The bulk of the hard X-ray background has been resolved into discrete sources by ultradeep surveys with the Chandra X-ray Observatory. We have carried out multiwavelength follow-up observations of the active galactic nuclei that make up the background to ascertain their nature. With the addition of optical, submillimeter, and radio data, the bolometric luminosities of the sources can be estimated and used to map the history of supermassive black hole accretion from the earliest times to the present.
Amy J Barger,
"Accretion history of supermassive black holes", Proc. SPIE 4835, Future Research Direction and Visions for Astronomy, (16 December 2002); doi: 10.1117/12.456501; https://doi.org/10.1117/12.456501