Two years ago the first images of a black hole’s event horizon made headlines. A follow-up study of a smaller, but closer, supermassive black hole at the heart of the Centaurus A galaxy has explored the enormous jets that make this among the noisiest objects in the sky at radio frequencies. In the process researchers have shed light on these immense, but poorly understood, phenomena.
Even at visible wavelengths, Centaurus A is among the brightest galaxies, seen with the naked eye by some with exceptional eyesight. However, its real claim to fame started in 1949, when early radio telescopes revealed it as one of the strongest radio wave sources, later found to also be true at the other end of the energy spectrum.
These emissions are the consequences of enormous jets spat out by the very active supermassive black hole at the heart of Centaurus A. These jets have now been imaged with previously impossible resolution and the observations analyzed in Nature Astronomy.
“There are bigger galaxies, but if we had radio eyes Centaurus A would be among the most spectacular thing to see in the night sky,” Dr Philip Edwards of Australia’s CSIRO told IFLScience. Its jets stretch across eight degrees of the sky (16 times the full Moon).
Centaurus A looks so large both because it’s relatively close – around 14 million light-years from Earth – and because it’s genuinely enormous. At two million light-years across, its lobes (as the jets are known once they spread out) would stretch from our own Milky Way to Andromeda.
Its southern sky location prevented really detailed observations of this leviathan. However, having observed the silhouettes of one of the largest known black holes, and the relatively modest object at the center of our own galaxy, the Event Horizon Telescope team wanted something in between.
M87, the black hole in the center of the Virgo A galaxy, has a mass around 6.5 billion times that of the Sun, while the Milky Way’s black hole Sagittarius A* is about 4 million solar masses. Centaurus A’s jets are powered by a 55 million solar mass black hole.
The Event Horizon Telescope is not a specific instrument, but rather a way of linking telescopes across continents to maximize resolution. The team responsible brought together radio observatories in Chile, South Africa, and Australia to study the Centaurus A jets with a resolution of 0.6 light days, 16 times sharper than ever before. “We see up close and personally how a monstrously gigantic jet launched by a supermassive black hole is being born”,” said Radboud University Nijmegen PhD student Michael Janssen in a statement.
The observations revealed the jets are brighter at their edges than in the center. Edwards told IFLScience this is not a complete surprise, since the same was seen for M87 and a number of other galaxies, but couldn’t be seen for Centaurus A before.
The discovery rules out some of the explanations for the jets’ brightness, leaving two main theories. “It could be the radiation is from the fast-moving jets interacting with the stationary, or even infalling, parts of the galaxy,” Edwards told IFLScience. “Alternatively it is thought to collimate a jet you need a strong magnetic field wrapped around it to constrain it, and that may make the edges brighter.”
Despite the outstanding resolution, the Event Horizon Telescope was unable to repeat the famous image of a black hole against the radiation from surrounding gas. Edwards explained Centaurus A’s black hole is four times closer than Messier 87, but 120 times smaller. To see it, the team believe they will need to go above the atmosphere, requiring a fleet of radio telescope satellites spaced at least an Earth radius apart.