What do the ultraviolet spectra of narrow line Seyfert 1 galaxies tell us about their broad-line regions?

We study the UV spectra of narrow-line Seyfert 1 (NLSy1) galaxies and compare them with "normal" active galactic nuclei. Similar to their optical lines, the NLSy1s show narrower UV lines. They are also characterized by weaker C IV lambda 1549 and C III lambda 1909 and stronger Al III lambda 1857 emission. These UV-line properties add to the optical and X-ray properties known to be part of the Boroson & Green eigenvector 1. We show that the steep soft X-rays, which characterize the NLSy1s SEDs, change the equilibrium of the two-phase cloud-intercloud medium, resulting in somewhat higher broad-line region cloud densities, lower ionization parameter, and larger broad-line region radii. These modified conditions can explain the unusual emission-line properties we find in NLSy1. Using a specific model of an accretion disk with corona presented by Witt, Czerny, & Zycki, we also show that the steep soft and hard X-ray continua can be explained if the L/L-Edd ratios are larger than in "normal" Sy1s/QSOs, strengthening earlier suggestions that the L/L-Edd is the physical parameter driving this eigenvector.