A Hungry Black Hole Sends a Beam Across Space

Pictor A galaxy with black hole and jet.
The Pictor A galaxy has a supermassive black hole at its center, and material falling onto the black hole is driving an enormous beam, or jet, of particles at nearly the speed of light into intergalactic space. These images show X-ray data obtained by Chandra at various times over 15 years and radio data from the Australia Telescope Compact Array. By studying the details of the structure seen in both X-rays and radio waves, scientists seek to gain a deeper understanding of these huge collimated blasts. NASA/CXC/Univ of Hertfordshire/M.Hardcastle et al., Radio: CSIRO/ATNF/ATCA

It's Bigger than a Death Star — WAY Bigger! 

Imagine a "death beam" stretching across 300,000 light-years of space, more than three times the width of the Milky Way Galaxy!  That's what astronomers studied streaming out of the heart of the distant galaxy Pictor A with the Chandra X-Ray telescope. This beam comes from the region around a superhungry supermassive black hole at the heart of the galaxy. Chandra has been watching this beam for the past 15 years, measuring how fast it's moving away from the black hole. In addition, a small array of radio telescopes in Australia, called the Australian Telescope Compact Array (ACTA) has been watching the same region. The data from both sets of observations were combined to produce a high-resolution "view" of the region. The joint results show features in the beam, and may hint at the existence of another jet, flowing away in the opposite direction from the one we can see. 

Anatomy of the Pictor A Black Hole

The x-ray and radio-wave data tell astronomers a lot about this jet. The x-ray emissions come from electrons that are spiraling around and around magnetic field lines. Those electrons come from the region around the black hole, where gas and other material are being sucked into the accretion disk around the black hole. The disk, which is swirling around quite fast, is superheated by magnetic activity and the friction generated as materials in the gas clouds swirl around and collide.

The electrons generated in this maelstrom escape along the lines of magnetic force, and that's what forms the jet. The magnetic field lines focus the heated material, and that's what shapes the long narrow jet. It's like focusing a beam of light through a tube. In this case, the tube is made up of magnetic field lines. As the electrons spiral out, they're constantly accelerated. The technical term for that shepherding action is "collimation" and the x-rays that are emitted by this spiral action come are created by a process called "synchrotron emission". Astronomers have seen these emissions in the core of the Milky Way, too, although it doesn't have a powerful jet like Pictor A does.

The jet is streaming through clouds of gas, which heats them up and they give off radio waves. The clouds are the pink-colored lobes on either side of the black hole in this image. The supermassive black hole does not actually give off light — instead what we're seeing are the x-rays from the heated material surrounding it. The jet appears to be slamming into a cloud of gas and lighting that up, too.

Monster Black Holes Light up the Hearts of Many Galaxies

To really understand the relationship between supermassive black holes at the hearts of galaxies, and the jets that some of them create, astronomers use whatever tools they can. X-rays and radio waves are always found around these hungry objects and indicate how hot and energetic the regions are. 
Many galaxies, including our own, have black holes feeding at their cores. Unlike the Milky Way, which has a rather quiet black hole at its heart, some galaxies have some real monsters hidden away. Their jets and associated x-ray and radio wave emissions give away their presence.

For astronomers, the jets are a clue to the activity of the black hole as it waxes and wanes. When there's a lot of gas, dust, or possibly even stars spiraling around the black hole, its superheated annihilation and disappearing act into the black hole spurs a strong jet, like the one Chandra and the ACTA studied. When the black hole runs out of food, the action in the accretion disk slows down, which affects the strength and density of the jet. Sometimes the jet can stop completely. So, the study of jets from black holes like the one in Pictor A can tell astronomers something about the environment in the near vicinity.