Abstract
X-rays probe the inner regions of active galactic nuclei (AGN) and can escape almost unimpeded from regions where the optical-ultraviolet emission is heavily reddened. Surveys in X-rays are the most efficient in detecting accretion-powered sources (e.g., AGN) and also detect clusters of galaxies unambiguously. The hard spectrum of the extragalactic X-ray background (produced by the superposition of emission from many AGN) indicates that up to 85% of the accretion power in the Universe is absorbed. Extragalactic surveys with XMM-Newton and Chandra probe the history of accretion in the Universe. The extragalactic X-ray background has been resolved with these observatories into three different types of objects in approximately equal numbers: objects with AGN signatures in their optical spectra, sources without them (luminous early galaxies, some of them probably hidden AGN), and I > 23.5 spectroscopically unidentified objects with colors compatible with evolved early galaxies. These identifications require 10 m class telescopes. The source densities of these surveys are ideal for multiobject optical spectrographs (such as OSIRIS). Near-IR cameras and spectrographs (such as EMIR) are also required, especially for those X-ray souces with optically faint or nonexistent counterparts.
