Astronomers dig out buried black holes

This result helps give astronomers a more accurate census of black holes in the universe.

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A group of astronomers was able to locate hundreds of previously undiscovered black holes by combining optical data from the Sloan Digital Sky Survey (SDSS) with data from the Chandra Source Catalog, a public database containing hundreds of thousands of X-ray sources discovered by the observatory over its first 15 years. They are in galaxies that have not previously been known to host quasars, extraordinarily luminous objects with fast-expanding supermassive black holes.

The black holes in this new study are the supermassive variety, thus containing millions or even billions of times the mass of the Sun. Although nearly all large galaxies are believed to contain massive black holes at their centers, only some will be actively absorbing radiation, and some will be hidden beneath dust and gas.

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

Galaxies that appear normal in optical light, with light from stars and gas but without the distinguishing optical signs of a quasar, but shine brilliantly in X-rays, have been known to astronomers for almost 40 years. These objects are referred to as “XBONGs,” or “X-ray brilliant optically normal galaxies.”

The scientists found 817 XBONG candidates by methodically sifting through the deep Chandra Source Catalog and comparing it to SDSS optical data, which is more than ten times the number of candidates previously known before Chandra was in service. This many XBONG candidates could be found thanks to Chandra’s crisp images, which were on par with those from SDSS, and the vast amount of data in the Chandra Source Catalog. Further investigation found that almost half of these XBONGs are black hole populations that were previously unknown.

Co-author Amanda Malnati, an undergraduate student at Smith College in Northampton, Massachusetts, said, “These results show how powerful it is to compare X-ray and optical data mines. The Chandra Source Catalog is a growing treasure that will help astronomers make discoveries for years.”

The researchers concluded that around half of the XBONG candidates feature X-ray sources hidden under thick gas after analyzing the number of X-rays detected at various energies for each source. Layers of the surrounding gas can more easily block these X-rays than they can higher energy ones.

These X-ray sources are so brilliant practically all of them must come from matter surrounding supermassive black holes that are rapidly expanding. Wide-Field Infrared Survey Explorer data offer further evidence that almost half of the XBONGs are buried within expanding supermassive black holes. From Earth, these black holes are located anywhere between 550 million and 7.8 billion light-years away.

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

Dong-Woo Kim presented these results at the 241st meeting of the American Astronomical Society in Seattle, WA.

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