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A multiwavelength study of near- and mid-infrared selected galaxies at high redshift: ERGs, AGN-identification and the contribution from dust

The main focus of this thesis is the IR spectral regime, which since the 70's and 80's has revolutionised our understanding of the Universe. A multi-wavelength analysis on Extremely Red Galaxy populations is first presented in one of the most intensively observed patch of the sky, the Chandra Deep Field South. By adopting a purely statistical methodology, we consider all the photometric and spectroscopic information available on large samples of Extremely Red Objects (EROs, 553 sources), IRAC EROs (IEROs, 259 sources), and Distant Red Galaxies (DRGs, 289 sources). We derive general properties: redshift distributions, AGN host fraction, star-formation rate densities, dust content, morphology, mass functions and mass densities. The results point to the fact that EROs, IEROs, and DRGs all belong to the same population, yet seen at different phases of galaxy evolution. The second part of this thesis is dedicated to the AGN selection in the IR, with particular relevance to the James Webb Space Telescope, to be launched in 2018. We develop an improved IR criterion (using K and IRAC bands) as an alternative to existing IR AGN criteria for the z<2.5 regime, and develop another IR criterion which reliably selects AGN hosts at 0<z<7 (using K, Spitzer-IRAC, and Spitzer-MIPS24um bands, KIM). The ability to track AGN activity since the end of reionization holds great advantages for the study of galaxy evolution. The thesis then focus on the importance of dust. Based on deep IR data on the Cosmological Survey, we derive rest-frame 1.6, 3.3, and 6.2um luminosity functions and their dependency on redshift. We estimate the dust contribution to those wavelengths and show that the hot dust luminosity density evolves since z=1-2 with a much steeper drop than the star-formation history of the Universe. Future prospects are finally discussed in the last chapter.