WHAT POWERS ULTRALUMINOUS IRAS GALAXIES

We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxies (L(IR)greater than or equal to 10(1 2) L.). We combine the survey results with a detailed case study, base d on arcsecond resolution, near-IR, and millimeter imaging spectroscop y, of one of the sample galaxies (UGC 5101). We compare the near-and m id-IR characteristics of these ultraluminous galaxies to ISO and liter ature data of 30 starburst and active galactic nuclei (AGN) template g alaxies. We find the following: 1. Of the ultraluminous IRAS galaxies in our sample, 70%-80% are predominantly powered by recently formed ma ssive stars, and 20%-30% are powered by a central AGN. These conclusio ns are based on a new infrared ''diagnostic diagram'' involving the ra tio of high-to low-excitation mid-IR emission lines on the one hand, a nd the strength of the 7.7 Can PAH feature on the other hand. 2. At le ast half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circumnuclear disk/ring. 3. The mid-IR emitting regions are highly obscured [A(v)(screen) simi lar to 5-50 or A(v)(mixed) similar to 50-1000]. In a model where star- forming regions and dense molecular clouds are fully mixed, the ISO-de rived, V-band dust extinctions approach the dust column densities infe rred from CO millimeter measurements. After correction for these extin ctions, we estimate that the star-forming regions in ultraluminous inf rared galaxies have ages between 10(7) and 10(8) yr, similar to but so mewhat larger than those found in lower luminosity starburst galaxies. 4. In the sample we have studied there is no obvious trend for the AG N component to dominate in the most compact, and thus most advanced me rgers. Instead, at any given time during the merger evolution, the tim e-dependent compression of the circumnuclear interstellar gas, the acc retion rate onto the central black hole, and the associated radiation efficiency may determine whether star formation or AGN activity domina tes the luminosity of the system.