Probing the Cosmic Dark Age in X-rays
  Empirical studies of the first generation of stars and quasars will likely become feasible within the next decade in several different wavelength bands. Microwave anisotropy experiments, such as MAP or Planck, will set constraints on the ionization history of the intergalactic medium due to these sources. In the infrared, the Next Generation Space Telescope (NGST) will be able to directly detect sub-galactic objects at redshifts z>10. In the optical, data from the Hubble Deep Field already places a constraint on the abundance of high-redshift quasars. However, the epoch of the first quasars might be first probed in X-ray bands, by instruments such as the Chandra X-ray Observatory (CXO) and the X-ray Multi-mirror Mission (XMM). In a 500 Ksec integration, CXO reaches a sensitivity of 2x10-16 erg/s cm^2. Based on simple hierarchical CDM models, we find that at this flux threshold approx. 100 quasars might be detectable from redshifts z>5, and approx. 1 quasar at z=10, in each 17x17 arcmin field. Measurement of the power spectrum of the unresolved soft X-ray background will further constrain models of faint, high-redshift quasars. 
Zoltan Haiman (Princeton University, Fermilab)