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We use the ratio of the 7.7 µm `PAH' emission feature to the local continuum as a discriminator between starburst and AGN activity. About 80 % of all the ULIRGs are found to be predominantly powered by star formation but the fraction of AGN powered objects increases with luminosity. Whereas only about 15 % of ULIRGs at luminosities below 2 x 1012 Lsun are AGN powered this fraction increases to about half at higher luminosity.
Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs.
The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9 µm continuum to the IRAS 60 µm continuum, confirming suggestions that strong mid-IR continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-SWS spectroscopy which implies significant extinction even in the mid-infrared.
We have searched for indications that ULIRGs which are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found amongst those objects for which near infrared images are available to assess their likely merger status.
Keywords: galaxies, galaxies: starburst, galaxies:active
Status: to appear in ApJ Letters issue 1.Oct.98 (accepted)
Address: mailto: lutz@mpe.mpg.de
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Last update: 01-Sep-1998 by D. Kunze