The discovery paper for the BE was Baldwin's (1977a) article, which noted an anti-correlation of the equivalent width W of the C IV emission line and the continuum luminosity L of quasars on the assumption that their redshifts were cosmological. A copy of his original diagram is shown in Figure 1. The effect was pronounced: the equivalent width of the lines decreased by an order of magnitude as the continuum luminosity increased by 1.6 dex.
Baldwin (1977b) published another seminal paper that same year in which he noted that the observed average Ly / H intensity ratio in quasar spectra was significantly less than predicted for a recombination spectrum. Subsequently Baldwin et al. (1978) showed that the Mg II 2798 emission line also exhibited an anti-correlation with luminosity and used the BE to constrain cosmological models. They concluded that their data ruled out the local hypothesis for quasars and zero-pressure models with q0 = 0.
The first two papers mentioned above stimulated much work on quasar emission lines in the subsequent two decades. The third one pointed out the cosmological possibilities of the BE; however, the uncertainties were large enough to limit the usefulness of the technique, a problem that has continued to this day. One of the goals of this meeting is to see if our knowledge has advanced enough for quasars to be useful cosmological probes.
|Figure 1. Baldwin's (1977a) plot of the BE for C IV.|