A starburst/AGN diagnostic for the Spitzer Space Telescope

Quantifying the contribution of massive star formation as well as the emission from an active nucleus to the bulk of the bolometric luminosity of a galaxy is one of the outstanding questions in extragalactic astronomy. Addressing this issue in IR luminous systems is of a particular interest since it has been shown that luminous and ultraluminous IR galaxies are responsible for nearly half of the cosmic IR background. The unprecedented sensitivity of the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope will enable us for the first time to obtain 5 to 38 micron spectra of IR luminous galaxies out to z 2. We have developed an AGN/starburst diagnostic based on IRS spectroscopy and 3-4 micron IRAC imaging of several well known nearby starburst and Seyfert galaxies. We will present our preliminary findings on the application of this diagnostic to a number of near and distant IR luminous systems observed with Spitzer.