Rays of light shooting out from the supermassive black hole at the center of a galaxy are lighting up surrounding dust even as we can't see the source. The discovery is significant for what it can teach us both about black holes and galactic structures, but also for the collaborative way in which it was made, after an image was uploaded to Twitter.
Last year astronomy image processor Judy Schmidt was trying to work out if she was seeing immense cones of light in a Hubble image of the galaxy IC 5063, or if her eyes were playing tricks on her. After some image enhancement, she realized what she was seeing was real and wondered what the dark areas in between were. “This is an active galaxy with a supermassive black hole in the middle. Is it...casting galaxy-sized shadows?” Schmidt asked.
Dr Peter Maksym of the Harvard/Smithsonian Center for Astrophysics was already studying a giant loop of ionized gas emerging from IC 5063's central region and was intrigued. He and other astronomers discussed possible explanations with Schmidt, obtained additional Hubble images, and have now published a paper in The Astrophysical Journal Letters reframing ideas about the light emitted around black holes.
Black holes are dark, but the accretion disks around them emit a lot of light. In the case of IC 5063, Maksym, Schmidt, and co-authors suspect the light comes from a torus, or donut shape, hidden from us. They compare the effect to the crepuscular rays seen at sunrise and sunset when thick clouds block the Sun, but atmospheric particles are lit up. Nebulae such as NGC 2261 show something similar; our view of a bright star is blocked but can see the material it illuminates.
“The torus, or ring, could be very thin—light seems to get out almost everywhere. If the torus is big enough it becomes unstable, the gravity and rotation holding it together point one direction near the black hole and in a different direction as influences from the galaxy start to become important. This looks like a warp or a bend," said Maksym in a statement.
When light leaving the torus hits dust strewn around the galaxy some is scattered our way. Every evolved galaxy has dust, but the densities vary greatly. IC 5063 is the product of a very recent galactic merger, which could, in Maksym's words, “Kick up dust everywhere,” allowing us to see an effect that could be happening more weakly in other galaxies.
"It's possible the warping creates big enough gaps for some of the light to shine through, and as the torus rotates, beams of light could sweep through across the galaxy like a lighthouse beams through fog,” Maksym added.
As Maksym's wording suggests, he and the other authors aren't sure they have nailed the explanation. The rays are up to 36,000 light-years long. To make such beams continuous the torus gaps would have to be stable for at least 36,000 years – short by most astronomical standards, but possibly quite long for the active region around a black hole. The possibility outflowing cones of hot gas are causing the effect has not been dismissed, or that the galactic merger herded bright stars into formations, lighting up surrounding dust.
Nevertheless, the authors have no doubt they have come across a new and interesting phenomenon, and hope the publication of their paper will open the discussion up further, possibly including explanations not yet raised.