A large mid-infrared spectroscopic and near-IR imaging survey of ULIRGs Their nature and evolution

We present a low resolution mid-infrared spectroscopic survey of an unbiased sample of 62 Ultraluminous Infrared Galaxies (ULIRGs) (L_I_R>10"1"2 L_sun, z#<=#0.3) using ISOPHOT-S on board the Infrared Space Observatory (ISO). For comparison we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGN) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 #mu#m PAH emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, L_P_A_H/L_I_R, for starburst dominated ULIRGs is very similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. (orig.)SIGLEAvailable from TIB Hannover: RN 9303(476) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman