O~III]. When the density of the cloud at the sublimation radius is as high as 10$^{11.5}$ cm$^{-3}$, the ILR should clearly be seen in the observations independently of the shape of the illuminating radiation. Moreover, our result is valid for low ionization nuclear emission regions of active galaxies.

Highlights

The emission lines in active galactic nuclei (AGNs) provide a unique opportunity to study the properties of the materials located in the environment of the supermassive black hole (SMBH)
There are subtle differences in emissivities corresponding to the broad line region (BLR) and narrow line region (NLR) due to the different amount of UV and soft Xray photons among the spectral energy distributions (SEDs). This result is in agreement with the conclusion of AD16, that the presence of intermediate line region (ILR) emission is not determined by the shape of the incident radiation
The results above confirm the conclusion of AD16, that the dust extinction of the emission lines in AGN introduced by NL93 is important only when the gas density is low

Summary

INTRODUCTION

The emission lines in active galactic nuclei (AGNs) provide a unique opportunity to study the properties of the materials located in the environment of the supermassive black hole (SMBH). Adhikari et al, (2016 hereafter AD16) have shown, using the framework put forward by NL93, that when the density of illuminated clouds is high enough, the dust does not suppress the gap between BLR and NLR and intermediate line emission is clearly visible. In NL93, the authors assumed a power law with slope −1.5 and normalization 109.4 cm−3 at Rd to describe the variation of the density of gas clouds with distance from the nucleus These clouds were illuminated by the mean AGN spectrum. AD16 made one step forward, showing that if the density at the sublimation radius is high, of the order of 1011.5 cm−3, the ILR is clearly visible Such result appeared to be independent on SED of illuminated radiation taken into account. We briefly outline the photoionisation model itself, and discuss the resulting line emissivity profiles focusing mainly on the visibility of ILR in different AGN

PHOTOIONISATION MODEL

LINE EMISSIVITIES

DISCUSSION