Chandra Unearths New Black Holes :: NASANET

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Chandra Unearths New Black Holes
A survey has revealed hundreds of previously unidentified black holes using data from the Chandra Source Catalog and the Sloan Digitized Sky Survey (SDSS). The researchers compared optical and X-ray data for a class of objects known as “XBONGs” (X-ray bright optically normal galaxies) to reveal around 400 supermassive black holes. These graphics show these XBONGs in X-rays from Chandra and optical light from SDSS. Credits: X-ray: NASA/CXC/SAO/D. Kim et al.; Optical/IR: Legacy Surveys/D. Lang (Perimeter Institute)

Thanks to NASA’s Chandra X-ray Observatory, astronomers have discovered hundreds of previously hidden or buried supermassive black holes. This result helps give astronomers a more accurate census of black holes in the universe.

The black holes in this new study are supermassive, containing millions or even billions of times the mass of the Sun. While astronomers think that almost all large galaxies harbor giant black holes at their centers, only some of the black holes will attract actively radiation-producing material, and some will be buried under dust and gas.

By combining data from the Chandra Source Catalog, a public repository that includes hundreds of thousands of X-ray sources detected by the observatory during its first 15 years, and optical data from the Sloan Digital Sky Survey (SDSS), a team of astronomers has been able to identify hundreds of black holes that until now had remained hidden. They are found in galaxies where quasars have not previously been detected, extremely bright objects with rapidly growing supermassive black holes.

“Astronomers have already identified a large number of black holes, but many remain elusive,” said Dong-Woo Kim of the Center for Astrophysics | Harvard & Smithsonian (CfA), which led the study. “Our research has uncovered a missing population and has helped us understand how they behave.”

For about 40 years, scientists have known of galaxies that look normal in optical light—with starlight and gas, but without the distinctive optical signatures of a quasar—but that glow brightly in X-rays. These objects are called ” optically normal galaxies bright in X-rays” or “XBONG”.

By systematically trawling Chandra’s deep Source Catalog and comparing it to SDSS optical data, the researchers identified 817 XBONG candidates, more than ten times the number known before Chandra went live. The sharpness of the Chandra images, which are equal in quality to those of SDSS, and the large amount of data in the Chandra Source Catalog have made it possible to detect so many XBONG candidates. Later studies revealed that about half of these XBONGs represent a population of previously hidden black holes.

“These results demonstrate how powerful it is to compare the mines of X-ray and optical data,” said co-author Amanda Malnati, an undergraduate at Smith College in Northampton, Massachusetts. “The Chandra Source Catalog is a growing treasure trove that will help astronomers make discoveries for years to come.”
X-rays are especially useful for looking for rapidly growing black holes because the material swirling around them superheats to millions of degrees and glows brightly at X-ray wavelengths. A thick layer of gas and dust that surrounds a a black hole blocks most or all of the light at optical wavelengths. However, X-rays pass through the cocoon much more easily and can be detected by Chandra.

After studying the amount of X-rays detected at different energies for each source, the team concluded that about half of the XBONG candidates involve X-ray sources that are buried under thick gas because relatively small amounts of X-rays were detected. low energy X-rays. These X-rays are more easily blocked by the surrounding gas layers than the higher energy ones.

These X-ray sources are so bright that almost all of them must come from material surrounding rapidly growing supermassive black holes. Data from NASA’s Wide-Field Infrared Survey Explorer provide additional evidence that about half of XBONGs are buried and growing supermassive black holes. These black holes are found at distances between 550 million and 7.8 billion light years from Earth.

“It’s not every day you can say you’ve discovered a black hole,” said co-author Alyssa Cassity, a graduate student at the University of British Columbia, “so it’s very exciting to realize we’ve discovered hundreds of them.”

The explanation for XBONGs not being buried under thick gas is less clear. About 100 of the X-ray sources may not be single point X-ray sources, but appear scattered. Some of these may be galaxies in previously unidentified groups or clusters of galaxies, which are known to contain large amounts of hot, X-ray-emitting gas. No more than 20% of XBONGs can be classified in this way. The remaining 30% may contain some supermassive black holes located in galaxies where the optical signals from supermassive black holes are diluted by relatively bright starlight. Scientists will need more research to determine the true nature of these XBONGs.

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