The ``broad'' components

At this stage, we removed from the blue and red spectra the best fitting line-core (the Hii region and the Seyfert 2 nebulosity, Fig. c and d), obtaining two spectra we shall call ``original data minus core''. The blue one was then fitted with a broad H$\beta$ Gaussian component and two sets of three components modelling the narrow H$\beta$ and O III$\lambda\lambda$4959, 5007 lines. The result is very suggestive: one set has a strong H$\beta$ line and very weak negative O III components, while the other set displays a strong O III contribution and a weak negative H$\beta$ component, showing that we have in fact a H$\beta$ component with no associated O III emission and O III lines with a very weak (undetected) associated H$\beta$; in other words, the region producing the H$\beta$ line does not emit forbidden lines, while the O III emitting region has a high $\lambda$5007/H$\beta$ ratio, which are, respectively, the characteristics of the ``broad'' and ``narrow'' line regions in Seyfert 1 galaxies.

Having these results in mind, we optimized this last fit by using a Lorentzian profile for the H$\beta$ line, with no associated O III emission, and a set of three Gaussians for the remaining contribution coming from the ``narrow'' components (this is not the first time Lorentzian profiles are used to fit AGN emission lines; for example Stüwe et al. (1992) found that, in the case of NGC 4258, the narrow lines were better fitted by Lorentzians, rather than Gaussians).

The best fit is presented in Fig. e: in this model, the flux of the ``narrow'' (Gaussian) H$\beta$ component is only 9% of the ``broad'' (Lorentzian) H$\beta$ component. The H$\beta$ Lorentzian component is blueshifted by 150 kms^-1 with a width of 915 kms^-1. The Gaussian components are blueshifted by $\sim$ 395 kms^-1 and their width is $\sim$ 1115 kms^-1. Mason et al. found a FWHM = 1030 $\pm$ 150 kms^-1 for this component which is blueshifted by 240 $\pm$ 30 kms^-1. This blueshift, however, is measured with respect to the O III lines core which is dominated by the Seyfert 2 cloud, itself blueshifted by 95 kms^-1 with respect to the Hii region; the blueshift of Mason et al.'s intermediate component is, therefore, 240 + 95 = 335 kms^-1, in agreement with our value of 395 kms^-1.

In Seyfert 1 galaxies, the Balmer decrement of the broad component is never smaller than that of the narrow component; in the present case, we therefore expect the ``narrow'' H$\alpha$ component flux to be less than 9% of the ``broad'' H$\alpha$ component flux. Moreover, the N II$\lambda$6583 line flux is, in Seyfert galaxies, equal or smaller than the narrow H$\alpha$ component flux. So, in KUG 1031+398, we expect the narrow lines to be quite weak compared to the ``broad'' H$\alpha$ component, and we fitted the ``original data minus core'' red spectrum with a single Lorentzian profile of 1205 kms^-1 FWHM, blueshifted by 65 kms^-1 with respect to the Hii region. This fit is shown in Fig. f.

Another model allowing, in addition, for a set of H$\alpha$ and N II Gaussian components was also tested, resulting in a fit of similar quality; the very small $\lambda$6583/H$\alpha$ ratio observed for this solution (0.2), shows that the nitrogen lines may be considered as undetectable.