Parametric modeling of the 3.6-8 mu m color distributions of galaxies in the swire survey

摘要

We fit a parametric model comprising a mixture of multidimensional Gaussian functions to the 3.6-8 mu m 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-a 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 C-b, in which polycyclic aromatic hydrocarbon emission dominates at 8 mu m, making these sources very red in longer wavelength Infrared Array Camera (IRAC) colors. Population C-c 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 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 highmid-infrared emission may be attributed to either dust heated by the AGNs or substantial star formation. These sources have z(ph) similar to 2-4, making them incredibly infrared-luminous, with L-IR similar to 10(12.6) - 10(14.1) L-circle dot.