Ultraviolet and optical properties of narrow-line Seyfert 1 galaxies

Narrow-line Seyfert 1 galaxies (NLS1s) are remarkable for their extreme continuum and emissionline properties, which are not well understood. New results bearing on the spectroscopic characteristics of these objects are presented here, with the aim of establishing their typical UV and optical spectral behavior. We employ Hubble Space Telescope observations of 22 NLS1s, which represent a substantial improvement over previous work in terms of data quality and sample size. High signal-to-noise ratio NLS1 composite spectra are constructed, allowing accurate measurements of the continuum shape and the strengths, ratios, and widths for lines, including weak features that are barely identifiable in other active galactic nuclei composites. We find that the NLS1 sources have redder UV-blue continua than those typically measured in other quasars and Seyfert galaxies. Objects with UV line absorption show redder spectra, suggesting that dust is important in modifying the continuum shapes. The data also permit a detailed investigation of the previously proposed link between NLS1s and z greater than or similar to 4 quasars. Direct comparison of their composite spectra, as well as a principal component analysis, suggests that high-z quasi-stellar objects do not show a strong preference toward NLS1 behavior.