The narrow-line region of narrow-line Seyfert 1 galaxies

This work studies the optical emission-line properties and physical conditi ons of the narrow-line region (NLR) of seven narrow-line Seyfert 1 galaxies (NLS1's) for which high signal-to-noise ratio spectroscopic observations w ere available. The resolution is 340 km s(-1) (at H alpha) over the wavelen gth interval 370-9500 Angstrom, enabling us to separate the broad and narro w components of the permitted emission lines. Our results show that the flu x carried out by the narrow component of H beta is, on average, 50% of the total line flux. As a result, the [O III] lambda 5007/H beta ratio emitted in the NLR varies from 1 to 5, instead of the universally adopted value of 10. This has strong implications for the required spectral energy distribut ion that ionizes the NLR gas. Photoionization models that consider a NLR co mposed of a combination of matter-bounded and ionization-bounded clouds are successful at explaining the low CO mi lambda 5007/H beta ratio and the we akness of low-ionization lines of NLS1's. Variation of the relative proport ion of these two type of clouds nicely reproduces the dispersion of narrow- line ratios found among the NLS1 sample. Assuming similar physical model pa rameters of both NLS1's and the normal Seyfert 1 galaxy NGC 5548, we show t hat the observed differences of emission-line ratios between these two grou ps of galaxies can be explained, to a first approximation,:in terms of the shape of the input ionizing continuum. Narrow emission-line ratios of NLS1' s are better reproduced by a steep power-law continuum in the EUV-soft X-ra y region, with spectral index alpha similar to -2. Flatter spectral indices (alpha similar to -1.5) match the observed line ratios of NGC 5548 but are unable to provide a good match to the NLS1. ratios. This result is consist ent with ROSAT observations of NLS1's, which show that these objects are ch aracterized by steeper power-law indices than those of Seyfert 1 galaxies w ith strong broad optical lines.