Probing the ionizing continuum of narrow-line Seyfert 1 galaxies. I. Observational results

We present optical spectra and emission-line ratios of 12 narrow-line Seyfert 1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A common feature in the EUV continuum of active galactic nuclei is the big blue bump (BBB), generally associated with thermal accretion disk emission. While Galactic absorption prevents direct access to the EUV range, it can be mapped by measuring the strength of a variety of forbidden optical emission lines that respond to different EUV continuum regions. We find that narrow emission line ratios involving [O II] lambda 3727, H beta, [O III] lambda 5007, [O I] lambda 6300, H alpha, [N II] lambda 6583, and [S II] lambda lambda 6716, 6731 indicate no significant difference between NLS1 galaxies and broad-line Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy distributions of their ionizing EUV-soft X-ray continua are similar. The relative strength of important forbidden high-ionization lines like [Ne V] lambda 3426 compared to He II lambda 4686 and the relative strength of [Fe X] lambda 6374 appear to show the same range as in BLS1 galaxies. However, a trend of weaker F([O I] lambda 6300)/F(H alpha) emission-line ratios is indicated for NLS1 galaxies compared to BLS1 galaxies. To recover the broad emission line profiles, we used Gaussian components. This approach indicates that the broad H beta profile can be well described with a broad component (FWHM similar or equal to 3275 +/- 800 km s(-1)) and an intermediate broad component (FWHM similar or equal to 1200 +/- 300 km s(-1)). The width of the broad component is in the typical range of normal BLS1 galaxies. The emission-line flux that is associated with the broad component in these NLS1 galaxies amounts to at least 60% of the total flux. Thus, it dominates the total line flux, similar to BLS1 galaxies.