Reverberation measurements for 17 quasars and the size-mass-luminosity relations in active galactic nuclei

Correlated variations in the line and continuum emission from active galact ic nuclei (AGNs) can be used to determine the size and geometry of the broa d emission-line regions (BLRs). We have spectrophotometrically monitored a well-defined sample of 28 Palomar-Green quasars in order to obtain measurem ents of their BLRs and to investigate the relationships between quasar lumi nosity, central black hole mass, and BLR size in AGNs. Spectrophotometry wa s obtained every 1-4 months for 7.5 yr, 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. Seven teen of the 28 objects were observed with adequate sampling (greater than o r similar to 20 independent observing epochs) to search for correlated vari ations between the Balmer emission lines and the continuum flux. For each o f these 17 objects, a significant correlation was observed, with the Balmer -line variations lagging those of the continuum by similar to 100 days (res t frame). Our work increases the available luminosity range for studying th e size-mass-luminosity relations in AGNs by 2 orders of magnitude and doubl es the number of objects suitable for such studies. Combining our results w ith comparable published data available for Seyfert 1 galaxies, we find the BLR size scales with the rest-frame 5100 Angstrom luminosity as L0.70+/-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 an d suggests that the effective ionization parameter in AGNs may be a decreas ing function of luminosity. We are also able to constrain, subject to our a ssumption that gravity dominates the motions of the BLR gas, the scaling re lationship between the mass of the central black holes and the luminosity i n AGNs. We find that the central mass scales with 5100 Angstrom luminosity as M proportional to L0.5+/-0.1. This is inconsistent with all AGNs having optical luminosity that is a constant fraction of the Eddington luminosity.