Simulating the Spitzer mid-infrared color-color diagrams

We use a simple parameterization of the mid-IR spectra of a wide range of galaxy types in order to predict their distribution in the Infrared Array Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 mum and Multiband Photometer for Spitzer 24 mum color-color diagrams. We distinguish three basic spectral types by the energetically dominant component in the 3-12 mum regime: stellar-dominated, polycyclic aromatic hydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markov chain Monte Carlo approach to arrive at a more systematic and robust representation of the mid-IR spectra of galaxies than do more traditional approaches. We find that IRAC color-color plots are well suited to distinguishing the above spectral types, while the addition of 24 mum data allows us to suggest practical three-color cuts that preferentially select higher redshift sources of a specific type. We compare our simulations with the color-color plot obtained by the Spitzer First Look Survey and find reasonable agreement. Lastly, we discuss other applications as well as future directions for this work.