Mysterious Light Originates Near A Galaxy's Black Hole
At top left, a VLA image of the M87 galaxy shows the radio-emitting jets at a scale of about 200,000 light-years. Subsequent zooms progress closer into the galaxy's core, where the supermassive black hole resides. In the artist's conception (background), the black hole illustrated at the center is about twice the size of our Solar System, a tiny fraction of the size of the galaxy, but holding some six billion times the mass of the Sun.
Credit: Bill Saxton, NRAO/AUI/NSF

Photons with a trillion times more energy than visible light are flying out of a relatively nearby galaxy.

Until now, scientists didn't understand this light's origin, but a new study shows that its source is a giant black hole inside the M87 galaxy.

The radiation takes the form of high-frequency gamma rays.

"We detect it in roughly 25 galaxies so far but we never knew where exactly it was coming from," said study team member Matthias Beilicke, an astrophysicist at Washington University in St. Louis. "Only in the case of M87 were we able to narrow it down to the black hole vicinity."

Matter funnels into the supermassive black hole from a ring of material circling around it and called an accretion disk. Some matter is also propelled out in very bright jets that stream across space.

One hypothesis suggests the gamma-ray flares occur when an extra amount of matter is pulled out of the accretion disk and injected into the jets. Either electrons or protons in the jet could interact with light photons or matter to create the extremely high-energy gamma rays.

"This acceleration mechanism requires strong magnetic fields which can be found around the black hole," Beilicke said. "We need charged particles accelerated to very high energies in order to emit gamma rays at these energies."

The observations could help scientists understand the environment around gigantic black holes like this one, which contains the mass of more than six billion suns.

"The fact that there are particles accelerated up to such high speeds gives us input for the modeling of these systems," Beilicke told "M87 is so far more or less the only source which really allows this particular kind of study."

The researchers were able to pin down the radiation's origination point by combining measurements of the gamma-rays with radio-wave observations. Even though gamma rays cannot usually pierce through Earth's atmosphere, ground-based telescopes can detect them by looking for telltale flashes of faint blue light that result when gamma rays hit the air. These observations also reflect the direction the gamma rays came from, though they are not very precise.

Astronomers also observed M87 with the National Science Foundation?s Very Long Baseline Array (VLBA), a group of radio telescopes spread out around the United States. These observations revealed a burst of radio light that coincided with the gamma-ray flares, which usually lasted a few days.

The radio flares, imaged with much more precision than gamma-ray light, could be traced back to the location of the supermassive black hole in the center of M87.

"Combining the gamma-ray observations with the supersharp radio 'vision' of the VLBA allowed us to see that the gamma rays are coming from a region very near the black hole itself," said team member Craig Walker of the National Radio Astronomy Observatory (NRAO).

M87, at 50 million light-years from Earth, is much closer than any similar gamma-ray flare sources, so it could be scientists best bet to study these systems for a while.

The new study is detailed in the July 2 issue of the journal Science.