The Intermediate Line Region

It is commonly accepted that line-emission in AGN comes from two well separated regions: one, compact (< 1 pc) and lying close to the central engine, has a high electron density ($N_{\rm e}$ > 10⁸ cm^-3) and is responsible for the production of broad (FHWM $\sim$ thousands of kms^-1) permitted lines--the BLR; the other, more extended and lying further away from the central source (10-1000 pc), has lower electron densities ($N_{\rm e}$ $\sim$ 10⁵ cm^-3) and emits lines with a lower velocity dispersion ($\sim$ hundreds of kms^-1)--the NLR. A ``gap'' of line-emission is usually observed between the two regions; most objects show optical spectra which can be fitted by line profiles corresponding to clouds belonging to one or the other line-emitting regions. This line-emission gap can be explained by the presence of dust mixed up with the gas (Netzer & Laor 1993). Nevertheless, the existence of an intermediate region, both in terms of velocity and density ($N_{\rm e}$ $\sim$ 10^6.5 cm^-3) is expected; at this density, the [OIII] lines are partially collisionally de-excited such that $\lambda$5007/H$\beta$ $\sim$ 1 [this ILR should not be confused with the ILR found in QSOs by Brotherton et al. (1994), which is much smaller and denser, with velocity dispersion of the order of 2000 kms^-1 and density $\sim$ 10¹⁰ cm^-3]. To the best of our knowledge, no uncontroversial report of the existence of an ILR has ever been made. Crenshaw & Peterson (1986) and Van Groningen & de Bruyn (1989) have found broad wings in the [OIII] lines of a number of Seyfert 1 galaxies, implying the presence of an ILR in these objects; however, they all show strong Fe II emission, and the observed broad [OIII] components could be due to the inaccurate removal of the FeII blends (Boroson & Green 1992). The claim by Mason et al. (1996) that KUG 1031+398 showed evidence for an ILR induced us to conduct new spectroscopic observations and modeling of its emission-line features.