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Music

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Narrator: Most big galaxies contain big black holes. Not just big,

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supersized, with millions of times the sun's mass

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Some of these black holes are actively devouring gas. This drives

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particle jets that can spew matter millions of light-years into space, and it

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also makes the holes a source of penetrating, or hard, X-rays.

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At these energies, the sky glows in every direction, even far away

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from bright sources. Astronomers have long suspected that active

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supermassive black holes in galaxies were responsible, but they just couldn't find

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enough of them to account for the X-ray glow--especially the peak of the energy spectrum.

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Now, astronomers using NASA's

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Swift satellite confirm that a largely unseen population of black-hole-powered

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galaxies is out there. There are so many that scientists say they might fully

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account for the cosmic X-ray background. What emission we

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detect from an active black hole is a function of how we see it--whether we're looking

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face-on and into one of it's jets, or viewing it from the side, through the disk of gas

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and dust that surrounds it. The brightest active black

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holes, which include quasars and blazars, are those we see face-on.

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But as the viewing angle increases, the surrounding disk absorbs increasing

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amounts of radiation. Astronomers have always assumed that many

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active galaxies were oriented edgewise to us, but because the disk of gas

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smothers most of their X-rays, these sideways black holes just weren't detected.

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And that's where Swift comes in. Since

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2004, the satellites Burst Alert Telescope has been building up the largest, most

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sensitive X-ray map of the sky. Using these data,

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astronomers found that the most heavily absorbed galaxies create the energy peak in the

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cosmic X-ray background. What does it all mean?

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When the universe was about half its present age, about 7 billion years ago, galaxies

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crashed together more frequently and these collisions produced gas rich galaxies

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with heavily obscured black holes. The Swift survey shows that

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galaxy mergers helped activate these black holes by feeding them torrents of fresh gas.

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The new findings are consistent with idea that the X-ray background

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peaked around this time, when our own galaxy was young and before our