Hidden in the Background To use the background radiation as a quality-control check, astronomers have had to develop ways to compare what is measured with what is expected. That is no easy task. The background represents a tangled mixture of light from various classes of astronomical objects. Starlight, which is produced by thermonuclear fusion, is mainly confined to near-infrared, optical and ultraviolet wavelengths. Quasars and other active galactic nuclei (AGN), whose black holes suck in matter and efficiently convert its gravitational energy into radiation, shine in a very broad band, from radio to gamma wavelengths. Clouds of dust absorb optical, ultraviolet and x-ray light and reradiate the energy in the far-infrared. To complicate matters further, the background blends together light from objects at vastly different cosmic distances and evolutionary stages.
One strategy is to conduct intensive surveys of the sky—to make observations with the highest possible resolution and sensitivity and thereby get a fix on the specific sources of the background. By comparing the findings made at different wavelengths, we can determine what kind of objects these sources are. This direct approach, however, can achieve the requisite precision only for relatively bright objects in very limited areas of the sky. For the broader picture, we turn to a second technique known as population synthesis: calculate the expected emission from possible combinations of objects, compare this prediction with the background measurements, and continue trying different combinations until one seems to fit.
Because the CXB was the first known background, it has been studied more than the other background components. The most basic question—does the CXB come from unresolved sources or a hitherto unknown type of diffuse gas?—was debated for three decades [see “The Origin of the Cosmic X-ray Background,” by Bruce Margon; Scientific American, January 1983]. In the 1990s an indirect line of argument finally settled the issue. If the CXB comes from hot intergalactic gas, the gas should also act as a screen that distorts our view of the cosmic microwave background. The spectrum of the CMB would then deviate from that of a perfect blackbody. Yet CMB observations, notably by the Cosmic Background Explorer satellite, saw no such deviation. Therefore, only a small fraction of the x-ray background can come from such gas; cooler gas might contribute, but for the most part, the CXB must represent unidentified discrete sources.
But what could these sources be? The first intensive surveys to answer this question were performed in the early 1980s with the Einstein x-ray satellite (HEAO-2) by Riccardo Giacconi, the discoverer of the CXB, and others. They resolved about a fifth of the x-ray background into discrete sources, including quasars. The ROSAT satellite followed up this work. In 1984 a group of scientists consisting of Giacconi, Maarten Schmidt (the discoverer of quasars), Joachim Trümper (the father of ROSAT) and one of us (Hasinger) met at the Max Planck Institute for Extraterrestial Physics in Garching, Germany, to start planning for deep surveys with that satellite. After the ROSAT launch in1990, the surveys became a major enterprise lasting over a decade and involving a large number of co-workers, more than we could possibly list here.
The ROSAT Deep Surveys of the so-called Lockman Hole—an area close to the Big Dipper that is almost free from foreground absorption—are among the longest and deepest x-ray-plus-optical observations ever performed. They have resolved 80 percent of the x-ray background at energies of less than 2 keV, a range that astronomers call soft x-rays. The main bottleneck has been making the optical identifications. We have to look for counterparts of the x-ray sources on deep optical images, and often they are extremely faint. Then we have to obtain their spectra, which reveal the properties of the objects as well as their redshift, a measure of distance. This work would not be possible without the giant Keck telescope, but even its 10-meter mirror has trouble collecting enough light to measure the spectra of the faintest optical counterparts.
About 80 percent of the ROSAT sources have turned out to be active galactic nuclei of various kinds—mostly luminous quasars and so-called Seyfert-1 galaxies. The broad emission lines in the spectra of these AGNs indicate that we have a clear view into their innermost regions, where the monstrous black holes are gorging themselves.