A group of astronomers has been 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 Catalogue, a public database of hundreds of thousands of X-ray sources discovered by the observatory over its first 15 years . They are in galaxies not previously known to host quasars, extraordinarily luminous objects with rapidly expanding supermassive black holes.

The black holes in this new study are of the supermassive variety, thus containing millions or even billions of times the mass of the sun. While almost all large galaxies are believed to have massive black holes at their centers, only a few will actively absorb radiation, and some will be hidden under dust and gas.

Dong-Woo Kim of the Center for Astrophysics | Harvard & Smithsonian (CfA), which 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 behave.”

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

The scientists found 817 XBONG candidates by methodically searching the deep Chandra source catalog and comparing them to optical SDSS data, which is more than ten times the number of candidates previously known before Chandra was employed. So many XBONG candidates could be found thanks to Chandra’s sharp images, which were similar to SDSS’s, and the vast amount of data in the Chandra source catalog. Further investigation revealed that nearly 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-rays and optical data mines. The Chandra Source Catalog is a growing treasure that will help astronomers make discoveries for years to come.”

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

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

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

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