The First Stars and Quasars in the Universe

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The transition between the nearly smooth initial state of the Universe and its clumpy state today occurred during the epoch when the first stars and low-luminosity quasars formed. For Cold Dark Matter cosmologies, the radiation produced by the first baryonic objects is expected to ionize the Universe at z=10-20 and consequently suppress by 10% the amplitude of microwave anisotropies on angular scales <10 degrees. Future microwave anisotropy satellites will be able to detect this signature. The production and mixing of metals by an early population of stars provides a natural explanation to the metallicity, 1% solar, found in the intergalactic medium at redshifts z<5. The Next Generation Space Telescope (NGST) will be able to image directly the ``first light'' from these stars. With its nJy sensitivity, NGST is expected to detect >10³ star clusters per square arcminute at z>10. The brightest sources, however, might be early quasars. The infrared flux from an Eddington luminosity, 10⁶ solar mass, black hole at z=10 is 10 nJy at 1 micron, easily detectable with NGST. The time it takes a black hole with a radiative efficiency of 10% to double its mass amounts to more than a tenth of the Hubble time at z=10, and so a fair fraction of all systems which harbor a central black hole at this redshift would appear active. The redshift of all sources can be determined from the Lyman-limit break in their spectrum, which overlaps with the NGST wavelength regime, 1-3.5 micron, for 10<z<35. Absorption spectra of the first generation of star clusters or quasars would reveal the reionization history of the Universe. The intergalactic medium might show a significant opacity to infrared sources at z>10 due to dust produced by the first supernovae.