A POPULATION OF FAINT GALAXIES THAT CONTRIBUTE TO THE COSMIC X-RAY-BACKGROUND

The origin of the cosmic X-ray background radiation(1,2) has remained mysterious since its discovery(3) thirty-five years ago. Investigation of its origin has been difficult because instruments have had insuffi cient resolution to distinguish small, faint sources in the hard X-ray band (above 2 keV) that dominates the background. Until now, only thr ee per cent of the flux in the 2-10 keV band could be attributed to in dividual sources(4,5). Here we report the results of a survey 100 time s more sensitive than previous studies in the 2-10 keV band. We find m any faint resolved sources, whose integrated flux accounts for 30 per cent of the X-ray background in this energy range. The average spectru m of the resolved sources is harder than those of nearby bright active galactic nuclei and is dose to the spectrum of the X-ray background r adiation. This means that a new dass of sources, with hard X-ray spect ra, dominate the sky at photon energies above 2 keV.