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 Seyfert1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. Acommon feature in the EUV continuum of active galactic nuclei is the big bluebump (BBB), generally associated with thermal accretion disk emission. WhileGalactic absorption prevents direct access to the EUV range, it can be mappedby measuring the strength of a variety of forbidden optical emission lines thatrespond to different EUV continuum regions. We find that narrow emission-lineratios involving [OII]3727, Hbeta, [OIII]5007, [OI]6300, Halpha,[NII]6583, and[SII]6716,6731 indicate no significant difference between NLS1s and Broad-LineSeyfert 1 (BLS1) galaxies, which suggests that the spectral energydistributions of their ionizing EUV - soft X-ray continua are similar. Therelative strength of important forbidden high ionization lines like [NeV]3426compared to HeII4686 and the relative strength of [FeX]6374 appear to show thesame range as in BLS1 galaxies. However, a trend of weakerF([OI]6300)/F(Halpha) emission-line ratios is indicated for NLS1s compared toBLS1s. To recover the broad emission-line profiles we used Gaussian components.This approach indicates that the broad Hbeta profile can be well described witha broad component (FWHM = 3275 +- 800 km/s) and an intermediate broad component(FWHM = 1200 +- 300 km/s). The width of the broad component is in the typicalrange of normal BLS1s. The emission-line flux that is associated with the broadcomponent in these NLS1s amounts to at least 60% of the total flux. Thus itdominates the total line flux, similar to BLS1 galaxies.Comment: 34 pages, 9 figures. accepted for publication in the Astrophys.Journa