The Spitzer Warm Mission:: Prospects for studies of the distant universe

IRAC excels at detecting distant objects. Due to a combination of the shapes of the spectral energy distributions of galaxies and the low background achieved from space, IRAC reaches greater depth in comparable exposure time at 3.6 and 4.5 mu m than any ground- or space-based facility currently can at 2.2 mu m. Furthermore, the longer wavelengths probed by IRAC enable studies of the rest-frame optical and near-infrared light of galaxies and AGN to much higher redshift than is possible from the ground. This white paper explores the merits of different survey strategies for studying the distant universe during the warm mission. A three-fiered approach serves a wide range of science goals and uses the spacecraft effectively: 1) an ultra-deep survey of approximate to 0.04 square degrees to a depth of 250 hrs (in conj unction with an HST/WFC3 program), to study the Universe at 7 < z < 14; 2) a survey of 2 square degrees to the GOODS depth of 20 hrs, to identify luminous galaxies at z > 6 and characterize the relation between the build-up of dark matter halos and their constituent galaxies at 2 < z < 6, and 3) a 500 square degree survey to the SWIRE depth of 120 s, to systematically study large scale structure at 1 < z < 2 and characterize high redshift AGN. One or more of these programs could conceivably be implemented by the SSC, following the example of the Hubble Deep Field campaigns. As priorities in this field continuously shift it is also crucial that a fraction of the exposure time remains unassigned, thus enabling science that will reflect the frontiers of 2010 and beyond rather than those of 2007.