Quasar Emission Lines

The following two papers were the first models of quasar emission line regions based upon the hypothesis of photoionization.

Some Models of the Emission-Line Region of 3C 273

Authors:John N. Bahcall and Ben-Zion Kozlovsky

Journal: ApJ, 155, 1077 (March 1969)

pdf file

Abstract: Models for the emission-line region of 3C 273 are derived by using ionization distributions calculated with extrapolated forms of the observed photon flux. The calculated ionization distributions, which are different from those assumed by previous workers, are used to make estimates of the relative abundances of H, He, O, Ne, Mg, and Fe. Estimates are also given of the physical parameters that characterize the emission-line region.

Some Models for the Emission-Line Region of 3C 48

Authors:John N. Bahcall and Ben-Zion Kozlovsky

Journal: ApJ, 158, 529 (November 1969)

pdf file

Abstract: Models for the emission-line region of 3C 48 are derived by using calculated ionization distributions. Estimates are given of the parameters that characterize the emission-line region and of the relative abundance of H, He, O, He, and Mg.

Does the fine-structure constant vary with cosmological epoch?

Authors:John N. Bahcall, Charles L. Steinhardt and David Schlegel

Journal: ApJ, 600, 520 (January 10, 2004), astro-ph/0301507

postscript file     pdf file    Spectra figures (gzipped file for downloading)

Abstract: We use the strong nebular lines of O III, 5007 A and 4959 A, to set a robust upper limit on the time dependence of the fine structure constant. We find |alpha^{-1} d alpha(t)/dt | < 10^{-13} yr, corresponding to Delta alpha/alpha(0) = (-2 +- 1.2) * 10^{-4} for quasars with 0.16 < z < 0.80 obtained from the SDSS Early Data Release. Using a blind analysis, we show that the upper limit given here is invariant with respect to 17 different ways of selecting the sample and analyzing the data. As a by-product, we show that the ratio of transition probabilities for the 5007 A and the 4959 A lines is 3.01 +- 0.02, in good agreement with (but more accurate than) theoretical estimates. We compare and contrast the O III emission line method used here with the Many-Multiplet method that has been used recently to suggest evidence for a time-dependent alpha.