10 November 2010

Massive gamma ray bubbles discovered in Milky Way

Agence France-Presse
Two huge, mysterious gamma ray-emitting bubbles have been discovered at the centre of the Milky Way galaxy, US astronomers said.
Gamma Ray Bubbles

Energies from 1 to 10 billion electron volts are shown here. The dumbbell-shaped feature (centre) emerges from the galactic centre and extends 50 degrees north and south from the plane of the Milky Way, spanning the sky from the constellation Virgo to the constellation Grus. Credit: NASA/DOE/Fermi LAT/D. Finkbeiner et al.

WASHINGTON: Two huge, mysterious gamma ray-emitting bubbles have been discovered at the centre of the Milky Way galaxy, U.S. astronomers said.

Masked by a fog of gamma rays that appears throughout the sky, the bubbles form a feature spanning 50,000 light-years and could be the remnant of a supersized black hole eruption or the outflows from a burst of star formation, astronomers said.

The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old, the astronomers said in a paper, accepted for publication in The Astrophysical Journal.

Each bubble 25,000 light years long

“What we see are two gamma ray emitting bubbles that extend 25,000 light-years north and south of the galactic centre,” said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Centre for Astrophysics in Boston, Massachusetts, who first recognized the feature.

“We don’t fully understand their nature or origin,” said the expert who along with Harvard graduate students Meng Su and Tracy Slatyer made the discovery while processing publicly available data from NASA’s Fermi Large Area Telescope (LAT).

Launched in 2008, the international Fermi project is the most sensitive and highest-resolution gamma ray detector ever devised. It scans the entire sky every three hours.

Hard to see through gamma-ray fog

Gamma rays are the highest-energy form of light. Other gamma rays studies failed to detect the bubbles partly due to a fog of gamma rays that permeates the sky.

The fog happens when particles moving near the speed of light interact with light and interstellar gas in the Milky Way.

Finkbeiner’s team team constantly refines models to uncover new gamma-ray sources obscured by this so-called diffuse emission and was able to isolate and unveil the bubbles from the LAT data.

Scientists now are conducting more analyses to better understand how the newly discovered structure was formed.

The bubbles’ gamma-ray emissions are much more powerful than the gamma-ray fog seen elsewhere in the Milky Way, the researchers said.

Large, rapid energy release

The bubbles also appear to have well-defined edges, suggesting they were formed as a result of a large and relatively rapid energy release.

While the source of the energy releases remain a mystery, the researchers said one possibility is a particle jet from a supermassive black hole at the galactic centre, as observed in other galaxies outside the Milky Way.

While the Milky Way’s black hole lacks such a jet – which is powered by matter falling inside the black hole, scientists believe it may have had one many millions of years ago.

Cause: burst of star formation?

The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way’s centre several million years ago, said Princeton University scientist David Spergel.

“In other galaxies, we see that starbursts can drive enormous gas outflows,” he added.

“Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics,” Spergel said.

More information:
NASA’s Fermi telescope

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