2006AJ....132.1818D

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 (C_a, C_b, C_c, and C_d). 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 C_ais 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 C_b, in which polycyclic aromatic hydrocarbon emission dominates at 8 µm, making these sources very red in longer wavelength Infrared Array Camera (IRAC) colors. Population C_cconsists 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 C_d. 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 z_ph∼2-4, making them incredibly infrared-luminous, with L_IR∼10^12.6-10^14.1 L_☉.