December 17, 2015
For the first time, scientists have observed gamma radiation from a well-known distant galaxy. In the center of the active galaxy PKS 1441+25 resides a massive black hole, which is surrounded by a luminous disk of matter. The latest observation leads not only to a better understanding of active galaxies: Also, because the light from PKS 1441+25 has been traveling for around 7.6 billion years on its way to Earth, researchers anticipate new insights out of the “middle ages” of the universe, which came into being 13.8 billion years ago.
PKS 1441+25 is among the roughly 10 percent of galaxies in the universe described as active. Their common characteristic: They produce more light than can be accounted for by the brightness of their stars and dust alone. In the center of each lies a supermassive black hole almost as heavy as a billion suns.
Black holes exert a strong gravitational force on matter in their surroundings. Before material is swallowed up by a black hole, it circles around the active nucleus as a brightly shining disk. PKS 1441+25 also belongs to the class of extremely bright quasars: In these objects, elementary particles are ejected as so-called jets into space at the speed of light – in the case of PKS 1441+25 the jets are pointed toward Earth.
With the MAGIC telescopes on the Spanish island of La Palma, scientists observed highly energetic gamma rays from PKS 1441+25. The quasar’s light reached Earth after roughly 7.6 billion years — thus locating the galaxy in the “middle ages” of the 13.8 billion-year-old universe.
“Besides PKS 1441+25, we know of only one active galaxy so far away from which the gamma radiation could be detected,” explains Dr. Razmik Mirzoyan, speaker of the MAGIC collaboration and a researcher at the Max Planck Institute for Physics. “We have also detected this galaxy, B0218+357, with MAGIC in 2014.”
The observation of PKS 1441+25 shows that the quasar’s activity is highly variable: The gamma-ray emissions detected were as high as 250 gigaelectronvolt (GeV). These outbursts were up to 100 times stronger than the gamma radiation profile otherwise observed. The reasons for this variability remain obscure.
Through the observations, however, scientists learnt where the source of the extremely intense outbursts lies. “They arise many billions of kilometers away from the active nucleus,” says Mirzoyan, “while the other emissions take shape much closer to the black hole.”
Apart from its unusual behavior, the quasar is interesting in one further, very important respect. The cosmos is filled with diffuse extragalactic background light – photons from all the stars and galaxies that have ever existed in the universe. Thus the cosmic haze holds important information about the universe’s past.
From our vantage point in the Milky Way, from which it is hardly possible to determine how dense the background light is, astrophysicists use indirect methods. They measure gamma rays from distant galaxies. On their way to Earth, the highly energetic rays become attenuated: When they encounter light particles, they are converted into one electron and one positron – and as a result are, for observational purposes, lost. The more dense the haze is, the more gamma rays will be absorbed by the background light.
“For the exact determination of the extragalactic background light, gamma rays from far-distant objects are required,” says Mirzoyan. “With PKS 1441+25, we now have ‘captured’ a gamma source that is twice as far away as previously studied objects. Essentially we have doubled our own record observation range established in 2007(*) and now we can obtain information about the state of the universe 7.6 billion years ago.”
More distant quasars show a loss of higher-energy gamma rays thanks to the extragalactic background light (EBL), a "cosmic fog" that permeates the universe. If a gamma ray on its way to Earth collides with lower-energy light in the EBL, it converts into a pair of particles and is lost to astronomers. As shown by the graphs at left in this illustration, the more distant the blazar, the fewer high-energy gamma rays we can detect.
© NASA Goddard Space Flight Center/MAGIC
These latest measurements are in compatible with the established models for the development of stars and galaxies. At 250 GeV, the gamma rays from PKS 1441+25 fall within the scale that fits the models.
“It would have been exciting if we had found gamma rays with significantly higher energies, for example from around 1000 GeV and up,” Mirzoyan adds. “Then we would have had to rethink our models – or assume that we are dealing with previously unknown physical processes that enable more gamma rays to make their way to us through the cosmic background light.”
The active galaxy’s strong gamma radiation was discovered in April 2015, when the quasar hurled an especially strong flare in the direction of Earth. It was first “seen” by the LAT instrument of NASA’s Fermi satellite. This one scans the entire night sky in just about three hours.
Because Fermi only measures the lower region of the gamma ray spectrum, the ground-based double telescope MAGIC, specializing in higher energies, was quickly trained on the object. A few days later the outburst was also observed by the VERITAS telescope in Arizona, USA. In all, MAGIC gathered observations over ten days.
BW / HOR
