What Powers Ultraluminous IRAS Galaxies?

R.Genzel, D.Lutz, E.Sturm, E.Egami, D.Kunze, H.W.W.Spoon (MPE)
A.F.M.Moorwood (ESO), A.Sternberg (Tel Aviv)

We have obtained ISO-SWS and ISOPHOT-S spectroscopy of fifteen ultraluminous IRAS galaxies (ULIRGs) and compare their characteristics to those of thirty starburst and AGN template galaxies. Evidence from various wavelength regimes has suggested the presence of both AGNs and strong star formation in many ULIRGs, but their relative importance remained unclear. ISO spectra provide two new diagnostics which can distinguish whether starburst or AGN powering prevails in these highly dust-obscured galaxies: >From a new diagnostic diagram involving this ratio of high and low excitation fine structure lines and the PAH line-to-continuum ratio, we find that 70 to 80% of the ultraluminous IRAS galaxies are predominantly powered by recently formed massive stars. The diagram is displayed below, with squares representing AGNs, triangles starbursts, and filled circles ULIRGs. The mid-infrared emitting regions are highly obscured. In a model where star forming regions and molecular clouds are fully mixed, extinctions approach average values inferred from CO mm measurements. After correcting for these extinctions, we estimate ages for the star forming regions of ULIRGs of 107 to 108, similar but somewhat larger than those found in lower luminosity starbursts.

Lutz, D., Genzel, R., Sternberg, A., et al.: Astron. Astrophys. 315, L133-L136 (1996).
Genzel, R., Lutz, D., Sturm, E., et al., submitted to ApJ.

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