Virtually all large galaxies have massive black holes at their center, or nucleus. About 5-10 percent of these galaxies are currently accreting matter at a high rate. In-spiraling gas piles up into a hot “accretion disk” near the edge of the black hole. These are known as “active galactic nuclei,” the brightest of which are also known as “quasars.” Quasar accretion disks can be very luminous and even outshine an entire normal galaxy of stars.
Quasars are bright at all wavelengths, from X-rays to radio. The hot accretion disks can thus be detected at very large distances, making them unique beacons that can be used to probe the farthest reaches of the universe. Studying the detailed structure of accretion disks and other matter in the vicinity of the black hole is difficult, however, because of their small sizes and great distances.
While the accretion disk and surrounding structures of active galaxy nuclei are too small to be imaged by any telescope, CCAPP scientists make use of the fact that the brightness of light from the accretion disk varies with time. Since the ultraviolet light from the accretion disk causes nearby gas clouds to glow, the brightness of the gas clouds also varies, following the same patterns of variation seen in the accretion disk, but with a time delay that how long it takes light to travel between the accretion disk and the gas clouds. By measuring this delay, we can determine how the gas clouds are arrayed around the accretion disk. This technique is known as “reverberation mapping,” as the light from the clouds of gas appears to echo—or reverberate—the brightness variations of the accretion disk. By combining this information with the motions of the gas clouds, CCAPP scientists can estimate the masses of the central black holes.
Reverberation mapping requires a long series of observations, sometimes lasting months or years, to measure the changes in the brightness of the accretion disk and the surrounding gas. CCAPP scientists and students recently led a large-scale reverberation-mapping program with the NASA-ESA Hubble Space Telescope, NASA’s Swift Observatory and an international network of ground-based telescopes. Observations were made at least a daily for six months. The data are still being analyzed; however, they have already yielded unprecedented detail about the structure of the accretion disk and the distribution of gas clouds around the central 70 million solar mass black hole in the active galaxy NGC 5548.
CCAPP scientists are working on innovative ways to use reverberation mapping to extend the measurement of black hole masses in active galactic nuclei across the universe. Programs using multi-object spectrographs on large telescopes are attempting to study hundreds of quasars simultaneously. Reverberation mapping results are also being used to refine shortcut methods that can yield quasar black hole mass estimates from a single observation.
Because the light from distant quasars can take billions of years to reach the Earth, we see these quasars as they were when the universe was still very young and the earliest black holes were in their extreme growth phases. Our ability to measure the masses of these black holes in the distant past will help us understand how black hole growth took place over cosmic history.