Observations of ULIRGs with the IRS on Spitzer

Ultraluminous Infrared Galaxies (ULIRGs), those systems with L_(IR) ≥ 10^(12)L_⊙, have the power output of quasars, yet (re-)emit the vast majority of their energy in the far-infrared part of the spectrum. Because this energy is reprocessed by dust, the origin of the power is often hidden from view. With the Infrared Spectrograph on the Spitzer Space Telescope, we have observed a large number of low-redshift ULIRGs in order to better understand the source of their power, the conditions in the dusty interstellar medium, and their relationship to optically selected quasi-stellar objects (QSOs), effectively increasing the number of ULIRGs with rest-frame mid-infrared spectra by two orders of magnitude. The IRS has also been used to obtain the first mid-infrared spectra of samples of ULIRGs at redshifts of z > 2. In this paper, we summarize the key findings from the IRS studies of ULIRGs at low and high redshifts, and look toward the future of far-infrared spectroscopy from space.