Hidden black holes.

Astronomers have new evidence that a majority of the biggest black holes in the universe lie hidden behind thick veils of dust.

Monster black holes, which reside at the centers of galaxies, betray their presence when their immense gravity induces surrounding matter to generate a brilliant beacon of light known as a quasar. Models of the quasar light suggest that these beacons make a large contribution to the overall X-ray glow of the sky. However, the number of individual quasars actually observed in X-ray and visible-light studies falls far short of the expected population.

Researchers have proposed that the shortfall comes about because many quasars are shrouded by their own dusty rings or because they reside in dust-drenched galaxies that keep the beacons' visible light and even their X rays hidden from astronomers.

Infrared light, however, can make it through even a thick layer of dust. Using a sensitive infrared observatory, the Spitzer Space Telescope, researchers have uncovered 21 previously unknown quasars in a small patch of sky. By extrapolating that number to the entire sky, Alejo Martinez-Sansigre of the University of Oxford in England and his collaborators came up with the predicted number of quasars in the cosmos.

Black holes that accumulate huge masses do so at the cores of dusty, starforming galaxies, the Oxford team asserts in the Aug. 4 Nature. Another team, led by Jennifer Donley of the University of Arizona in Tucson, describes similar Spitzer findings in an upcoming Astrophysical Journal.