Correlated variations in the line and continuum emission from active galactic nuclei ( AGN ) can be used to determine the size and geometry of the broad emission line regions ( BLRs ) .
We have spectrophotometrically monitored a well-defined sample of 28 Palomar-Green quasars in order to obtain measurements of their BLRs and to investigate the relationships between quasar luminosity , central black hole mass , and BLR size in AGN .
Spectrophotometry was obtained every 1–4 months for 7.5 years , yielding 20–70 observing epochs per object .
Both the continuum and emission line fluxes of all of the quasars were observed to change during the duration of the observing program .
Seventeen of the 28 objects were observed with adequate sampling ( \mathrel { \raise 1.29 pt \hbox { $ > $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 20 independent observing epochs ) to search for correlated variations between the Balmer emission lines and the continuum flux .
For each of these 17 objects , a significant correlation was observed , with the Balmer line variations lagging those of the continuum by \sim 100 days ( rest frame ) .
Our work increases the available luminosity range for studying the size–mass–luminosity relations in AGN by two orders of magnitude and doubles the number of objects suitable for such studies .
Combining our results with comparable published data available for Seyfert 1 galaxies , we find the BLR size scales with the rest-frame 5100 Å luminosity as L ^ { 0.70 \pm 0.03 } .
This determination of the scaling of the size of the BLR as a function of luminosity is significantly different from those previously published , and suggests that the effective ionization parameter in AGN may be a decreasing function of luminosity .
We are also able to constrain , subject to our assumption that gravity dominates the motions of the BLR gas , the scaling relationship between the mass of the central black holes and the luminosity in AGN .
We find that the central mass scales with 5100 Å luminosity as M \propto L ^ { 0.5 \pm 0.1 } .
This is inconsistent with all AGN having optical luminosity that is a constant fraction of the Eddington luminosity .