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Cosmic Downsizing and Quasar Extinction

by Amy Barger

Chandra Deep Field North “true-color” X-ray image (2 million second exposure). Red denotes soft X-rays, green denotes hard X-rays, and blue denotes ultra-hard X-rays. Optical image taken at three wavelengths (B, R, and Z) that corresponds to the Chandra Deep Field North X-ray image above. Each color is a two-hour exposure with the wide-field Suprime-Cam camera on the Subaru Telescope. Note that we can detect many more objects emitting visible light than those emitting X-ray radiation.

When the Universe was young, the enormously luminous compact sources we call quasars were common in the cosmos. However, as time passed, these dinosaurs of the Universe died out. Trying to understand what caused this quasar extinction is a team of researchers that includes IfA astronomer Len Cowie, IfA graduate student Peter Capak, and myself, as well as researchers from other institutions.

Quasars outshine galaxies that contain hundreds of billions of stars. The only known mechanism that can explain their extreme brightness is the release of gravitational energy by matter falling toward a "supermassive" black hole—a black hole whose mass is millions to billions of times the mass of our Sun.

Prior to our research, scientists concluded from optical observations that the Universe went through a quasar era. That is, several billion years after the Big Bang, quasars were many times more numerous than they are today. Since supermassive black holes that powered distant quasar activity cannot be destroyed, scientists theorized that the local part of the Universe must be filled with "dead" quasars, or black holes that have exhausted their fuel supply. These dormant supermassive black holes have indeed been detected through the orbital motions of stars and gas that are affected by the strong gravitational field of a black hole.

These facts led astronomers to infer that supermassive black holes, formed during the quasar era, proceeded to consume all of the material surrounding them in a violent fit of growth, and then became fainter as their fuel supply ran out so they could no longer be detected.

This turns out not to be the full story. Many supermassive black holes in the distant Universe are cocooned in gas and dust, making them difficult to see. Thus, optical observations give us only a partial view into black hole evolution.

X-rays reveal the presence of active supermassive black holes, even when they are highly obscured. This is because high-energy X-rays are able to penetrate a dust cocoon relatively unhindered. With the launch of the Chandra X-ray Observatory by NASA in 1999, our team now has the ability, for the first time, to directly detect X-rays emitted during the supermassive black hole accretion process, even if the black hole is obscured.

After we detect obscured black holes at X-ray wavelengths using Chandra, the next step in our investigation is to determine how far away they are. To do this, we align our X-ray images with deep optical images obtained with the Subaru 8.2-meter (27-foot) Telescope on Mauna Kea (bottom image, this page) and identify the optical counterparts to the X-ray sources. We then obtain the spectra of the optical counterparts using the Keck 10-meter (32-foot) Telescopes (also on Mauna Kea) to determine the distances to the supermassive black holes and their level of accretion activity.

The astounding new result from our combined Chandra and optical observations is that supermassive black holes have assembled from the earliest times until the present, and not just during the quasar era, as previously assumed. Moreover, our Chandra observations have shown us that although the quasars that formed in the distant past have the most vigorous X-ray activity, at more recent times there is a more numerous population of moderately accreting sources whose combined light is comparable to that of the distant sources. In other words, the Universe has gone from a small number of bright objects to a large number of dim ones. Nowadays supermassive black holes are built smaller and cheaper, but the overall rate at which they accumulate mass is undiminished.

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The radiation that makes up the electromagnetic spectrum ranges from extremely short and energetic gamma rays on one end to very long radio waves on the other, with visible light somewhere in the middle. The shorter the wavelength is, the higher the frequency and the more energetic the radiation. The wavelengths of X-rays are measured in billionths of a meter (nanometers). "Hard" X-rays are shorter and more energetic than "soft" X-rays. The wavelengths of visible light are relatively long, 0.3 to 0.75 millionths of a meter. (A meter is about 39 inches.)


Amy Barger is an assistant professor in the Department of Astronomy, University of Wisconsin-Madison. As a visiting adjunct astronomer in the Department of Physics and Astronomy at the University of Hawaii, she spends part of each year at IfA, where she was formerly a postdoctoral fellow.