The X-ray background and the census of quasars

Abstract

The Chandra X-ray Observatory detects X-rays emitted during the accretion of matter onto supermassive black holes, even when they are highly obscured. A cosmic census of AGN contributing to the X-ray background is nearing completion. Follow-up observations with ground-based telescopes provide new knowledge about distant supermassive black holes. Here we discuss deep optical, near-infrared, submillimeter, and radio data, as well as high quality optical spectra, of a complete sample of twenty 2–10 keV X-ray sources selected in a deep Chandra observation of the Hawaii Deep Survey Field SSA13. The data allow us to estimate the duration and times of black hole activity. Surprisingly, the duty cycle is about a billion years, and accretion is still occurring at cosmologically recent times. These conclusions challenge conventional wisdom that supermassive black hole growth is only associated with host galaxy formation or violent galaxy mergers. With the optical, submillimeter, and radio data, we estimate the bolometric luminosities of the black holes and infer their masses. We find that the emitted bolometric energy from accretion onto supermassive black holes is about a third of the emitted energy from all the stars in the optical.