Giant Radio Galaxy Supercharges Big Bang Leftovers

The radio galaxy Centaurus A, which is one of the brightest sources of radio waves in the sky, also radiates extremely high-energy gamma-rays, new observations from NASA's Fermi Gamma-Ray Telescope have found.

"This is something we've never seen before in gamma-rays," said Teddy Cheung, a Fermi team member at the Naval Research Laboratory in Washington, D.C.

Scientists think these gamma-rays, the highest-energy form of light in the universe, began as remnant radiation from the Big Bang thought to be the beginning of our universe about 14 billion years ago. But this radiation has been amped up to higher energy levels by the energetic particles zooming around magnetic fields in huge lobes that extend from either side of the galaxy, researchers said. This ramp-up in energy of photons is thought to be a fairly common process in the universe, but is the first time it has been observed in this particular situation.

Active radio galaxy

Centaurus A, also known as NGC 5128, is the closest so-called radio galaxy to Earth, located about 12 million light-years away in the constellation Centaurus. (One light-year is the distance that light travels in one year, about 6 trillion miles, or 9 trillion kilometers.)

Centaurus A is also an active galaxy, which means that its central region that exhibits strong emission across many wavelengths of the light spectrum. Active galaxies are powered by an actively feeding black hole millions of times more massive than the sun.

In a radio galaxy, "the black hole somehow diverts some of the matter falling toward it into two oppositely directed jets that stream away from the center," explained Yasushi Fukazawa of Hiroshima University in Japan, and a member of the team that studied the gamma-ray emission.

These jets contain magnetized particles that move near the speed of light. Over the course of tens of millions of years, the jets expand out into two large lobes that straddle the central source of the galaxy and extend out into space about 1 million light-years. The radio waves from the lobes arise as high-speed electrons spiral through the lobes' tangled magnetic fields.

When Fermi turned its eye on Centaurus A during its first 10 months of data collection, it found quite a surprise: In addition to the radio waves known to emanate from the lobes, Fermi also detected gamma-rays radiating from the galaxy.

"Not only do we see the extended radio lobes, but their gamma-ray output is more than 10 times greater than their radio output," Cheung said.

How to get gamma-rays

The gamma-rays arise through a process that involves the spiraling magnetic fields of the radio lobes and other low-energy radiation that permeates the universe: Photons from the cosmic microwave background (the remnant radiation of the Big Bang), as well as infrared and visible light from stars and galaxies, can collide with the high-energy particles zooming through the lobes of Centaurus A.

"When one of these photons collides with a super-fast particle in the radio lobes, the photon receives such an energy boost, it becomes a gamma ray," said team member Lukasz Stawarz of the Japan Aerospace Exploration Agency (JAXA) in Japan.

The process that accelerates the photons up to gamma-rays is called inverse Compton scattering and is a common way of making cosmic gamma-rays.

The process had been known to produce X-rays in dozens of active galaxies, but Centaurus A is the first case where astronomers have solid evidence that that microwaves can be boosted up to gamma-ray energies. Cheung had theorized before Fermi launched (in June 2008) that the process would work and produce gamma-rays in this way for galaxies like Centaurus A.

The team is also looking for the gamma-ray emission from another active radio galaxy, Fornax A.

The discovery is detailed in the April 2 issue of the journal Science.

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