The nature of emission-line profile variability and the structure of the line-emitting regions in active galactic nuclei

The standard model of active galactic nuclei (AGN) suggests that the broad-line regions (BLRs) of high luminosity AGN (HLAGN) should be so large that any variability in the broad emission-line (BEL) fluxes should be on timescales of {dollar}sim{dollar}5 years or more. Spectrophotometric observations of 8 radio-quiet QSOs over a period of 1 year revealed definite BEL flux variability in one of these objects and possibly in two others. This may be an indication that the BLRs in HLAGN may be much smaller than that predicted by the standard model. Follow-up observations to monitor HLAGN should be made.; The structure of the BLRs in two Seyfert 1 galaxies was investigated through the analysis of their broad emission-line profiles and profile variability. Multiple broad-line emitting components were found for Arakelian 120 and Markarian 590 through the construction of difference spectra and profile fitting. In the case of Markarian 590 two distinct components could be well isolated from one another: a narrow variable core, which we associated with the classical BLR, and a strong and constant very broad-base (VBB). We interpreted the VBB to be emitted in optically thin gas lying nearer to the central engine. Such gas may account for several properties of AGN. Two of the more important are the observed big blue bump and the soft X-ray absorption. The Balmer broad-line profiles in Akn 120 are more complicated, and require at least 4 Gaussian-like components. We also determined that FeII can account for most or all of the emission in the red-shelf of the H{dollar}beta{dollar} profile in Akn 120 as well as some of the structure and variability in the H{dollar}beta{dollar} profile.; Finally, the origin of the coronal emission-lines was investigated, utilizing a photoionization code. We found that the high ionization iron lines, such as (FeX) {dollar}lambda{dollar}6375, may have their origin in the {dollar}rm Nsb{lcub}e{rcub}{lcub}sim{rcub}{dollar}1 cm{dollar}sp{lcub}-3{rcub}{dollar} interstellar medium photoionized by the AGN central continuum source. The soft X-ray extensions may be due to nuclear X-ray emission electron-scattered off the coronal-line gas.