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

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Published in The Astrophysical Journal, v. 856, no. 1, 78, p. 1-13.

© 2018. The American Astronomical Society. All rights reserved.

The copyright holder has granted the permission for posting the article here.

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This research was supported by Polish National Science Center grants No. 2015/17/B/ST9/03422, 2015/18/M/ST9/00541, 2015/17/B/ST9/03436, 2016/21/N/ST9/03311 and by Ministry of Science and Higher Education grant W30/7.PR/2013. It received funding from the European Union Seventh Framework Program (FP7/2007-2013) under the grant agreement No.312789. T.P.A. received funding from NCAC PAS grant for young researchers. G.J.F. thanks the Nicolaus Copernicus Astronomical Center for its hospitality and acknowledges support by NSF (1108928, 1109061, and 1412155), NASA (10-ATP10-0053, 10-ADAP10-0073, NNX12AH73G, and ATP13-0153), and STScI (HST-AR-13245, GO-12560, HST-GO-12309, GO-13310.002-A, HST-AR-13914, and HST-AR-14286.001).