Optical emission-line properties of narrow-line Seyfert 1 galaxies and comparison active galactic nuclei

Based on a new spectroscopic sample observed using the William Herschel Telescope, we examine the kinematic properties of the various emission-line regions in narrow-line Seyfert 1 galaxies (NLS1s) by modelling their profiles using multiple component fits. We interpret these results by comparison with velocity components observed for different lines species covered in the same spectrum, and equivalent components measured in the spectra of some broad-line Seyfert 1s and a representative Seyfert 2 galaxy. We find that the fits to the Hα and Hβ line profiles in NLS1s require an additional broad (≳3000 km s−1) component that might correspond to a suppressed broad-line region (BLR) with similar kinematics to those of typical broad-line Seyfert 1s. From the profiles of the forbidden high-ionization lines (FHILs) in NLS1s, we find evidence that they appear to trace an ‘intermediate’ velocity region with kinematics between the standard broad and narrow-line regions. Weaker evidence of this region is also present in the profiles of the permitted Balmer lines. Finally, we note that despite having similar ionization potentials, the relative intensities of the highly ionized lines of [Fe x]λ6374 and [Fe xi]λ7892 show considerable dispersion from one galaxy to another. The interpretation of this requires further modelling, but suggests the possibility of using the ratio as a diagnostic to constrain the physical conditions of the FHIL emitting region and possibly the shape of the spectral energy distribution in the vicinity of 200 eV. This spectral region is very difficult to observe directly due to photoelectric absorption both in our Galaxy and intrinsic to the source.