Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

3.4.1. Signature for AGN versus Starburst Activity: The ISO Diagnostic Diagram

We discussed in Section 3.1.1 that AGN and starburst template galaxies have qualitatively very different spectral characteristics (Figure 4). In the "ISO SWS diagnostic diagram," 13 bright ULIRGs studied with SWS and ISOPHOT-S (Genzel et al 1998) are located between pure AGNs and pure starbursts (Figure 5a). They appear to be composite objects, but star formation dominates in most objects (especially when taking into account that the [OIV]/[NeII] line ratios are typically upper limits). In the simple mixing model shown in Figure 5a, the average bright ULIRG in the study of Genzel et al (1998) has a 30% AGN contribution with 70% coming from star formation. In 4 of the 13 bright ULIRGs, an energetically significant AGN is required by the measurements, either on the basis of a detection of strong high excitation lines (Mrk 273, NGC 6240), or on the basis of low UIB strength (Mrk 231, 05189-2524). In Mrk 231, 05189-2524, and Mrk 273 the AGN contribution may range between 40 and 80%.