Astronomers from Western Sydney University have discovered one of the largest black hole jets in the sky.
Spanning more than a million light-years from one end to the other, the jet shoots out of a black hole with enormous energy at almost the speed of light. But in the vast space between galaxies, it doesn’t always go its own way.
In cosmic terms, the galaxy NGC 2663 is only 93 million light-years away. If our galaxy were a house, NGC 2663 would be a suburb or two.
Looking at its starlight with ordinary telescopes, we see the familiar elliptical “typical” elliptical galaxy with about ten times as many stars as our Milky Way.
Typically, that is, until we observed NGC 2663 in Western Australia with CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) – a network of 36 linked radio antennas forming a super telescope.
Radio waves revealed a jet of matter that was ejected from the Milky Way by a central black hole. This stream of energetic material is about 50 times larger than the Milky Way: if our eyes could see it in the night sky, it would be bigger than the moon.
While astronomers have spotted such jets before, the sheer size (over a million light-years) and relative proximity of NGC 2663 make these jets the largest known in the sky.
So what did we see when the precision and power of ASKAP resulted in a “close-up” (astronomically speaking!) view of extragalactic jets?
The research was led by Western Sydney University PhD student Velibor Velović and has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society (Preprint available here). Our Evolutionary Map of the Universe (EMU) survey has seen evidence of material pushing back on either side of the jet between galaxies.
This process is similar to the effect seen in jet engines. As the exhaust plume passes through the atmosphere, it is pushed sideways by ambient pressure. This causes the jet to expand and contract, creating pulsations as it travels.
As you can see in the image below, we see regular bright spots in the jet, known as “impact diamonds” because of their shape. As the flow compresses, it glows brighter.
the biggest one
As with jet engines, impact diamonds can also be seen in smaller galaxy-sized jets. We’ve seen jets slam into dense clouds of gas, illuminating them as they pass. But the jets retracting from the side are a more subtle effect, making it harder to observe.
However, until NGC2663, we had not seen this effect on such a huge scale.
This tells us that there is enough material in the intergalactic space around NGC 2663 to push the sides of the jet. In turn, the jet heats and pressurizes the substance.
It’s a feedback loop: intergalactic matter enters the galaxy, the galaxy creates a black hole, the black hole emits a jet, and the jet slows the supply of intergalactic matter to the galaxy.
As the universe evolves, these jets affect how the gas forms galaxies. It’s exciting to see such a direct illustration of this interaction.
Meanwhile, the EMU survey, which is responsible for identifying a new type of mysterious astronomical object called an “odd radio circle,” is continuing to scan the sky. More discoveries to come soon from this remarkable radio jet.
As we do, we will better understand how black holes closely shape the galaxies that form around them.
Luke Barnes, Lecturer in Physics, Western Sydney University; Miroslav Filipovic, Professor, Western Sydney University; Ray Norris, Professor, Faculty of Science, Western Sydney University and Velibor Velović, PhD student, Western Sydney University
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