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Session 98 - AGN-Absorption/Emission.
Display session, Thursday, January 16
Metropolitan Ballroom,

[98.08] Can Quasars Be Used As Cosmological Candles?

K. Korista, G. Ferland (Univ. Kentucky), J. Baldwin (CTIO)

Since their discovery as the most distant and luminous discrete objects in the Universe, it has been hoped that somehow quasars could be used as ``cosmological candles'' to measure the expansion parameters (q_o and H_o) of the universe. With the discovery of the Baldwin Effect (Baldwin 1977), whereby the emission line equivalent widths scale inversely with the quasar luminosity, that hope is half-realized. What remains is to understand the formation of the quasar emission lines and their observed properties to the extent that we can predict the quasar luminosity from its spectrum. Past analyses of the origin of the Baldwin effect indicate that some sort of evolution in the continuum spectral energy distribution (SED) and possibly metallicity (e.g., Zheng and Malkan 1993; Hamann amp; Ferland 1993; Ferland et al.\ 1996) with luminosity and/or redshift occurs. However, the theoretical investigations have been hampered by the consideration of a single ionization parameter cloud --- unfortunately, significant changes in the SED or metallicity result in significant changes in the effective ionization parameter, producing misleading results. Recent observational analyses of Seyfert nuclei and quasars indicate a broad range in cloud parameters (density, column density, ionizing flux) is present within the broad emission line region. In light of this Baldwin et al.\ (1995) suggested that the observed emission lines are emitted from those clouds which emit them at high efficiency --- a powerful selection effect at work. Here we consider the effects on the line spectrum of a changing SED and metallicity upon a broad distribution in clouds, mitigating the problems associated with single cloud analyses. Once their spectra are understood, quasars will serve as probes of the universe back to z \approx 5.

Program listing for Thursday