Structure and Kinematics of the Broad-Line Regions in Active Galaxies from IUE Variability Data

Abstract

IUE archival data are used to investigate the structure and kinematics of the broad-line regions (BLRs) in nine active galactic nuclei (AGNs). The cross-correlation technique, for unevenly sampled data, advocated by Gaskell and Sparke has been applied to spectra taken between 1978 and 1988. An extensive investigation of the errors has been carried out. Monte Carlo simulations conducted to estimate the errors in the analysis indicate that the theoretical errors are slight overestimates on average when the measurement errors are small and tend to be underestimates when the measurement errors are significant. The error estimates from the simulations are comparable to error estimates from the subsets for the weaker features where measurement errors are more significant. Three of the objects fail to satisfy the Gaskell and Peterson sampling criterion, i.e., the width of the sampling autocorrelation function (ACF) is much smaller than the width of the true continuum ACF. These AGNs only yield upper limits to the sizes of the emitting region from the line ACF. For the remaining six AGNs we show that luminosity-weighted radii of the emitting region can be obtained from the centroids of the cross-correlation functions (CCFs). We also get estimates of the inner radii of the emitting region from the Positions of the peaks in the CCFs and upper limits to the overall sizes from the line ACFs. We find that the regions extend radially by probably less than a factor of 1O. We confirm that the high-ionization lines are emitted from regions significantly smaller than predicted by the "standard" photoionization model and that the lower ionization lines come from further away from the central source. C III] λ 1909 comes from a region twice as far away as C IV λ1549 and Mg II λ2798 can come from even further away, especially in the low-luminosity sources. We confirm that the motions of the gas clouds are dominated by gravity, and we exclude pure radial motion (either inflow or outflow) at a high confidence level. The rapid variability of the 3C 273 BLR suggests that we might be seeing anisotropic continuum variability in this AGN.