Powerful gamma rays seen in distant galaxy

The light from a blazar could help find the density of the Universe’s photon fog

Blazars are very compact quazars. A supermassive black hole accrets gas and dust, superheating it to the point it gives off intense light. 
Credit:NASA/JPL-Caltech/T. Pyle (SSC)

Bright gamma rays have been spotted from a galaxy that should be too far away for the energetic light to reach us. The light is emitted from a blazar, a rare type of galaxy that emits huge amounts of energy that’s powered by a central black hole.

Light from the object travelled over 7.6 billion light years before being observed with VERITAS, an array of telescopes that detect gamma rays and NASA's Fermi Gamma Ray Telescope. It was a shock that any gamma rays from this galaxy reached us at all as they should have been drowned out in the fog of visible light that fills the Universe; the extragalactic background light (EBL).

“We were surprised to detect high-energy gamma rays,” says Manel Errando from Washington University in St Louis, “because they don’t usually make it through. It’s like turning on your high beams in a fog. The fact that the gamma rays get through means that there is not as much fog as you go farther out.”

How far gamma rays can travel through the EBL can be used to work out how dense the fog is, and this new quasar will help refine measurements even further.

“With PKS 1441+25, we can now place tight constraints on this loose net of photons,” said Jonathan Biteau from University of California, Santa Cruz. “This is clearly the opening of a new era where we can compare source-by-source measurements and start to probe the cosmic evolution of the extragalactic background light.”

It also seems that the emitted gamma rays are not from the centre of the galaxy. When the galaxy was imaged at radio wavelengths, it was found the radio pulsed along with the gamma rays suggesting they originated from the same place.

“Whenever we see radio waves, we assume they are coming from far away from the black hole,” says Errando. “If they are created in a very dense environment, they are immediately absorbed. Only when the density gets low enough are they able to propagate outward. So we knew that the radio emission was coming from far up the jet, quite far away from the black hole.”

This is surprising, as the central black hole, the most energetic part of the galaxy, is where it is typically assumed gamma rays come from. Instead it appears the rays came from a region roughly four light years away, the distance between Earth and our nearest star.

Whatever is going on, it’s a system that’s unlike anything we have seen before.

“These observations constitute a fantastic step forward in our understanding of blazars as cosmic accelerators and as light beacons for gamma ray cosmology,” Biteau said.


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