Gamma rays from a galaxy halfway across the visible universe have been detected by a team of astronomers.
In April 2015, after traveling for about half the age of the universe, a flood of powerful gamma rays from a distant galaxy slammed into Earth’s atmosphere. That torrent generated a cascade of light – a shower that fell onto the waiting mirrors of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona. The resulting data have given astronomers a unique look into that faraway galaxy and the black hole engine at its heart.
Gamma rays are photons of light with very high energies. These gamma rays came from a galaxy known as PKS 1441+25, which is a rare type of galaxy known as a blazar. At its center it hosts a supermassive black hole surrounded by a disk of hot gas and dust.
As material from the disk swirls toward the black hole, some of it gets channeled into twin jets that blast outward like water from a fire hose only much faster – close to the speed of light. One of those jets is aimed nearly in our direction, giving us a view straight into the galaxy’s core.
“We’re looking down the barrel of this relativistic jet,” explains Wystan Benbow of the Harvard-Smithsonian Center for Astrophysics (CfA). “That’s why we’re able to see the gamma rays at all.”
The emitting region, which is at least a tenth of a light-year away and most likely is 5 light-years away, was larger than typically seen in an active galaxy, measuring about a third of a light-year across.
Measuring high-energy gamma rays at all was a surprise. They tend to be either absorbed at the source or on their long journey to Earth. When the galaxy flared to life, it must have generated a huge flood of gamma rays.
The study appears in Astrophysical Journal Letters.