Key Found to Birth of Super-Galaxies

Illustration depicts a quasar in the center of a galaxy that has turned on and is expelling gas at high speeds in a galactic superwind. CREDIT: NASA/CXC/M.Weiss

Astronomers have spotted what they think is evidence for the ignition switch that turns on super-bright galaxies called quasars.

The trigger was likely the merger of two galaxies, researchers said today.

Quasars were first detected in the 1950s based on emissions of radio waves. Astronomers thought they were strange nearby stars. Later they figured out the brilliant beacons were billions of light-years away.

That's how they got the name quasar, which is short for quasi-stellar radio sources.

A frenzy

A quasar is in fact a large galaxy anchored by a supermassive black hole that is actively--very actively-- feeding on surrounding gas. The frenzy causes the gas to heat up and glow so brightly that it outshines the galaxy itself.

How quasars are born has remained a mystery.

NASA's Chandra X-ray Observatory detected hot, X-ray regions around two distant quasars. Each feature is tens of thousands of light-years from the central supermassive black hole thought to power the quasar.

"The X-ray features are likely shock waves that could be a direct result of the turning on of the quasar about 4 billion years ago," said Alan Stockton of the University of Hawaii in Honolulu, and lead author of a report on the findings in the Astrophysical Journal.

The quasars, 4C37.43 and 3C249.1, do not appear to be surrounded by any larger gas envelopes, and the X-ray regions were not associated with radio waves from the quasars.

"The best explanation for our observations is that a burst of star formation, or the activation of the quasar itself, is driving an enormous amount of gas away from the quasar's host galaxy at extremely high speeds," said co-author Hai Fu, also from the University of Hawaii.

A merger

Computer simulations led by Tiziana Di Matteo of Carnegie-Mellon University in Pittsburgh, Pennsylvania suggest a specific mechanism that might have triggered the activity: The merger of two galaxies drives gas toward the central regions where it triggers a burst of star formation and provides fuel for the black hole.

The inflow of gas releases a tremendous amount of energy, and a quasar is born.

The quasar's output dwarfs the rest of the galaxy and generates a superwind that drives material into intergalactic space. The Chandra data provide the best evidence yet for a quasar-produced superwind, the researchers conclude.

After about 100 million years in this scenario, the superwind will drive all the gas out. The quasar phase will end and the galaxy will settle down to a relatively quiet life, more like the situation in our own Milky Way.

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