By viewing energetic galaxies called quasars through a high-tech equivalent of polarised sunglasses, astronomers have clinched the case that they are fuelled by discs of hot matter swirling onto huge black holes. The trick could allow them to better understand why the environs of a black hole can shine with the light of a trillion Suns.
鈥淎fter many years of controversy, we finally have very convincing evidence that the expected disc is truly there,鈥� says Makoto Kishimoto from the in Bonn, Germany.
Quasars are the brilliant cores of galaxies that have supermassive black holes at their hearts. Somehow they manage to radiate as much light as a large galaxy of stars, but from a region only about the size of our solar system.
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For decades, physicists have theorised that the radiation comes from a hot 鈥渁ccretion disc鈥� around the black hole. The black hole鈥檚 gravity pulls in surrounding gas and dust, which swirls into a disc and heats up to hundreds of thousands of degrees.
Dense dust
A key prediction of this theory is that infrared light from the outermost, coolest region of the disc should show a very distinctive spectrum. It had never been possible to test this prediction because infrared light from dust clouds outside the disc completely swamps the disc鈥檚 faint glow.
鈥淭he emission from hot dust grains in the surroundings is so strong that it buries emission from the accretion disc, so we just couldn鈥檛 see it,鈥� Kishimoto told 快猫短视频.
However, there was some evidence that light emerging from the accretion disc is slightly polarised, just as light scattering from an ocean surface becomes polarised. So Kishimoto鈥檚 team reasoned that by looking at just the polarised part of a quasar鈥檚 infrared light, they could measure the glow of the disc alone.
Telltale spectrum
The team used the in Hawaii and the European in Chile to look at the faint polarised component of infrared light from six different quasars at an average distance of 5 billion light years. Sure enough, all showed the telltale spectrum predicted for an accretion disc.
鈥淚t looks like the standard picture is correct, which really moves the field of quasars forward,鈥� says Kishimoto.
Now that astronomers have polarised specs that can home in on the outer portions of accretion discs, they hope to answer long-standing questions about their anatomy, including how large the discs are, and how matter falls onto them 鈥� whether smoothly or in sudden bursts.
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