Motivation
With the launch of the InfraRed Spectrograph (IRS) on the Spitzer Space Telescope, mid-infrared spectroscopists have been handed a powerful tool to study extragalactic objects at high signal-to-noise and over a wide mid-infrared wavelength range, previously only available for the study of Galactic sources and a few nearby galaxies.
Especially rich and varied are the spectra of the class of infrared galaxies with infrared luminosities exceeding 10^12 Lsun: UltraLuminous InfraRed Galaxies (ULIRGs). If powered entirely by star formation, ULIRGs are the most spectacular starburst galaxies in the universe, building up an entire population in a few short bursts. If, on the other hand, ULIRGs are partly powered by accretion onto a supermassive black hole, the study of ULIRGs would catch the active galactic nucleus (AGN) at its most enshrouded phase.
Now, 2.5 years into the Spitzer mission, the availability of a sizeable sample of high quality mid-infrared spectra of ULIRGs begs for a comparison to results obtained at other wavelengths and for other galaxies, especially the Milky Way.