The model used by Mason et al. to fit the emission-line spectrum of KUG 1031+398 seems to support the existence of an ILR emitting lines of intermediate width (FWHM 1000 kms-1); this component dominates the Balmer line profiles, being also a significant contributor to the [OIII] lines, with a flux ratio 5007/H = 1.4.
There are two main reasons why our analysis yields different results. First, KUG 1031+398 having a redshift of 0.043, the [NII]6583 line coincides with the atmospheric B band. When correcting for this absorption feature, the [NII] true intensity is recovered (Fig. 1b) and our red spectrum appears different from the published one; different line-ratios and widths are therefore not unexpected.
Second, the line-profile analysis of Mason et al. differs from ours in that, while we force each Balmer component to be associated with forbidden lines having the same velocity and width, Mason et al. allow these parameters to have different values for the Balmer and forbidden line components. As a result, they found three H components (a narrow, an intermediate and a broad one), as well as two [OIII] components (a narrow and an intermediate one); they also detected three H components (again a narrow, an intermediate and a broad one), but only a single [NII] component (narrow). The measured width of the narrow H component is 150 20 km s-1 FWHM, while the width of the narrow [OIII] lines is 265 10 km s-1; this last value, significantly larger than the narrow H line width, suggests that the [OIII] lines may have a complex profile. Moreover, the width of the [NII] lines is found to be significantly larger (400 60 km s-1) than that of the narrow H component (190 40 km s-1); this could be due to an inaccurate correction of the atmospheric B band, as we have seen before.
Although our spectra have a lower resolution than those obtained by Mason et al. (3.4 Å compared to 2 Å FWHM), this does not affect the analysis; the narrow core components being identified and subtracted, all the discussion is centered on the broader components, well resolved even with our lower resolution. Similarly, the larger slit width used in our observations (21 compared to 15 for Mason et al.) does not affect the study of these broader components, since only the contribution from the extended emitting region (the HII region), removed with the core, changes with the slit width.
We disagree with Mason et al. on the result of the line profile analysis of KUG 1031+398, in the sense that we find no evidence for the presence of an ILR. Nevertheless, we find that this object is exceptional in having a NLR (defined as a region where 5007/H 5) with almost the same width as the BLR (Balmer lines with no detectable associated forbidden lines).
Several authors have suggested that the small width of the broad Balmer lines and the soft X-ray excess characteristic of the NLS1 galaxies could be the effect of a high accretion rate onto an abnormally small mass black hole. Mason et al. (1996) have argued that, although the emission line spectrum in this object is dominated by the ILR, a weak broad component is present with line-widths of the order of 2500 kms-1 FWHM and that, therefore, at least in this object, such a model is not required. Our analysis of the spectra shows that in the BLR, the Balmer lines are well fitted by a Lorentzian profile with 1000 kms-1 FWHM. We have shown (Gonçalves et al. in preparation) that in NLS1s the broad component of the Balmer lines is generally better fitted by a Lorentzian than by a Gaussian; so, in this respect, KUG 1031+398 is a normal NLS1 and could be explained by the same small black hole mass model suggested for the other objects of the same class.
Anabela C. Gonçalves acknowledges support from the Fundação para a Ciência e a Tecnologia, Portugal, during the course of this work (PRAXIS XXI/BD/5117/95 PhD. grant).