We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxiesWe combine the survey results with a detailed case study, based on arc-(L IR º 1012 L _ ). second resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near-and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We Ðnd the following :

Using a sample of ∼28,000 sources selected at 3.6-4.5 microns with Spitzer observations of the HDF-N, the CDF-S, and the Lockman Hole (surveyed area: ∼664 arcmin 2 ), we study the evolution of the stellar mass content of the Universe at 010 12.0 M ⊙ ) assembled the bulk of their stellar content rapidly (in 1-2 Gyr) beyond z∼3 in very intense star formation events (producing high specific SFRs). Galaxies with 10 11.5 2.5 is dominated by optically faint (R 25) red galaxies (Distant Red Galaxies or BzK sources) which account for ∼30% of the global population of galaxies, but contribute at least 60% to the cosmic stellar mass density. Bluer galaxies (e.g., Lyman Break Galaxies) are more numerous but less massive, contributing less than 50% to the global stellar mass density at high redshift.

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We present a first analysis of deep 24 m observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0:65 z < 0:75. We combine the infrared data with redshifts, rest-frame luminosities, and colors from COMBO-17 and with morphologies from Hubble Space Telescope images collected by the Galaxy Evolution from Morphology and SEDs (GEMS) and Great Observatories Origins Deep Survey (GOODS) projects. To characterize the decline in star formation rate (SFR) since z $ 0:7, we estimate the total thermal IR luminosities, SFRs, and stellar masses for the galaxies in this sample. At z $ 0:7, nearly 40% of intermediate-and high-mass galaxies (with stellar masses !2 ; 10 10 M ) are undergoing a period of intense star formation above their past-averaged SFR. In contrast, less than 1% of equally massive galaxies in the local universe have similarly intense star formation activity. Morphologically undisturbed galaxies dominate the total infrared luminosity density and SFR density: at z $ 0:7, more than half of the intensely star-forming galaxies have spiral morphologies, whereas less than $30% are strongly interacting. Thus, a decline in major merger rate is not the underlying cause of the rapid decline in cosmic SFR since z $ 0:7. Physical properties that do not strongly affect galaxy morphology-for example, gas consumption and weak interactions with small satellite galaxies-appear to be responsible.

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