The UV spectrum of PKS 2251+113 and physical conditions in the broad line region.

Abstract

The IUE satellite's UV spectrum of PKS 2251+113 is discussed, in view of previous IR, optical and X-ray observations which indicate that one cannot fit a single power law to the entire continuum. The values of the intensity ratios of Lyman-alpha to Balmer lines and Si II 1196/1263 indicate that PKS 2251+113 exemplifies objects for which the Broad Line Region must be explained in terms of a composite model. In such a model, there is a standard photoionized region in which the lines of H I (Lyman) and the high ionization lines of C III, C IV, He II, etc, are emitted. Most of the Balmer line and low ionization line emissions come from a physicaly distinct, collisionally ionized and excited region.