Wallowing in Dust The rest of the AGNs, however, show only narrow emission lines or no emission lines at all—suggesting that gas and dust block our view of their central black holes. They are classified as type 2 quasars or Seyfert-2 galaxies. The existence of a second type makes sense in the framework of the “unified model” for AGNs. Proposed in the mid-1980s, the unified model posits that all AGNs contain not only a central black hole but also a torus of gas and dust. Depending on how this torus is oriented, it can hide the black hole. The model has since been updated, but the basic prediction has stayed the same: we perceive either an unobscured (type 1) or an obscured (type 2) AGN.
Although these soft-x-ray surveys showed that AGNs are the dominant sources of the x-ray background, an apparent paradox emerged as astronomers began to employ their second strategy to understand the background—namely, population synthesis. When astronomers added together the spectra of different types of AGNs according to their observed proportions, the result should have equaled the spectrum of the CXB. It did not. AGN spectra have a flat or bowl-like shape, whereas the CXB spectrum has a peak at 30 keV.
A solution to this discrepancy was proposed in 1989 by Giancarlo Setti of Bologna University in Italy and Lo Woltjer of Haute-Provence Observatory in France, who at that time were working together at the European Southern Observatory in Garching. They hypothesized that population-synthesis modeling had not added the AGNs in their correct proportions. Contrary to what people had thought, most sources of the x-ray background could be type 2 AGNs. Higher-energy (so-called hard) x-rays can penetrate the dust and gas around these black holes, whereas the soft x-rays are absorbed. In this way, the overall CXB spectrum would differ from that of bright AGNs.
Picking up on this idea, population-synthesis modelers sought the right mixture of type 1 and type 2 AGNs that would explain the CXB spectrum, taking into account how these objects might evolve over time. As shown in 1995 by Andrea Comastri, then at the Max Planck Institute for Extraterrestrial Physics, and his co-workers, such models can reproduce the spectrum up to about 300 keV if the vast majority—80 to 90 percent—of the energy produced by black holes is veiled by thick clouds of gas and dust. If so, these beasts were 100 times more abundant in the early universe than today—a figure consistent with their forming in almost all galaxies. They might have gone unnoticed were it not for the cosmic x-ray background.
A related paradox concerns the optical and infrared backgrounds (the COB and CIB, respectively). The COB is most likely the summed emission of stars, redshifted as the universe expands. The CIB, on the other hand, has the spectrum of dust at a temperature of 10 to 100 kelvins, also redshifted. The energy represented by the dust emission must ultimately originate in stars and AGNs. Yet the CIB is as bright as or brighter than the COB. It is as though the moon (which merely reflects sunlight) were brighter than the sun (the source of that light). The logical resolution of this paradox, like that of the xray paradox, is that a substantial fraction of radiation sources in the universe is shrouded by gas and dust.
To confirm these inferences, astronomers have been studying the background radiation at wavelengths that would be unaffected by any obscuring material—namely, hard x-rays. This potent radiation passes through dust as though the dust were not even there. The two great new x-ray observatories now in orbit, the Chandra X-ray Observatory (with superb angular resolution) and XMM-Newton (with a large telescope area), have extended the band covered by ROSAT to substantially higher energies, up to 10 keV, though not yet to the peak of the x-ray background. The most sensitive x-ray surveys to date have been performed with Chandra in two sky areas, the Chandra Deep Field South and the Hubble Deep Field North, by the groups led by Giacconi, who is now at Johns Hopkins University, and by Gordon P. Garmire of Pennsylvania State University. These surveys have resolved at least 80 percent of the hard x-ray background.
The optical matchup work has just started. So far the sources are a mixture of type 1 and type 2 AGNs, in excellent agreement with the models. Interestingly, about 10 percent of the x-ray sources discovered by Chandra are very faint galaxies—presumably normal galaxies that contain no AGNs. Their x-ray emission is associated mainly with gas heated by star formation.