FROM THE FIRST MASSIVE BLACK HOLES

X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field. INTRODUCTION AND IMPORTANCE OF HIGH-REDSHIFT X-RAY STUDIES Understanding of the X-ray emission from z > 4 active galactic nuclei (AGN) has advanced rapidly over the past few years. The high sensitivities of Chandra and XMM-Newton have allowed the efficient detection of many z > 4 AGN, and wide-field AGN surveys (e.g., the Sloan Digital Sky Survey, hereafter SDSS; York et al. 2000) have greatly enlarged the number of suitable X-ray targets. The number of X-ray detections at z > 4 has increased to 57 (see Figure 1a).1 This increase has allowed the first reliable X-ray population studies of z > 4 AGN, and it has been possible to obtain X-ray detections at redshifts up to z = 6.28 (e.g., Brandt et al. 2002a). The X-ray emission from AGN provides direct information about their black hole regions, where accretion and black hole growth occur; the emission is thought to be produced by the inner accretion disk and its corona (e.g., Poutanen 1998). Do the early black holes seen at high redshift feed and grow in the same way as local ones? X-ray observations of high-redshift AGN can address this fundamental question. It is plausible that high-redshift AGN could be feeding and growing differently. The comoving number density of luminous quasars changes by a factor of ∼ 100 over the history of the Universe (e.g., Figure 9 of Fan et al. 2001 and references therein), and part of this strong evolution is believed to be due to environmental changes that might also impact the X-ray emission region. There have also been theoretical predictions that AGN accretion rates, relative to the Eddington rate, should change with redshift (e.g., Kauffmann and Haehnelt 2000); such changes can lead to accretion-disk instabilities (e.g., Gammie 1998 and references therein) and radiation “trapping” effects (e.g., Begelman 1979). X-ray absorption measurements can also be used to probe the large-scale environments of high-redshift AGN. Changes in the amount of X-ray absorption with redshift have been discussed by many authors. The fraction of radio-loud quasars (RLQs) with heavy X-ray absorption appears to rise with redshift, with column densities of ∼ 2 × 10 22 cm−2 being seen at z ∼ 3 (e.g., Elvis et al. 1998; Fiore et al. 1998; Reeves and Turner 2000). The absorbing gas may be circumnuclear, located in the host galaxy, or entrained by the radio jets. Radio-quiet quasars (RQQs) definitely show less of an absorption increase with redshift than do See http://www.astro.psu.edu/users/niel/papers/highz-xray-detected.dat for a regularly updated list of z > 4 X-ray detections.