Observational signatures of the first quasars

We study the observational signatures of a potential population of low-luminosity quasars at high redshifts in a ΛCDM cosmology. We derive the evolution of the quasar luminosity function at fainter luminosities and higher redshifts than currently detected based on three assumptions: (1) the formation of dark matter halos follows the Press-Schechter theory, (2) the ratio of central black hole mass to halo mass is the same for all halos, and (3) the light curve of quasars, in Eddington units, is universal. We show that a universal light curve provides an excellent fit to the observed quasar luminosity function at redshifts 2.6 < z < 4.5. By extrapolating the evolution of this luminosity function to higher redshifts (4.5 < z < 20), we find that the associated early population of low-luminosity quasars reionizes the universe at a redshift z ~ 12. The reprocessing of the UV light of these quasars by dust from early Type II supernovae distorts the microwave background spectrum by a Compton y-parameter y ~ 10-5, comparable to the lower limit set by COBE. The Next Generation Space Telescope could detect tens of quasars per arcmin-2 from redshifts z > 10 with its proposed 1 nJy sensitivity at 1-3.5 μm. Absorption spectra of several such quasars would reveal the reionization history of the universe.