The First Stars and Quasars in the Universe

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 >10^3 star clusters per square arcminute at z>10. The brightest sources, however, might be early quasars. The infrared flux from an Eddington luminosity, 10^6 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.