The intermediate line region in active galactic nuclei

TL;DR

This paper explains the observed intermediate line region in some active galactic nuclei by modeling increased gas density near the dust sublimation radius, showing it can produce continuous line emission between the broad and narrow line regions.

Contribution

It introduces a model where high gas density in the disk atmosphere accounts for the intermediate line region, resolving the apparent gap in emission lines in AGN.

Findings

High gas density (~10^{11.5} cm^{-3}) explains continuous line emission.

The model aligns with the density of upper layers of accretion disk atmospheres.

The shape of the spectral energy distribution does not affect the gap presence.

Abstract

We show that the recently observed suppression of the gap between the broad line region (BLR) and the narrow line region (NLR) in some AGN can be fully explained by an increase of the gas density in the emitting region. Our model predicts the formation of the intermediate line region (ILR) that is observed in some Seyfert galaxies by the detection of emission lines with intermediate velocity full width half maximum (FWHM) $\sim$ 700 - 1200 km s$^{-1}$. These lines are believed to be originating from an ILR located somewhere between the BLR and NLR. As it was previously proved, the apparent gap is assumed to be caused by the presence of dust beyond the sublimation radius. Our computations with the use of {\sc cloudy} photoionization code, show that the differences in the shape of spectral energy distribution (SED) from the central region of AGN, do not diminish the apparent gap in the line emission in those objects. A strong discontinuity in the line emission vs radius exists for all lines at the dust sublimation radius. However, increasing the gas density to $\sim$ 10$^{11.5}$ cm$^{-3}$ at the sublimation radius provides the continuous line emission vs radius and fully explains the recently observed lack of apparent gap in some AGN. We show that such a high density is consistent with the density of upper layers of an accretion disk atmosphere. Therefore, the upper layers of the disk atmosphere can give rise to the formation of observed emission line clouds.