Tour: Tour: Astronomers Dig Out Buried Black Holes With NASA's Chandra

Hundreds of black holes previously hidden, or buried, have been found using NASA’s Chandra X-ray Observatory. This result helps give astronomers a more accurate census of black holes in the Universe.

The black holes in this new study are the supermassive variety that contain millions or even billions of times the mass of the Sun. While astronomers think that almost all large galaxies harbor giant black holes in their centers, only some of the black holes will be actively pulling in material that produces radiation and some will be obscured by dust and gas.

The result was made possible by using data from the Chandra Source Catalog, a public repository including hundreds of thousands of X-ray sources detected by the observatory over its first 15 years. They combined the X-ray information with optical data from the Sloan Digital Sky Survey, or SDSS. From this, a team of astronomers was able to identify hundreds of black holes that had previously been hidden. They are in galaxies not previously identified to contain quasars, extremely bright objects containing rapidly growing supermassive black holes.

While astronomers have already identified huge numbers of black holes over the years, many of these exotic objects remain elusive. This research has uncovered a missing population of black holes and helped scientists understand how they are behaving.

For about 40 years scientists have known about galaxies that look normal in optical light — with light from stars and gas but not the distinctive optical signatures of a quasar — but shine brightly in X-rays. They refer to these objects as “X-ray bright optically normal galaxies” or “XBONGs”.

By systematically combing through the deep Chandra Source Catalog and comparing to SDSS optical data, the researchers identified 817 XBONG candidates, more than ten times the number known before Chandra was in operation.

X-rays are particularly useful to search for rapidly growing black holes because material swirling around them is superheated to millions of degrees and then glows strongly in X-ray wavelengths. A thick cocoon of gas and dust surrounding a black hole will block most or all the light at optical wavelengths. X-rays, however, pass through the cocoon much more easily to be detected by Chandra.

However, X-rays could not make this discovery alone. Only by combining Chandra data with the optical was the team able to identify these previously unknown black holes. It is yet another example of how telescopes spanning the electromagnetic spectrum often work together to make exciting progress in exploring the Universe.