Saturday, March 27, 2010

Black hole/Quasar combos, out of the dust

Quasars, those super-bright light sources in space, are much more common than anyone thought.

It's just that we haven't been able to see them.

(Image: Examples of strongly interacting/merging galaxies containing a heavily obscured growing supermassive black hole nearby (top panels) and in the early Universe (bottom panels), as observed by the Hubble Space Telescope. Source: University of Hawai'i Institute for Astronomy.)

Like the lights of a city obscured by fog, a new study by astronomers—including ones from the University of Hawai'i—shows that many of them are obscured by vast clouds of interstellar dust.

The study was led by Ezequiel Treister from the University of Hawaii, and published in journal Science March 25.

First, some overly simple definitions.

At the center of activity is a black hole, a super-compact, super-dense phenomenon that sucks in everything around it, including light.

But as matter is sucked in, before the black hole takes complete control, the matter speeds up, experiences friction, and releases large amounts of radiation. You can't detect the black hole direction, but you can detect the radiation around it.

When a black hole collides with, and sucks up a big gas cloud, the energetic process releases lots of radiation, including light—creating a quasar.

Quasars used to be thought of as relatively rare. Not any more.

Now, it turns out that many of the youngest black hole-quasar combinations are difficult to see because they are obscured within immense clouds of dust. Over time, like a fog being cleared by the wind, the dust is blown away, revealing the quasar inside.

Said Treister: "We find that these growing black holes are originally hidden by large amounts of dust, but after 10-100 million years this dust is blown out by the strong radiation pressure, leaving a naked quasar, that is visible in optical wavelengths and keeps shining for another 100 million years.”

The project used combined data from the Hubble, Chandra and Speitzer space observatories to detect the signals of quasars inside dust clouds. One finding: obscured quasars were much more common in the early universe than they are now.

The paper's authors are Treister, an Einstein postdoctoral fellow at the University of Hawaii, Priyamvada Natarajan of Yale, David Sanders of the University of Hawaii, Meg Urry and Kevin Schawinski of Yale and former University of Hawaii's graduate student Jeyhan Kartaltepe, who is now with the National Optical Astronomy Observatory.

© Jan TenBruggencate 2010

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