The impact of star formation and active nuclei on the interstellar medium in ultraluminous infrared galaxies

The energy input into the interstellar medium in Ultraluminous Infrared Gal axies (ULIRGs) is enormous, regardless of the nature of the power source. I discuss some of the major consequences for the structure and energetics of the ISM in these galaxies. Observationally, the column densities in the nu clear regions of ULIRGs are known to be very high, which makes distinguishi ng starbursts from AGN quite difficult. The level of energy and momentum in jection means that the pressure in the ISM must be extremely high, at least 3-4 orders of magnitude larger than in the local ISM or typical giant mole cular clouds. It also means that the luminosity of GMCs in ULIRGs must be v ery high, as they must radiate many times their binding energy over their l ifetimes. I briefly review the influence which X-ray irradiation can have o n the ISM in AGN-powered ULIRGs. Finally, I show that the presence of PAH f eatures in ULIRGs does not imply that they must be starburst-dominated, sin ce at the column densities and pressures typical of the ISM in ULIRGs PAHs can survive even at tens of parsec distances from the AGN.