PRINCIPAL COMPONENT ANALYSIS OF 2 ULTRAVIOLET EMISSION-LINES IN 18 ACTIVE GALACTIC NUCLEI

We apply a principal component analysis (PCA) to the spectra of each o f the 18 Seyfert 1-like objects observed more than 15 times by the int ernational ultraviolet explorer (IUE) from 1978 until the end of 1991. PCA allows us to decompose the Ly alpha and C IV lambda 1549 emission -lines of each object into two components with uncorrelated variations . We find that the principal component describes correlated continuum and line variations, whereas the rest component reveals the line-parts that do not vary in tune with the continuum. A cross-correlation anal ysis in NGC 5548 reveals that variable line features in the rest compo nent follow the continuum by about 25 days. The symmetry usually obser ved in the component's line profiles excludes that the velocity field of the line-emitting region is dominated by radial motion. In some obj ects, the principal component reveals a clearly double-peaked line pro file. The results suggest that the variable line feature in the princi pal and the rest component is respectively emitted in the inner and ou ter parts of an extended broad-line region (BLR). The fraction of the line that varies together with the continuum seems to decrease signifi cantly with increasing luminosity. We interpret this trend as a conseq uence of a larger BLR in more luminous objects. Finally, we propose a solution to the C IV/Ly alpha ratio problem without referring to optic ally thin emission-line clouds.