From Science:
Astronomers are missing as many as one-third of black holes by looking with the wrong telescopes, according to a new study which finds that massive black holes may be hiding behind thick clouds of dust and gas in the centers of galaxies. Astronomers speculate that every galaxy has a supermassive black hole at its center. Our own Milky Way has one, although it’s not actively sucking in matter. Researchers know that there are millions of galaxies in which a glowing disc of particles circles the central black hole, and sometimes jets of ions burst from inside the doughnut-shaped hole under twisty magnetic forces. Most of these so-called active galactic nuclei (AGN) have been found using ground-based optical telescopes, which are cheaper than space-borne x-ray instruments. But when it became clear that huge amounts of x-rays were coming from galaxies that astronomers didn’t think had these active cores, scientists raced to understand why these AGNs had evaded detection. An international team of astronomers used the Suzaku telescope, which is sensitive to x-rays, and an optical telescope to peer at two objects identified by a previous survey as AGNs. They found that the disk of debris around the black hole smothers all but the highest energy radiation and thus renders the black holes undetectable by optical telescopes. However, x-rays were powerful enough to penetrate the thick dust and gas cloud.
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