Parametric Modeling of the 3.6-8 μm Color Distributions of Galaxies in the SWIRE Survey

We fit a parametric model comprising a mixture of multidimensional Gaussian functions to the 3.6-8 μm color and optical photometric redshift distribution of galaxy populations in the ELAIS-N1 and Lockman Fields of the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy survey. For 16,698 sources in ELAIS-N1 we find our data are best modeled (in the sense of the Bayesian information criterion) by the sum of four Gaussian distributions or modes (Ca, Cb, Cc, and Cd). We compare the fit of our empirical model with predictions from existing semianalytic and phenomenological models. We infer that our empirical model provides a better description of the mid-infrared color distribution of the SWIRE survey than these existing models. This color distribution test is thus a powerful model discriminator and is entirely complementary to comparisons of number counts. We use our model to provide a galaxy classification scheme and explore the nature of the galaxies in the different modes of the model. Population Ca is found to consist of dusty star-forming systems such as ultraluminous infrared galaxies, over a broad redshift range. Low-redshift late-type spiral galaxies are found in population Cb, in which polycyclic aromatic hydrocarbon emission dominates at 8 μm, making these sources very red in longer wavelength Infrared Array Camera (IRAC) colors. Population Cc consists of dusty starburst systems with high levels of star formation activity at intermediate redshifts. Low-redshift early-type spiral and elliptical galaxies are found to dominate population Cd. We thus find a greater variety of galaxy types than one can with optical photometry alone. Finally, we develop a new technique to identify unusual objects and find a selection of outliers with very red IRAC colors. These objects are not detected in the optical but have very strong detections in the mid-infrared. These sources are modeled as dust-enshrouded, strongly obscured active galactic nuclei (AGNs), for which the high mid-infrared emission may be attributed to either dust heated by the AGNs or substantial star formation. These sources have zph ∼ 2-4, making them incredibly infrared-luminous, with LIR ∼ 1012.6-1014.1 L⊙.