Something is wrong in the southern constellation of Phoenix.
Strange radio emissions from a distant cluster of galaxies take the form of a giant jellyfish with a head and tentacles. In addition, the cosmic jellyfish only emits the lowest radio frequencies and cannot be detected at higher frequencies. The unusual shape and the radio spectrum tell of intergalactic gas that washed over galaxies and gently swirls up electrons that were spit out from gigantic black holes a long time ago, researchers report on March 10th Astrophysical Journal.
The strange being spans 1.2 million light years and resides in Abell 2877, a cluster of galaxies that is 340 million light years from Earth. Researchers have named the object the USS Jellyfish because of its ultra-steep spectrum (USS Jellyfish) from low to high radio frequencies.
“This is a source that is invisible to most of the radio telescopes we’ve been using for 40 years,” says Melanie Johnston-Hollitt, an astrophysicist at Curtin University in Perth, Australia. “It holds the record of falling the fastest” with increasing radio frequency.
Johnston-Hollitt’s colleague Torrance Hodgson, a PhD student at Curtin, discovered the USS Jellyfish while analyzing data from the Murchison Widefield Array, a complex of radio telescopes in Australia that detect low-frequency radio waves. These radio waves are more than a meter long and correspond to photons, light particles with the lowest energies. Notably, at 87.5 megahertz – a frequency similar to that of an FM radio station – the USS Jellyfish is about 30 times brighter than 185.5 MHz.
“That’s pretty spectacular,” says Reinout van Weeren, an astronomer at Leiden University in the Netherlands who was not involved in the work. “It’s a pretty decent result because it’s really extreme.”
The USS Jellyfish has no relationship with previously discovered jellyfish galaxies. “That’s absolutely huge compared to these other things,” says Johnston-Hollitt. Indeed, jellyfish galaxies are an entirely different cauldron from celestial fish. Although they also inhabit clusters of galaxies, they are single galaxies flowing through hot gas in a cluster. The hot gas tears out the galaxy’s own gas, creating a trail of tentacles. The much larger USS jellyfish, on the other hand, appears to have formed when intergalactic gas and electrons interacted.
Hodgson and colleagues find that two galaxies coincide in the Abell 2877 cluster with the brightest spots of radio waves in the head of the USS Jellyfish. These galaxies, the researchers say, likely have supermassive black holes at their centers. The team ran computer simulations and found that the black holes likely accumulated material about 2 billion years ago. In the process, discs of hot gas formed around each of them, which spat huge jets of material into the surrounding galaxy clusters.
This ejected material had electrons whirling around magnetic fields at almost the speed of light, causing the electrons to emit radio waves. Over time, however, the electrons lost energy, and the most energetic electrons, which had emitted the highest radio frequencies, faded the most. Then a wave of gas sloshed through the entire cluster and accelerated the electrons around the two galaxies again.
“It’s a very gentle process,” says Johnston-Hollitt. “The electrons don’t get that much energy, which means they don’t glow at high frequencies.” Instead, the gentle wave of gas caused electrons to emit radio waves at the lowest energies and frequencies, giving the USS Jellyfish the extreme spectrum it has today.