| This dissertation searches for common grounds for the diversity of properties exhibited by the emission-line nuclei of galaxies, from large look-back times to the local universe. I present results of (1) a program of high signal-to-noise spectroscopy for 44 z ≳ 4 quasars using the MMT and Keck observatories; (2) a detailed analysis of the ultraviolet and optical spectral behavior of 22 Narrow Line Seyfert 1 (NLS1) galaxies based on archival Hubble Space Telescope (HST) spectra; (3) an in-depth investigation of the proposed link between NLS1s and z ≳ 4 quasars, by means of comparison of composite spectra, and a Principal Component Analysis; (4) a simulation of Seyferts/quasars designed to explore the role of dust in modifying their observed spectral energy distribution; and (5) a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, employing a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures.; The low and high redshift quasars are found very similar in their emission characteristics, although differences exist. In particular, the data bolster indications of supersolar metallicities in the luminous, z ≳ 4 sources, which support scenarios that assume substantial star formation concurrent or preceding the quasar phenomena. Because high-z sources are more metal enriched and more spectroscopically heterogeneous than the NLS1s, a close connection between these objects remains doubtful. The results show that NLS1s have redder UV-blue continua than those measured in other quasars and Seyferts. The sources with UV line absorption are in general less powerful and show redder spectra, indicating that a luminosity-dependent dust absorption may be important in modifying their continua. A receding-torus-like geometry seems to explain these trends and other observed correlations between quasar luminosity and continuum slope. Finally, in most of the nearby emission-line nuclei, the expected increased AGN-like behavior at smaller scales is not seen, although the nuclear emission is resolved. This suggests that these sources are not necessarily powered by accretion onto a compact object, and that the composite model proposed for the LINER/H II transition nuclei (that assumes a central accreting-type nucleus surrounded by star-forming regions) is not generally supported. |
