Dimitra Rigopoulou 1, D. Lutz 1, R. Genzel 1, H.W.W. Spoon 1, D. Tran 1& A. Moorwood 2
1 Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany
2 ESO, Garching, Germany
We report results of a low resolution mid-infrared spectroscopic
survey of an unbiased sample of 70 Ultraluminous Infrared Galaxies (ULIRGs
with
), carried out with ISOPHOT-S on board the
Infrared Space
Observatory ISO.
We find that the ratio of the 7.7 micron emission feature, which is usually
ascribed to polycyclic aromatic hydrocarbons (PAH), to the local continuum
can
be used as an indicator of the dominant powering source in ULIRGs: Starburst
or AGN.
Our results indicate that the majority of the ULIRGs, about 80%, are
starburst
powered. A shift towards AGN powered appears to occur at higher luminosities.
Only about 15% of ULIRGs are AGN powered at luminosities below
,
while about half at higher luminosities.
The observed ratios of PAH features in ULIRGs are only slightly different than those in starbursts. The differences could be attributed to higher extinction and the physical conditions in the interstellar medium of ULIRGs.
Finally, we investigate whether there is a trend for advanced merger ULIRGs to be more AGN-like, as postulated by the classical evolutionary scenario for ULIRGs. We find that no such trend exists in the current dataset, however, we are complementing the existing data with further high resolution near-infrared imaging.