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A. Constantin (Drexel), J. C. Shields (Ohio U)
We present the results of a search 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. A program of high signal-to-noise spectroscopy for 44 z > 4 quasars using the MMT and Keck observatories reveals that the low and high redshift quasars are very similar in their 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. By combining these z > 4 quasar data with archival Hubble Space Telescope (HST) spectra of 22 Narrow Line Seyfert 1 (NLS1) galaxies, we conducted an in-depth investigation of the proposed analogy between these two types of objects. By means of comparison of composite spectra and a Principal Component Analysis, it is found that the high-z sources are more metal enriched and more spectroscopically heterogeneous than the NLS1s, and thus a close connection between them remains doubtful. Among the NLS1s, 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. Based on Monte Carlo simulations, we show that a receding-torus --like geometry may explain these trends and other observed correlations between quasar luminosity and continuum slope. We also proceeded with a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, based on a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures. We find that, in most of the local emission-line nuclei, the expected increased AGN-like behavior at smaller spatial 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.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.